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-Injectable Aqueous Suspensions of Sex Hormones and How Depot Injectables Work - Transfeminine Science
Injectable Aqueous Suspensions of Sex Hormones and How Depot Injectables Work
By Aly | First published June 14, 2019 | Last modified August 2, 2022
Abstract / TL;DR
Depot injectable steroid preparations are formulated mainly as oil solutions and crystalline aqueous suspensions. Oil solutions of steroids are more well-known and widely used but aqueous suspensions also exist and continue to be employed in medicine. For example, depot medroxyprogesterone acetate and estradiol cypionate-containing combined injectable contraceptives are aqueous suspensions. Both oil solutions and aqueous suspensions produce a depot effect. However, they function in very different ways to produce this depot effect. Oil solutions form an oil depot at the site of injection which is slowly absorbed and from which the steroid slowly escapes, whereas aqueous suspensions deliver crystals of pure steroid which slowly dissolve and are absorbed. Aqueous suspensions are essentially like tiny pellet implants that can be injected instead of surgically inserted. Aqueous suspensions tend to have longer durations than oil solutions and can allow for much wider dosing intervals in comparison. The dissolution rates and durations of aqueous suspensions are highly dependent on crystal size. A limitation of aqueous suspensions is that they can cause pain and irritation at the site of injection in a way that oil solutions have not been associated with and this is likely responsible for their less widespread use. Aqueous suspensions of estradiol benzoate (Agofollin Depot) and progesterone (Agolutin Depot) remain limitedly and sporadically available and have much longer durations than the more common oil solutions of these steroids. These preparations may be potential options for use by transfeminine people.
Introduction
Depot injectable sex steroids are administered by intramuscular injection (into muscle) or subcutaneous injection (into fat) and are formulated in two main ways: 1) as oil solutions; and 2) as crystalline aqueous suspensions. What most transfeminine people are familiar with when it comes to injectable steroid preparations are oil solutions. This is how most common preparations like injectable estradiol valerate (Delestrogen, Progynon-Depot), estradiol cypionate (Depo-Estradiol), testosterone esters, progesterone, and hydroxyprogesterone caproate are formulated. However, injectable aqueous suspensions of sex steroids also exist and are used medically. The most major and well-known formulations include depot medroxyprogesterone acetate (Depo-Provera) and combined injectable contraceptives containing estradiol cypionate/medroxyprogesterone acetate (Cyclofem, Lunelle). Additionally, there exist more obscure preparations of aqueous suspensions such as estradiol benzoate (Agofollin Depot) and progesterone (Agolutin Depot) among others which are marketed in addition to the more common oil solutions of these steroids. These preparations may remain commercially available, including online from sites like EU Aibolit (Reddit). Many transfeminine people are unaware of what aqueous suspensions are or their medical availability. It is notable in this regard that aqueous suspensions are very different from oil solutions in their properties but can have much longer durations in comparison. This is potentially of therapeutic value especially in the case of otherwise shorter-acting injectables like progesterone. The purpose of this article is to shed light on injectable aqueous suspensions, describe how injectable oil solutions and aqueous suspensions work to achieve their depot effect and how they are different, and discuss how relevant aqueous suspensions can be obtained for medical use.
How Depot Injectables Work
Sex steroids are typically lipophilic (fat-soluble or “lipid-loving”) and hydrophobic (water-insoluble or “water-hating”). In other words, they dissolve in and are “attracted to” lipids (e.g., fats), and they are poorly soluble in and repelled by water. From chemistry, this is because sex steroids are very non-polar, whereas water is a quite polar molecule. As a result of their lipophilicity, sex steroids readily form clear homogenous solutions when mixed into oil—that is, they form oil solutions. In contrast, because sex steroids are hydrophobic, they do not easily form solutions when mixed into water—that is, they do not easily form aqueous solutions (like, e.g., salt and water). Instead of aqueous solutions, solid “clumps” or crystal particles of sex steroids can be mixed into and thereby suspended in water—that is, aqueous suspensions of sex steroid crystals can be made. Depot injectables are formulated as oil solutions or aqueous suspensions and these preparations have very different properties.
Injectable Oil Solutions
When an oil solution of a sex steroid is administered by intramuscular or subcutaneous injection, the solution is trapped within the tissue compartment it is injected into and remains there. As the tissue fluid is a water mixture, the oil solution stays together inside the tissue compartment and does not easily separate or distribute. This is because the lipophilic fats and sex steroids within the solution are attracted to each other and are repelled by water. Instead of rapidly dissolving, the fats and sex steroids at the edges of the oil solution are very slowly absorbed into the surrounding water. Once they have escaped the oil depot into the surrounding tissue fluid, they can be distributed into the bloodstream and then into other tissues to exert their biological effects. Eventually, the whole oil solution will be absorbed.
Oftentimes sex steroids that are used by intramuscular or subcutaneous injection are esterified with one or more lipophilic hydrocarbonesters. These esters include fatty acids like propanoic acid (propionate), pentanoic acid (valerate), hexanoic acid (caproate), heptanoic acid (enanthate), decanoic acid (decanote or decylate), and undecanoic acid (undecylate or undecanoate) as well as cyclic compounds like benzoic acid (benzoate), cyclopentylpropanoic acid (cypionate), and phenylpropanoic acid (phenylpropionate), among many others. Examples of these sex steroid esters include the well-known estradiol valerate, estradiol cypionate, estradiol benzoate, hydroxyprogesterone caproate, and numerous others. The attachment of a lipophilic ester (e.g., valeric acid) to a sex steroid (e.g., estradiol) will increase the lipophilicity of the sex steroid compared to merely injecting the unesterified sex steroid in an oil solution. The longer the carbon atom chain in the case of the simple fatty acid esters (e.g., propionate, valerate, enanthate, undecylate), the more lipophilic the resulting esterified sex steroid will be. As a result, the injected sex steroid ester will escape the oil tissue depot more slowly, lengthening the amount of time it takes for the sex steroid ester to be absorbed and therefore its duration in the body. The tables here and here show the lengthening duration of estradiol with longer or bulkier and and more lipophilic esters. Whereas an intramuscular injection of estradiol or progesterone in oil solution has a duration of only around 2 days, an intramuscular injection of an oil solution of estradiol undecylate, an ester of estradiol with a long fatty acid chain, has a duration measured in months. And an intramuscular injection of an oil solution of hydroxyprogesterone caproate, an ester of a derivative of progesterone that has a medium-length fatty acid chain, has a duration measured in weeks.
Most sex steroid esters themselves are biologically inactive. Once they have left the oil tissue depot, they are rapidly cleaved by esteraseenzymes into free unesterified steroid (e.g., estradiol, testosterone) and the previously connected ester moiety (e.g., valeric acid). Hence, most sex steroid esters are prodrugs and are otherwise identical to their parent sex steroids in their biological actions. In the case of esters of estradiol and testosterone, this means that they are bioidentical just like the unesterified steroids. Certain synthetic progesterone derivatives like hydroxyprogesterone caproate and medroxyprogesterone acetate are however not prodrugs and are not meaningfully cleaved into the unesterified parent compound. Instead, they have intrinsic hormonal activity of their own and act without bioactivation.
Injectable Aqueous Suspensions
Aqueous suspensions of sex steroids also form an injection-site depot and achieve a long-lasting depot effect when administered by subcutaneous or intramuscular injection. However, they work in a completely different way than oil solutions. Aqueous suspensions of sex steroids consist of tiny crystal particles of pure sex steroid that are suspended in water. These sex steroid particles are highly lipophilic and hydrophobic. When injected, the hydrophilic water vehicle is rapidly mixed into the fluid of the tissue compartment and absorbed by the body. But the hydrophobic sex steroid crystals are not, and instead float about in the fluid of the tissue compartment. As with oil solutions, the sex steroids at the edges of the crystals very slowly dissolve off the surface of the crystals into the surrounding water and are then distributed into the circulation and tissues. Eventually, the crystal will be fully absorbed into the body, but only after a long period of time. The rate of absorption of the particles is dependent on the properties of the particle crystal lattice and varies depending on the compound.
In the case of aqueous suspensions, the duration of the sex steroid is additionally highly dependent on particle size. These particle sizes have ranged from nanocrystalline to microcrystalline to macrocrystalline in their range. Almost always however it is microcrystalline particle sizes that have been used in injectable aqueous suspensions of sex steroids. (The present author has seen macrocrystalline preparations described a few times, specifically in research on combined injectable contraceptives (Garza-Flores, Del, & Perez-Palacios, 1992; Newton, d’Arcangues, & Hall, 1994; Sang, 1994), and is fairly sure that no such preparations have ever been marketed. On the other hand, nanocrystalline aqueous suspensions have been used with depot antipsychotics (Spanarello & Ferla, 2014; Correll et al., 2021).) Typically, there is a given particle size range for the formulation, such as 0.01 to 0.1 mm in diameter. The larger the particle sizes, the slower the absorption into the body, and the longer the duration of the preparation; the smaller the particle sizes, the faster the absorption, and the shorter the duration. When microcrystalline aqueous suspensions of sex steroids are manufactured nowadays, the particle sizes are defined and carefully controlled. Particle sizes influence the duration of injectable aqueous suspensions because they result in different surface areas from which sex steroid ester can escape particles. A single large particle has a smaller total surface area and hence dissolution rate than the same particle divided up into many smaller particles.
Particle sizes are manipulated during manufacturing via micronization—the process of decreasing the diameter of larger particles, such as via milling or grinding. Whereas more micronization improves the absorption and bioavailability of estradiol and progesterone with oral administration by increasing the surface area available for absorption into the body (Wiki; Wiki), less micronization decreases the rate of absorption of crystalline aqueous suspensions via depot injection and thereby extends the durations of these preparations by decreasing the total surface area for absorption.
There is a notable similarity of injectable aqueous suspensions of sex steroid to implantable sex steroid pellets, for instance of estradiol, testosterone, and progesterone (Wiki; Wiki; Wiki). Pellet implants are basically just pure crystalline sex steroid compressed into the shape of a small cylinder (Photo; Photo). They are inserted into subcutaneous fat in the body through a small incision using a large needle-like instrument called a trocar (Diagram). Once implanted, pellets slowly dissolve and absorb into the body over time, eventually disappearing completely. As they are nothing but pure crystalline hormone, there is no need for them to be removed or retrieved later on. In other words, implantation of a pellet is in a way the same thing as a subcutaneous injection of an aqueous suspension of sex steroid crystals—a single pellet is just one massive crystal instead of many tiny crystals suspended in water. And with very large crystals comes a very long duration—typically 6 months or more for each subcutaneous pellet of estradiol or testosterone (Kuhl, 2005; Wiki; Wiki). However, though injectable aqueous suspensions are typically much less prolonged than pellet implants, they have the advantages over pellets of being less expensive and not requiring a surgical incision. Due to the similarity between aqueous suspensions and pellet implants, aqueous suspensions have been described and marketed as “micropellets” in the past.
Medical Use of Injectable Aqueous Suspensions
Clinical Durations of Injectable Aqueous Suspensions
Studies that have compared sex steroids in injectable oil solutions versus injectable aqueous suspensions have generally found that the durations are considerably longer with aqueous suspensions than with oil solutions. Whereas injectable estradiol and estrone in oil solution have durations of only about 1 to 2 days, aqueous suspensions of these steroids have durations of 2 to 7 days (Table). Moreover, whereas injectable estradiol benzoate in oil solution has a duration of 4 to 6 days, injectable estradiol benzoate as a microcrystalline aqueous suspension has a duration of 2 to 3 weeks (Table). Such a duration is on par with the duration of longer-acting injectable estradiol esters like estradiol cypionate in oil solution and estradiol enanthate in oil solution. In addition, whereas injectable progesterone in oil solution has a duration of about 2 to 3 days, injectable aqueous suspensions of progesterone have a duration in the range of 1 to 2 weeks (Table). This is a duration that is comparable to that of injectable hydroxyprogesterone caproate in oil solution. The prolonged duration of injectable progesterone suspensions is particularly notable as progesterone cannot be esterified and hence injectable progesterone in oil solution cannot be prolonged except with structural modification (as in progestins like hydroxyprogesterone caproate and dihydroxyprogesterone acetophenide) (Wiki). The duration of injectable testosterone propionate in oil solution is 3 to 4 days, in a suspension of small crystals (0.04–0.1 mm) is 8 days, and as commercial-size crystals (0.05–0.2 mm) is 12 days (Sinkula, 1978). Testosterone isobutyrate as an aqueous suspension is said to have a duration of 2 to 3 weeks (Sinkula, 1978; Table). Some injectable aqueous suspensions can have extremely prolonged durations. Depot medroxyprogesterone acetate for instance has a duration of at least 3 months and as long as 6 to 9 months (Wiki). It greatly outlasts the related injectable progestogen norethisterone enanthate in oil solution (Noristerat) (Bassol & Garza-Flores, 1994; Paulen & Curtis, 2009).
Availability of Injectable Aqueous Suspensions
A list of injectable aqueous suspensions of sex steroids that are known to have been marketed can be found here. Most of them were introduced in the 1950s and many of them have been discontinued, but several of the preparations remain available today. These include the single-drug preparations estradiol benzoate (Agofollin Depot, Ovocyclin M), progesterone (Agolutin Depot), testosterone isobutyrate (Agovirin Depot), and medroxyprogesterone acetate (Depo-Provera, Depo-SubQ Provera 104) as well as the combination preparations estradiol benzoate/testosterone isobutyrate (Folivirin, Femandren M) and estradiol cypionate/medroxyprogesterone acetate (Cyclofem, Lunelle). Among the more notable injectable aqueous suspensions that are no longer marketed include estradiol (Aquadiol, Diogyn, Progynon Aqueous Suspension, Progynon Micropellets), estrone (Estrone Aqueous Suspension, Kestrone, Theelin Aqueous), and testosterone (Andronaq, Sterotate, Virosterone) as well as the combination preparation estradiol benzoate/progesterone (Sistocyclin).
Folivirin (estradiol benzoate/testosterone isobutyrate suspension): PDF; Google Translate
The company that manufactures these products, BB Pharma, is notably also the producer of the well-known Neofollin (injectable estradiol valerate in oil solution) in Europe.
Injection Procedure for Injectable Aqueous Suspensions
Aqueous suspensions require larger needle gauges (e.g., 20 or 21 gauge) than oil or aqueous solutions in order to allow the steroid crystals to pass through the needle lumen. The needle sizes may vary depending on the preparation and its crystal sizes. Injectable aqueous suspensions should be shaken adequately prior to injection as the crystals tend to aggregate. Aqueous suspensions are known to be more irritating and prone to causing pain and injection site reactions like redness and swelling than oil solutions, although this may vary depending on the preparation (Wiki; Wiki). This property is likely responsible for the discontinuation of many injectable suspensions and the greater popularity of injectable oil solutions. On the other hand, an advantage of aqueous suspensions over oil solutions is that in contrast them, there is no oil, and hence there is no risk of allergic reaction to the oil. As with injectable oil solutions, injectable aqueous suspensions are indicated for use specifically by intramuscular injection. Depot medroxyprogesterone acetate, which is available in both formulations for both intramuscular injection (Depo-Provera) and subcutaneous injection (Depo-SubQ Provera 104), is suggestive that aqueous suspensions have similar properties both by intramuscular and subcutaneous injection analogously to the case of injectable oil solutions (Wiki; Wiki). However, it should be noted that depot medroxyprogesterone acetate is formulated differently between these preparations.
Update: Pharmaceutical Suspensions Discontinued
Since this article was posted, Agofollin Depot (estradiol benzoate), Agolutin Depot (progesterone), and Folivirin (estradiol benzoate/testosterone isobutyrate) appear to have been discontinued by their manufacturer and seem to no longer be available from online pharmacies.
References
Bassol, S., & Garza-Flores, J. (1994). Review of ovulation return upon discontinuation of once-a-month injectable contraceptives. Contraception, 49(5), 441–453. [DOI:10.1016/0010-7824(94)90003-5]
Correll, C. U., Kim, E., Sliwa, J. K., Hamm, W., Gopal, S., Mathews, M., Venkatasubramanian, R., & Saklad, S. R. (2021). Pharmacokinetic Characteristics of Long-Acting Injectable Antipsychotics for Schizophrenia: An Overview. CNS Drugs, 35(1), 39–59. [DOI:10.1007/s40263-020-00779-5]
Garza-Flores, J., Cravioto, M. C., & Pérez-Palacios, G. (1992). Steroid injectable contraception: Current concepts and perspectives. In Sitruk-Ware, L. R., & Bardin, C. W. (Eds.). Contraception: Newer Pharmacological Agents, Devices, and Delivery Systems (pp. 41–70). New York: M. Dekker. [Google Scholar] [Google Books] [OpenLibrary] [WorldCat]
Kuhl, H. (2005). Pharmacology of estrogens and progestogens: influence of different routes of administration. Climacteric, 8(Suppl 1), 3–63. [DOI:10.1080/13697130500148875] [PDF]
Newton, J. R., d’Arcangues, C., & Hall, P. E. (1994). A review of ‘once-a-month’ combined injectable contraceptives. Journal of Obstetrics and Gynaecology, 14(Suppl 1), S1–S34. [DOI:10.3109/01443619409027641]
Paulen, M. E., & Curtis, K. M. (2009). When can a woman have repeat progestogen-only injectables–depot medroxyprogesterone acetate or norethisterone enantate? Contraception, 80(4), 391–408. [DOI:10.1016/j.contraception.2009.03.023]
Sang, G. (1994). Pharmacodynamic effects of once-a-month combined injectable contraceptives. Contraception, 49(4), 361–385. [DOI:10.1016/0010-7824(94)90033-7]
Sinkula, A. A. (1978). Methods to Achieve Sustained Drug Delivery. The Chemical Approach. In Robinson, J. R. (Ed.). Sustained and Controlled Release Drug Delivery Systems (pp. 411–555). New York/Basel: Marcel Dekker. [Google Scholar] [Google Books] [PDF]
Spanarello, S., & Ferla, T. (2014). The Pharmacokinetics of Long-Acting Antipsychotic Medications. Current Clinical Pharmacology, 9(3), 310–317. [DOI:10.2174/15748847113089990051]
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+Injectable Aqueous Suspensions of Sex Hormones and How Depot Injectables Work - Transfeminine Science
Injectable Aqueous Suspensions of Sex Hormones and How Depot Injectables Work
By Aly | First published June 14, 2019 | Last modified August 2, 2022
Abstract / TL;DR
Depot injectable steroid preparations are formulated mainly as oil solutions and crystalline aqueous suspensions. Oil solutions of steroids are more well-known and widely used but aqueous suspensions also exist and continue to be employed in medicine. For example, depot medroxyprogesterone acetate and estradiol cypionate-containing combined injectable contraceptives are aqueous suspensions. Both oil solutions and aqueous suspensions produce a depot effect. However, they function in very different ways to produce this depot effect. Oil solutions form an oil depot at the site of injection which is slowly absorbed and from which the steroid slowly escapes, whereas aqueous suspensions deliver crystals of pure steroid which slowly dissolve and are absorbed. Aqueous suspensions are essentially like tiny pellet implants that can be injected instead of surgically inserted. Aqueous suspensions tend to have longer durations than oil solutions and can allow for much wider dosing intervals in comparison. The dissolution rates and durations of aqueous suspensions are highly dependent on crystal size. A limitation of aqueous suspensions is that they can cause pain and irritation at the site of injection in a way that oil solutions have not been associated with and this is likely responsible for their less widespread use. Aqueous suspensions of estradiol benzoate (Agofollin Depot) and progesterone (Agolutin Depot) remain limitedly and sporadically available and have much longer durations than the more common oil solutions of these steroids. These preparations may be potential options for use by transfeminine people.
Introduction
Depot injectable sex steroids are administered by intramuscular injection (into muscle) or subcutaneous injection (into fat) and are formulated in two main ways: 1) as oil solutions; and 2) as crystalline aqueous suspensions. What most transfeminine people are familiar with when it comes to injectable steroid preparations are oil solutions. This is how most common preparations like injectable estradiol valerate (Delestrogen, Progynon-Depot), estradiol cypionate (Depo-Estradiol), testosterone esters, progesterone, and hydroxyprogesterone caproate are formulated. However, injectable aqueous suspensions of sex steroids also exist and are used medically. The most major and well-known formulations include depot medroxyprogesterone acetate (Depo-Provera) and combined injectable contraceptives containing estradiol cypionate/medroxyprogesterone acetate (Cyclofem, Lunelle). Additionally, there exist more obscure preparations of aqueous suspensions such as estradiol benzoate (Agofollin Depot) and progesterone (Agolutin Depot) among others which are marketed in addition to the more common oil solutions of these steroids. These preparations may remain commercially available, including online from sites like EU Aibolit (Reddit). Many transfeminine people are unaware of what aqueous suspensions are or their medical availability. It is notable in this regard that aqueous suspensions are very different from oil solutions in their properties but can have much longer durations in comparison. This is potentially of therapeutic value especially in the case of otherwise shorter-acting injectables like progesterone. The purpose of this article is to shed light on injectable aqueous suspensions, describe how injectable oil solutions and aqueous suspensions work to achieve their depot effect and how they are different, and discuss how relevant aqueous suspensions can be obtained for medical use.
How Depot Injectables Work
Sex steroids are typically lipophilic (fat-soluble or “lipid-loving”) and hydrophobic (water-insoluble or “water-hating”). In other words, they dissolve in and are “attracted to” lipids (e.g., fats), and they are poorly soluble in and repelled by water. From chemistry, this is because sex steroids are very non-polar, whereas water is a quite polar molecule. As a result of their lipophilicity, sex steroids readily form clear homogenous solutions when mixed into oil—that is, they form oil solutions. In contrast, because sex steroids are hydrophobic, they do not easily form solutions when mixed into water—that is, they do not easily form aqueous solutions (like, e.g., salt and water). Instead of aqueous solutions, solid “clumps” or crystal particles of sex steroids can be mixed into and thereby suspended in water—that is, aqueous suspensions of sex steroid crystals can be made. Depot injectables are formulated as oil solutions or aqueous suspensions and these preparations have very different properties.
Injectable Oil Solutions
When an oil solution of a sex steroid is administered by intramuscular or subcutaneous injection, the solution is trapped within the tissue compartment it is injected into and remains there. As the tissue fluid is a water mixture, the oil solution stays together inside the tissue compartment and does not easily separate or distribute. This is because the lipophilic fats and sex steroids within the solution are attracted to each other and are repelled by water. Instead of rapidly dissolving, the fats and sex steroids at the edges of the oil solution are very slowly absorbed into the surrounding water. Once they have escaped the oil depot into the surrounding tissue fluid, they can be distributed into the bloodstream and then into other tissues to exert their biological effects. Eventually, the whole oil solution will be absorbed.
Oftentimes sex steroids that are used by intramuscular or subcutaneous injection are esterified with one or more lipophilic hydrocarbonesters. These esters include fatty acids like propanoic acid (propionate), pentanoic acid (valerate), hexanoic acid (caproate), heptanoic acid (enanthate), decanoic acid (decanote or decylate), and undecanoic acid (undecylate or undecanoate) as well as cyclic compounds like benzoic acid (benzoate), cyclopentylpropanoic acid (cypionate), and phenylpropanoic acid (phenylpropionate), among many others. Examples of these sex steroid esters include the well-known estradiol valerate, estradiol cypionate, estradiol benzoate, hydroxyprogesterone caproate, and numerous others. The attachment of a lipophilic ester (e.g., valeric acid) to a sex steroid (e.g., estradiol) will increase the lipophilicity of the sex steroid compared to merely injecting the unesterified sex steroid in an oil solution. The longer the carbon atom chain in the case of the simple fatty acid esters (e.g., propionate, valerate, enanthate, undecylate), the more lipophilic the resulting esterified sex steroid will be. As a result, the injected sex steroid ester will escape the oil tissue depot more slowly, lengthening the amount of time it takes for the sex steroid ester to be absorbed and therefore its duration in the body. The tables here and here show the lengthening duration of estradiol with longer or bulkier and and more lipophilic esters. Whereas an intramuscular injection of estradiol or progesterone in oil solution has a duration of only around 2 days, an intramuscular injection of an oil solution of estradiol undecylate, an ester of estradiol with a long fatty acid chain, has a duration measured in months. And an intramuscular injection of an oil solution of hydroxyprogesterone caproate, an ester of a derivative of progesterone that has a medium-length fatty acid chain, has a duration measured in weeks.
Most sex steroid esters themselves are biologically inactive. Once they have left the oil tissue depot, they are rapidly cleaved by esteraseenzymes into free unesterified steroid (e.g., estradiol, testosterone) and the previously connected ester moiety (e.g., valeric acid). Hence, most sex steroid esters are prodrugs and are otherwise identical to their parent sex steroids in their biological actions. In the case of esters of estradiol and testosterone, this means that they are bioidentical just like the unesterified steroids. Certain synthetic progesterone derivatives like hydroxyprogesterone caproate and medroxyprogesterone acetate are however not prodrugs and are not meaningfully cleaved into the unesterified parent compound. Instead, they have intrinsic hormonal activity of their own and act without bioactivation.
Injectable Aqueous Suspensions
Aqueous suspensions of sex steroids also form an injection-site depot and achieve a long-lasting depot effect when administered by subcutaneous or intramuscular injection. However, they work in a completely different way than oil solutions. Aqueous suspensions of sex steroids consist of tiny crystal particles of pure sex steroid that are suspended in water. These sex steroid particles are highly lipophilic and hydrophobic. When injected, the hydrophilic water vehicle is rapidly mixed into the fluid of the tissue compartment and absorbed by the body. But the hydrophobic sex steroid crystals are not, and instead float about in the fluid of the tissue compartment. As with oil solutions, the sex steroids at the edges of the crystals very slowly dissolve off the surface of the crystals into the surrounding water and are then distributed into the circulation and tissues. Eventually, the crystal will be fully absorbed into the body, but only after a long period of time. The rate of absorption of the particles is dependent on the properties of the particle crystal lattice and varies depending on the compound.
In the case of aqueous suspensions, the duration of the sex steroid is additionally highly dependent on particle size. These particle sizes have ranged from nanocrystalline to microcrystalline to macrocrystalline in their range. Almost always however it is microcrystalline particle sizes that have been used in injectable aqueous suspensions of sex steroids. (The present author has seen macrocrystalline preparations described a few times, specifically in research on combined injectable contraceptives (Garza-Flores, Del, & Perez-Palacios, 1992; Newton, d’Arcangues, & Hall, 1994; Sang, 1994), and is fairly sure that no such preparations have ever been marketed. On the other hand, nanocrystalline aqueous suspensions have been used with depot antipsychotics (Spanarello & Ferla, 2014; Correll et al., 2021).) Typically, there is a given particle size range for the formulation, such as 0.01 to 0.1 mm in diameter. The larger the particle sizes, the slower the absorption into the body, and the longer the duration of the preparation; the smaller the particle sizes, the faster the absorption, and the shorter the duration. When microcrystalline aqueous suspensions of sex steroids are manufactured nowadays, the particle sizes are defined and carefully controlled. Particle sizes influence the duration of injectable aqueous suspensions because they result in different surface areas from which sex steroid ester can escape particles. A single large particle has a smaller total surface area and hence dissolution rate than the same particle divided up into many smaller particles.
Particle sizes are manipulated during manufacturing via micronization—the process of decreasing the diameter of larger particles, such as via milling or grinding. Whereas more micronization improves the absorption and bioavailability of estradiol and progesterone with oral administration by increasing the surface area available for absorption into the body (Wiki; Wiki), less micronization decreases the rate of absorption of crystalline aqueous suspensions via depot injection and thereby extends the durations of these preparations by decreasing the total surface area for absorption.
There is a notable similarity of injectable aqueous suspensions of sex steroid to implantable sex steroid pellets, for instance of estradiol, testosterone, and progesterone (Wiki; Wiki; Wiki). Pellet implants are basically just pure crystalline sex steroid compressed into the shape of a small cylinder (Photo; Photo). They are inserted into subcutaneous fat in the body through a small incision using a large needle-like instrument called a trocar (Diagram). Once implanted, pellets slowly dissolve and absorb into the body over time, eventually disappearing completely. As they are nothing but pure crystalline hormone, there is no need for them to be removed or retrieved later on. In other words, implantation of a pellet is in a way the same thing as a subcutaneous injection of an aqueous suspension of sex steroid crystals—a single pellet is just one massive crystal instead of many tiny crystals suspended in water. And with very large crystals comes a very long duration—typically 6 months or more for each subcutaneous pellet of estradiol or testosterone (Kuhl, 2005; Wiki; Wiki). However, though injectable aqueous suspensions are typically much less prolonged than pellet implants, they have the advantages over pellets of being less expensive and not requiring a surgical incision. Due to the similarity between aqueous suspensions and pellet implants, aqueous suspensions have been described and marketed as “micropellets” in the past.
Medical Use of Injectable Aqueous Suspensions
Clinical Durations of Injectable Aqueous Suspensions
Studies that have compared sex steroids in injectable oil solutions versus injectable aqueous suspensions have generally found that the durations are considerably longer with aqueous suspensions than with oil solutions. Whereas injectable estradiol and estrone in oil solution have durations of only about 1 to 2 days, aqueous suspensions of these steroids have durations of 2 to 7 days (Table). Moreover, whereas injectable estradiol benzoate in oil solution has a duration of 4 to 6 days, injectable estradiol benzoate as a microcrystalline aqueous suspension has a duration of 2 to 3 weeks (Table). Such a duration is on par with the duration of longer-acting injectable estradiol esters like estradiol cypionate in oil solution and estradiol enanthate in oil solution. In addition, whereas injectable progesterone in oil solution has a duration of about 2 to 3 days, injectable aqueous suspensions of progesterone have a duration in the range of 1 to 2 weeks (Table). This is a duration that is comparable to that of injectable hydroxyprogesterone caproate in oil solution. The prolonged duration of injectable progesterone suspensions is particularly notable as progesterone cannot be esterified and hence injectable progesterone in oil solution cannot be prolonged except with structural modification (as in progestins like hydroxyprogesterone caproate and dihydroxyprogesterone acetophenide) (Wiki). The duration of injectable testosterone propionate in oil solution is 3 to 4 days, in a suspension of small crystals (0.04–0.1 mm) is 8 days, and as commercial-size crystals (0.05–0.2 mm) is 12 days (Sinkula, 1978). Testosterone isobutyrate as an aqueous suspension is said to have a duration of 2 to 3 weeks (Sinkula, 1978; Table). Some injectable aqueous suspensions can have extremely prolonged durations. Depot medroxyprogesterone acetate for instance has a duration of at least 3 months and as long as 6 to 9 months (Wiki). It greatly outlasts the related injectable progestogen norethisterone enanthate in oil solution (Noristerat) (Bassol & Garza-Flores, 1994; Paulen & Curtis, 2009).
Availability of Injectable Aqueous Suspensions
A list of injectable aqueous suspensions of sex steroids that are known to have been marketed can be found here. Most of them were introduced in the 1950s and many of them have been discontinued, but several of the preparations remain available today. These include the single-drug preparations estradiol benzoate (Agofollin Depot, Ovocyclin M), progesterone (Agolutin Depot), testosterone isobutyrate (Agovirin Depot), and medroxyprogesterone acetate (Depo-Provera, Depo-SubQ Provera 104) as well as the combination preparations estradiol benzoate/testosterone isobutyrate (Folivirin, Femandren M) and estradiol cypionate/medroxyprogesterone acetate (Cyclofem, Lunelle). Among the more notable injectable aqueous suspensions that are no longer marketed include estradiol (Aquadiol, Diogyn, Progynon Aqueous Suspension, Progynon Micropellets), estrone (Estrone Aqueous Suspension, Kestrone, Theelin Aqueous), and testosterone (Andronaq, Sterotate, Virosterone) as well as the combination preparation estradiol benzoate/progesterone (Sistocyclin).
Folivirin (estradiol benzoate/testosterone isobutyrate suspension): PDF; Google Translate
The company that manufactures these products, BB Pharma, is notably also the producer of the well-known Neofollin (injectable estradiol valerate in oil solution) in Europe.
Injection Procedure for Injectable Aqueous Suspensions
Aqueous suspensions require larger needle gauges (e.g., 20 or 21 gauge) than oil or aqueous solutions in order to allow the steroid crystals to pass through the needle lumen. The needle sizes may vary depending on the preparation and its crystal sizes. Injectable aqueous suspensions should be shaken adequately prior to injection as the crystals tend to aggregate. Aqueous suspensions are known to be more irritating and prone to causing pain and injection site reactions like redness and swelling than oil solutions, although this may vary depending on the preparation (Wiki; Wiki). This property is likely responsible for the discontinuation of many injectable suspensions and the greater popularity of injectable oil solutions. On the other hand, an advantage of aqueous suspensions over oil solutions is that in contrast them, there is no oil, and hence there is no risk of allergic reaction to the oil. As with injectable oil solutions, injectable aqueous suspensions are indicated for use specifically by intramuscular injection. Depot medroxyprogesterone acetate, which is available in both formulations for both intramuscular injection (Depo-Provera) and subcutaneous injection (Depo-SubQ Provera 104), is suggestive that aqueous suspensions have similar properties both by intramuscular and subcutaneous injection analogously to the case of injectable oil solutions (Wiki; Wiki). However, it should be noted that depot medroxyprogesterone acetate is formulated differently between these preparations.
Update: Pharmaceutical Suspensions Discontinued
Since this article was posted, Agofollin Depot (estradiol benzoate), Agolutin Depot (progesterone), and Folivirin (estradiol benzoate/testosterone isobutyrate) appear to have been discontinued by their manufacturer and seem to no longer be available from online pharmacies.
References
Bassol, S., & Garza-Flores, J. (1994). Review of ovulation return upon discontinuation of once-a-month injectable contraceptives. Contraception, 49(5), 441–453. [DOI:10.1016/0010-7824(94)90003-5]
Correll, C. U., Kim, E., Sliwa, J. K., Hamm, W., Gopal, S., Mathews, M., Venkatasubramanian, R., & Saklad, S. R. (2021). Pharmacokinetic Characteristics of Long-Acting Injectable Antipsychotics for Schizophrenia: An Overview. CNS Drugs, 35(1), 39–59. [DOI:10.1007/s40263-020-00779-5]
Garza-Flores, J., Cravioto, M. C., & Pérez-Palacios, G. (1992). Steroid injectable contraception: Current concepts and perspectives. In Sitruk-Ware, L. R., & Bardin, C. W. (Eds.). Contraception: Newer Pharmacological Agents, Devices, and Delivery Systems (pp. 41–70). New York: M. Dekker. [Google Scholar] [Google Books] [OpenLibrary] [WorldCat]
Kuhl, H. (2005). Pharmacology of estrogens and progestogens: influence of different routes of administration. Climacteric, 8(Suppl 1), 3–63. [DOI:10.1080/13697130500148875] [PDF]
Newton, J. R., d’Arcangues, C., & Hall, P. E. (1994). A review of ‘once-a-month’ combined injectable contraceptives. Journal of Obstetrics and Gynaecology, 14(Suppl 1), S1–S34. [DOI:10.3109/01443619409027641]
Paulen, M. E., & Curtis, K. M. (2009). When can a woman have repeat progestogen-only injectables–depot medroxyprogesterone acetate or norethisterone enantate? Contraception, 80(4), 391–408. [DOI:10.1016/j.contraception.2009.03.023]
Sang, G. (1994). Pharmacodynamic effects of once-a-month combined injectable contraceptives. Contraception, 49(4), 361–385. [DOI:10.1016/0010-7824(94)90033-7]
Sinkula, A. A. (1978). Methods to Achieve Sustained Drug Delivery. The Chemical Approach. In Robinson, J. R. (Ed.). Sustained and Controlled Release Drug Delivery Systems (pp. 411–555). New York/Basel: Marcel Dekker. [Google Scholar] [Google Books] [PDF]
Spanarello, S., & Ferla, T. (2014). The Pharmacokinetics of Long-Acting Antipsychotic Medications. Current Clinical Pharmacology, 9(3), 310–317. [DOI:10.2174/15748847113089990051]
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-An Informal Meta-Analysis of Estradiol Curves with Injectable Estradiol Preparations - Transfeminine Science
An Informal Meta-Analysis of Estradiol Curves with Injectable Estradiol Preparations
By Aly | First published July 16, 2021 | Last modified May 8, 2025
Abstract / TL;DR
Injectable estradiol preparations such as estradiol valerate and estradiol cypionate in oil are frequently used as estrogens in transfeminine hormone therapy. However, there is little characterization of these preparations in transfeminine people and dosing recommendations by transgender health guidelines appear to be based on expert opinion rather than on clinical data. To help shed light on the properties of injectable estradiol and to better inform dosing considerations in transfeminine people, an informal meta-analysis of available clinical data on estradiol concentration–time curves with major injectable estradiol formulations was conducted. The included preparations were injectable estradiol benzoate in oil, estradiol valerate in oil, estradiol cypionate both in oil and as a suspension, estradiol enanthate in oil, estradiol undecylate in oil, and polyestradiol phosphate. The literature was searched for clinical concentration–time data with these injectable estradiol esters and these data were collected and analyzed. Meta-analysis consisted of data for each injectable estradiol preparation being processed and fit with pharmacokinetic models. Selected pharmacokinetic parameters were additionally determined and reported. The results of this work were discussed with regard to characteristics of injectable estradiol preparations like curve shapes, durations, estrogenic exposure, and variability between people and studies. Recommendations for injectable estradiol preparations by transgender health guidelines were also explored in light of the present results. Current guidelines recommend doses of these preparations that appear to be highly excessive with injection intervals that are too widely spaced. Based on the findings of the present meta-analysis, recommendations by guidelines should be reassessed. Finally, the fitted curves in this work were incorporated into an interactive web-based injectable estradiol simulator intended for use by transfeminine people and their medical providers to help guide therapeutic decisions.
Introduction
Estradiol is the main estrogen used in transfeminine hormone therapy and is available in a variety of different forms for use by different routes of administration. The most commonly employed forms are oral, sublingual, transdermal, and injectable preparations. Injectable estradiol preparations have been discontinued in many countries and hence are unavailable for use in transfeminine hormone therapy in many parts of the world, for instance in most of Europe (Glintborg et al., 2021). However, they are still used by many transfeminine people particularly in the United States and in the do-it-yourself (DIY) community. The most commonly used forms include estradiol valerate, estradiol cypionate, and estradiol enanthate all in oil. Injectable estradiol preparations have certain advantages over other estradiol forms that make them a popular choice for use in transfeminine hormone therapy. These include often lower cost, capacity to easily achieve higher estradiol levels that can be useful for testosterone suppression, less frequent administration, and theoretically reduced health risks relative to oral estradiol at equivalent doses due to the lack of the first pass with this route (Aly, 2020). The higher estradiol levels with injections are particularly useful for estradiol monotherapy, in which an antiandrogen is not used.
Clinically used injectable estradiol preparations are formulated not as estradiol but as estradiol esters. When injected into muscle or fat in oil solutions or crystalline aqueous suspensions, these estradiol esters form depots at the injection site from which they are slowly released. Subsequent to release, estradiol esters are rapidly metabolized into estradiol and hence act as prodrugs. When estradiol itself is given by intramuscular injection in an aqueous solution or oil solution, it is rapidly absorbed and has a very short duration. Due to having lipophilic esters, most clinically used injectable estradiol esters are more fat-soluble than estradiol (as measured by oil–water partition coefficient (P)) (Table). When these esters are administered as oil solutions by intramuscular or subcutaneous injection, their increased lipophilicity causes them to be released from the injection-site depot more slowly than estradiol and to therefore have longer durations. In the case of fatty acid esters, the longer the chain length of the ester—as in e.g. estradiol valerate (5 carbons) vs. estradiol enanthate (7 carbons) vs. estradiol undecylate (10 carbons)—the greater the fat solubility, the slower the rate of release from the depot, and the longer the time to peak levels and duration (Edkins, 1959; Sinkula, 1978; Chien, 1981; Kuhl, 2005; Kalicharan, 2017; Vhora et al., 2019). The durations of both injectable oil solutions and aqueous suspensions depend on the ester and its particular physicochemical properties, but the characteristics of these preparations are different and they work in distinct ways to produce their depot effects (Enever et al., 1983; Aly, 2019). The durations of oil solutions are dependent on the lipophilicity of the ester as well as oil vehicle, whereas the durations of aqueous suspensions depend on the properties of the ester crystal lattice as well as crystal sizes (Chien, 1981; Enever et al., 1983; Aly, 2019). The polymeric estradiol ester polyestradiol phosphate is more hydrophilic (water-soluble) than estradiol and works differently than other injectable estradiol preparations. Ιt is composed of many estradiol molecules linked together via phosphate esters (on average 13 molecules of estradiol per one molecule of polyestradiol phosphate) and has a prolonged duration due to slow cleavage into estradiol following injection. Estradiol esters are able to substantially prolong the duration of estradiol when used as injectables and these preparations have durations ranging from days to months depending on the ester and how it is formulated (Table).
In order to aid understanding of concentration–time profiles with injectable estradiol preparations, I’ve developed an interactive web-based injectable estradiol simulator for transfeminine people and their medical providers. During work on this simulator, it became apparent that there is substantial variability in estradiol levels and curve shapes between different studies even with the same injectable estradiol ester. The injectable estradiol simulator was originally designed to simulate curves from only a single well-known pharmacokinetic study that directly compared estradiol benzoate, estradiol valerate, and estradiol cypionate in oil (Oriowo et al., 1980 [Graph]). However, due to the considerable differences in estradiol levels and curves across studies, it was decided that relying on only one study for such a project would be untenable. Instead, for the simulations to be reasonably accurate to the available data, many studies would need to be incorporated. Including additional studies would also allow for inclusion of other injectable estradiol esters in the simulator. As a result, the present work—an informal meta-analysis of estradiol curves with injectable estradiol formulations—was conducted for the simulator project.
Methods
A literature search was performed to identify studies reporting clinical estradiol concentration–time data with major injectable estradiol formulations (Table 1). All of these preparations have been used in transfeminine hormone therapy at one time or another in different parts of the world, although only estradiol valerate in oil and estradiol cypionate in oil are widely used today. Some of the injectable preparations included have notably been discontinued. Acceptable data for the search included mean and individual estradiol concentration data and Cmax estradiol levels (mean peak estradiol levels of individual subjects at time Tmax). Databases like PubMed, Google Scholar, and WorldCat were searched using relevant keywords (e.g., estradiol ester names and variations thereof as well as major brand names). Publications with relevant information were catalogued for data collection. Only single-dose data and multi-dose data that allowed estradiol levels to return to baseline between doses (as in e.g. repeated once-monthly combined injectable contraceptives) were included. Studies were included regardless of the hypothalamic–pituitary–gonadal axis (HPG axis) status of the participants. The study selection criteria aimed to maximize data inclusion due to scarcity of data for several preparations. If however there were many studies for a specific preparation, studies with only 1 or 2 subjects were generally skipped due to the limited additional value that they would provide. When data were in figures in papers—as was generally the case—they were extracted from the graphs using WebPlotDigitizer.
Table 1: Major injectable estradiol formulations (ordered roughly from shortest- to longest-acting):
Following their collection, data were processed, aggregated, and modeled. Data were adjusted for endogenous estradiol production and were normalized by dose. Adjustment for endogenous estradiol production was generally done via subtraction of baseline estradiol levels. In a number of cases however, subtraction of trough estradiol levels or of estradiol levels from a control group was required instead. Data were also weighted by sample size. In a handful of instances, certain missing information (e.g., time to peak levels, baseline levels, subject body weights) was filled in with reasonable assumptions to help maximize data inclusion. Data were processed in the form of mean estradiol curve data rather than individual-subject data (except for rare n=1 studies). The combined processed data from all studies for each injectable estradiol preparation were fit via least squares regression to one-, two-, and three-compartmentpharmacokinetic models with first-order absorption and elimination that were obtained from the literature and other sources (e.g., Colburn, 1981; Wagner, 1993; Fisher & Shafer, 2007; Lixoft, 2008; Abuhelwa, Foster, & Upton, 2015; Certara, 2020). These models fit most curves from individual studies very well. Fitting the combined curve fits of all individual studies (as opposed to fitting all of the combined processed data directly) was additionally evaluated for each injectable estradiol preparation, and if it was feasible for the preparation and allowed for better fitting results, was employed instead. Fitting directly to the combined processed data has the effect of weighting individual studies by quantity of time points, whereas fitting the combined curve fits of studies eliminates this. The Akaike information criterion (AIC) was used to help guide model selection for fitting of the preparations. Curve fitting was performed using the Python library Lmfit with the Levenberg–Marquardt algorithm. Cmax concentrations are a different form of data than mean curve estradiol concentration–time data, and for this reason, were not included in the fitting unless data were very limited for a given injectable estradiol preparation. Outlying data were also excluded from fitting in a number of instances and this allowed for improved curve fits with more uniform area-under-the-curve levels. The main criterion used for excluding curves was fit area-under-the-curve levels that deviated considerably from what was typical for the injectable estradiol preparations (generally less than about 50% of the average or greater than about 150% of the average).
A selection of pharmacokinetic parameters were calculated for each injectable estradiol preparation using the single-dose fit curves and compartmental pharmacokinetic analyses. These parameters included maximal or peak concentrations of estradiol after a single dose scaled to 5 mg (Cmax), time to maximal concentrations of estradiol after a single dose (Tmax), total area-under-the-curve concentrations of estradiol after a single dose (AUC0–∞), terminal elimination half-life after a single dose (t1/2), and the terminal 90% life after a single dose (t90%) (calculated as t1/2 × 3.322). In addition, selected pharmacokinetic parameters were calculated for simulated repeated administration of each injectable preparation at steady state with a dose and dose interval of 5 mg once every 7 days using the single-dose fit curves and compartmental pharmacokinetic analyses. These parameters included time to peak concentrations of estradiol (Tmax), peak and trough concentrations of estradiol (Cmax and Cmin, respectively), peak–trough difference (PTD; Cmax – Cmin), peak–trough ratio (PTR; Cmax ÷ Cmin), and integratedmean concentrations of estradiol (Cavg). Simulation of repeated administration was performed by stacking estradiol levels for multiple injections. Cmax and Tmax were defined and calculated in general as peak estradiol level and time to peak level of the fit mean curve as opposed to the mean peak level and mean time to peak level of individual subjects. This is because the latter would not be possible to compute as most studies reported only estradiol mean curve data. Pharmacokinetic parameters were calculated using relevant pharmacokinetic equations and, as a sanity check, were compared against those computed by PKSolver, a Microsoft Excel pharmacokinetics add-in program (Zhang et al., 2010).
Results
The figures in the subsequent sections show the original data from studies adjusted for endogenous estradiol levels and normalized to a common dose as well as the curve fits to the data (or alternatively the curve fits of the fits of the data depending on the preparation) for the included injectable estradiol preparations. Estradiol benzoate, estradiol cypionate in oil, and estradiol cypionate suspension were fit to the fits of all individual studies for these preparations, whereas estradiol enanthate, estradiol undecylate, and polyestradiol phosphate were fit directly to the combined processed data for these esters. In the case of estradiol valerate, the two fitting approaches gave nearly identical curves, and so fitting the combined processed original data was done for simplicity for this preparation. Cmax studies were excluded in the fitting for all preparations except estradiol enanthate, for which available estradiol concentration–time data were otherwise very limited. The data for the injectable estradiol preparations were generally fit best by a three-compartment pharmacokinetic model (Desmos). As a result, and for consistency, this model was used in the fitting of all preparations.
Estradiol Benzoate
Injectable estradiol benzoate has been extensively used in the past in scientific research, most notably in studies elucidating the function and dynamics of the HPG axis. One such use of estradiol benzoate has been the estrogen provocation test, a diagnostic test of HPG axis function. Due to its use in research, substantial estradiol concentration–time data with injectable estradiol benzoate exists. A total of 26 publications and concentration–time data for 355 individual injections were identified (Table 2).
Table 2: Studies of injectable estradiol benzoate (Spreadsheet; Plotly):
a Total number of injections, not total number of subjects.
A number of studies were excluded from fitting due to much higher or lower area-under-the-curve levels than average. A couple of studies were omitted from the meta-analysis as they only reported total estrogen levels rather than estradiol levels with estradiol benzoate (Akande, 1974; Weiss, Nachtigall, & Ganguly, 1976). Two studies were omitted due partly to being very old and using very early and inaccurate blood tests (Varangot & Cedard, 1957; Ittrich & Pots, 1965 [Graph]). The processed original data and fit of fits curve for estradiol benzoate are shown in Figure 1.
Figure 1: Published estradiol concentration–time curves and fit of fit curves (thick black or white line) with a single intramuscular injection of estradiol benzoate in oil solution over a period of 7 days. Each curve was adjusted for endogenous estradiol levels, normalized to a dose of 5 mg, and fit with a compartmental pharmacokinetic model. Following this, the combined fit curves of the individual studies were fit using the same pharmacokinetic model. The original data from the studies for estradiol benzoate are also provided elsewhere (Spreadsheet; Plotly).
Estradiol Valerate
Studies with curve data on injectable estradiol valerate come from its use in menopausal hormone therapy and other therapeutic indications for estrogens, its use in combined injectable contraceptives, and use in scientific research. A total of 28 publications and concentration–time data for 309 individual injections were identified for estradiol valerate (Table 3).
Table 3: Studies of injectable estradiol valerate (Spreadsheet; Plotly):
Study
na
Subjects
Dose
Reference(s)
S7175
12
Premenopausal women with menstrual migraine (n=10) and amenorrheic/postmenopausal women with history of menstrual migraine (n=2)
Normally cycling transmasculine people not on hormone therapy (n=31), transfeminine people not on hormone therapy (n=14), and gonadally intact transfeminine people on oral estrogen therapy (n=9)
a Total number of injections, not total number of subjects.
A few of these studies were excluded from fitting due generally to much higher or lower area-under-the-curve levels than average or due to being Cmax data. One study was omitted as it only reported estrone levels rather than estradiol levels (Ibrahim, 1996). Another study was not included due to being in pregnant women with concomitant pregnancy termination (Garner & Armstrong, 1977). One last study was omitted due partly to being very old and using very early and inaccurate blood tests (Ittrich & Pots, 1965 [Graph]). The processed original data and fit curve for estradiol valerate are shown in Figure 2.
Figure 2: Published estradiol concentration–time curves and fit curve (thick black or white line) with a single intramuscular injection of estradiol valerate in oil solution over a period of 30 days. Curves were adjusted for endogenous estradiol levels, normalized to a dose of 10 mg, and fit with a compartmental pharmacokinetic model. Fitting of the combined fits of individual studies for this preparation was explored but gave a nearly identical overall curve, so the overall fit curve for the combined processed original data was used for simplicity for this preparation. The original data from the studies for estradiol valerate are also provided elsewhere (Spreadsheet; Plotly).
Estradiol Cypionate Oil
Estradiol cypionate in oil is used in menopausal hormone therapy and for other estrogen indications. However, its use has been more limited relative to other injectable estradiol preparations, like estradiol valerate. Only a handful of studies with relevant data were identified for estradiol cypionate in oil. This included 4 publications and estradiol concentration–time data for 49 individual injections (Table 4).
Table 4: Studies of injectable estradiol cypionate in oil (Spreadsheet; Plotly):
a Total number of injections, not total number of subjects.
No curves were excluded from fitting in the case of this preparation. The processed original data and fit of fit curves for estradiol cypionate in oil are shown in Figure 3.
Figure 3: Published estradiol concentration–time curves and fit of fit curves (thick black or white line) with a single intramuscular injection of estradiol cypionate in oil solution over a period of 30 days. Each curve was adjusted for endogenous estradiol levels, normalized to a dose of 5 mg, and fit with a compartmental pharmacokinetic model. Following this, the combined fit curves of the individual studies were fit using the same pharmacokinetic model. The original data from the studies for estradiol cypionate in oil are also provided elsewhere (Spreadsheet; Plotly).
Estradiol Cypionate Suspension
Estradiol cypionate suspension has been used exclusively in combined injectable contraceptives. For this reason, many relatively high quality pharmacokinetic studies with this injectable preparation have been conducted. A total of 9 publications and estradiol concentration–time data for 131 individual injections were identified for estradiol cypionate suspension (Table 5).
Table 5: Studies of injectable estradiol cypionate suspension (Spreadsheet; Plotly):
a Total number of injections, not total number of subjects. b By subcutaneous injection rather than intramuscular injection.
One of these studies used subcutaneous injection instead of the usual intramuscular injection but the resulting curve was very similar to the curve for intramuscular injection in the same study (Sierra-Ramírez et al., 2011 [Graph]). Several Cmax studies were excluded from fitting for this preparation. One pharmacokinetic study only measured estradiol cypionate levels rather than estradiol levels and hence was not included (Martins et al., 2019 [Graph]). The processed original data and fit of fit curves for estradiol cypionate suspension are shown in Figure 4.
Figure 4: Published estradiol concentration–time curves and fit of fits curve (thick black or white line) with a single intramuscular (or in one case subcutaneous) injection of a microcrystalline aqueous suspension of estradiol cypionate over a period of 30 days. Each curve was adjusted for endogenous estradiol levels, normalized to a dose of 5 mg, and fit with a compartmental pharmacokinetic model. Following this, the combined fit curves of the individual studies were fit using the same pharmacokinetic model. The original data from the studies for estradiol cypionate suspension are also provided elsewhere (Spreadsheet; Plotly).
Estradiol Enanthate
Estradiol enanthate has been used exclusively in combined injectable contraceptives. Several pharmacokinetic studies have been conducted with it because of this. A total of 7 publications and concentration–time data for 270 individual injections were identified for estradiol enanthate (Table 6).
Table 6: Studies of injectable estradiol enanthate (Spreadsheet; Plotly):
a Total number of injections, not total number of subjects.
Of the available data, 216 of the injections were from a single study and mainly included only Cmax levels. Wiemeyer et al. (1986) was excluded from fitting due to having unusually high area-under-the-curve levels with a small sample size (n=3). Because of the scarcity of estradiol concentration–time data available for estradiol enanthate, Cmax studies were included in the fitting for this preparation. The processed original data and fit curve for estradiol enanthate are shown in Figure 5.
Figure 5: Published estradiol concentration–time curves and fit curve (thick black or white line) with a single intramuscular injection of estradiol enanthate in oil solution over a period of 30 days. Curves were adjusted for endogenous estradiol levels, normalized to a dose of 10 mg, and fit with a compartmental pharmacokinetic model. The original data from the studies for estradiol enanthate are also provided elsewhere (Spreadsheet; Plotly).
Estradiol Undecylate
Estradiol undecylate was formerly used in the treatment of prostate cancer and in menopausal hormone therapy as well as for other estrogen therapeutic indications. However, it was discontinued many years ago and is no longer used today. Nonetheless, estradiol undecylate is of significant historical interest as an injectable estradiol preparation. A total of 4 publications and estradiol concentration–time data for 7 individual injections were identified for estradiol undecylate (Table 7).
Table 7: Studies of injectable estradiol undecylate (Spreadsheet; Plotly):
a Total number of injections, not total number of subjects.
Unfortunately, the identified data were of very low quality, with small sample sizes and considerable variations in estradiol levels. Moreover, estradiol undecylate is a very long-acting injectable estradiol ester with a duration measured in months, and the follow up in these studies only went to about 2 weeks post-injection. For these reasons, it was not possible to fit the data for estradiol undecylate in a reasonably accurate way—as suggested by area-under-the-curve estradiol levels that were only around one-third those of the other non-polymeric injectable estradiol esters. Limited multi-dose hormone concentration–time data also exist for estradiol undecylate, but these data could not be incorporated (Jacobi & Altwein, 1979 [Graph]; Jacobi et al., 1980 [Graph]; Derra, 1981 [Graph]). The processed original data and fit curve for estradiol undecylate are shown in Figure 6.
Figure 6: Published estradiol concentration–time curves and fit curve (thick black or white line) with a single intramuscular injection of estradiol undecylate in oil solution over a period of 90 days. Curves were adjusted for endogenous estradiol levels, normalized to a dose of 50 mg, and fit with a compartmental pharmacokinetic model. The original data from the studies for estradiol undecylate are also provided elsewhere (Spreadsheet; Plotly).
Polyestradiol Phosphate
Polyestradiol phosphate has been used primarily in the treatment of prostate cancer but has also been used for estrogen therapeutic indications like treatment of breast cancer and menopausal hormone therapy. While this injectable estradiol preparation has been used widely in the past, it appears to have recently been discontinued. All of the identified studies with estradiol concentration–time data on polyestradiol phosphate were in men with prostate cancer. A total of 11 publications and concentration–time data for 114 individual injections were identified for polyestradiol phosphate (Table 8).
Table 8: Studies of injectable polyestradiol phosphate (Spreadsheet; Plotly):
a Total number of injections, not total number of subjects.
A few older and strongly outlying studies were excluded from the fitting. The processed original data and fit curve for polyestradiol phosphate are shown in Figure 7.
Figure 7: Published estradiol concentration–time curves and fit curve (thick black or white line) with a single intramuscular injection of an aqueous solution of polyestradiol phosphate over a period of 90 days. The graph was clipped to maximum estradiol levels of 600 pg/mL (~2,200 pmol/L) for better viewability. Curves were adjusted for endogenous estradiol levels, normalized to a dose of 160 mg, and fit with a compartmental pharmacokinetic model. The original data from the studies for polyestradiol phosphate are also provided elsewhere (Spreadsheet; Plotly).
Other Injectable Estradiol Preparations
A number of clinical studies with estradiol concentration–time data for other injectable estradiol preparations were also identified during literature search:
Estradiol (unesterified) in an “aqueous” preparation (type of aqueous preparation unspecified but probably a microcrystalline aqueous suspension) (Jones et al., 1978 [Graph])
These preparations were not included in the present meta-analysis due to their relative obscurity and the limited data available for them. In addition, there were concerns about fitting the used pharmacokinetic models to the formulations with multiple estradiol components and to the microsphere formulations.
No estradiol concentration–time data were identified for certain other injectable estradiol forms of interest, like unesterified estradiol in aqueous solution, estradiol benzoate as a microcrystalline aqueous suspension (Agofollin Depot; Ovocyclin M), or estradiol benzoate butyrate/dihydroxyprogesterone acetophenide in oil (Redimen, Soluna, Unijab) (another lesser-known combined injectable contraceptive).
All Injectable Estradiol Preparations Together
Figure 8 shows the curve fits for all of the injectable estradiol preparations scaled to a single dose of 5 mg (or equivalent) together in the same figure. The dose for polyestradiol phosphate was scaled to be about 6.5 times higher than the other injectable estradiol preparations in order to make it roughly equivalent to them in terms of total estradiol exposure. This was because polyestradiol phosphate was found to produce much lower area-under-the-curve estradiol levels than the other injectable estradiol preparations (see the Discussion section). Estradiol undecylate was not included in Figure 8 as a decent fit curve could not be obtained for it due to the very limited data available for this preparation.
Figure 8: Curve fits of published estradiol concentration–time data with different injectable estradiol preparations by intramuscular injection scaled to equivalent doses and plotted over a period of 20 days in a single combined graph. Polyestradiol phosphate is scaled to a 6.5-fold higher dose that is roughly equivalent to that for the other esters as it gave total estradiol levels that were around 6 or 7 times lower than the other esters at the same dose. An alternative version of this figure without estradiol benzoate and with the x-axis spanning 30 days is also provided (Graph).
Figure 9 shows simulated curves at steady state for repeated administration of all of the injectable estradiol preparations scaled to a dose of 5 mg (or equivalent) once every 7 days. As with the previous figure, the dose for polyestradiol phosphate was scaled to be about 6.5 times higher than the other injectable estradiol preparations and estradiol undecylate was not included in the figure.
Figure 9: Simulated curves at steady state for repeated administration of different injectable estradiol preparations by intramuscular injection scaled to equivalent doses and plotted over three injection cycles. This simulation was based on the fit curves of the published single-dose estradiol concentration–time data reported in this meta-analysis. Polyestradiol phosphate is scaled to a 6.5-fold higher dose that is roughly equivalent to that for the other esters as it gave total estradiol levels that were around 6 or 7 times lower than the other esters at the same dose. An alternative version of this figure without estradiol benzoate is also provided (Graph).
For more simulated estradiol concentration–time curves with repeated injections of these injectable estradiol preparations, please see the accompanying interactive web simulator.
Selected Pharmacokinetic Parameters
The table below shows selected pharmacokinetic parameters for the fit curves of the included injectable estradiol preparations (Table 9). Estradiol undecylate was not included in the table due to the lack of data needed to achieve a decent curve fit for this preparation and the uncertainty of its parameters.
Table 9: Selected pharmacokinetic parameters for estradiol with injectable estradiol preparations following a single 5 mg dose by intramuscular injection:
Estradiol preparation
Tmax (d)
Cmax (pg/mL)
t1/2 (d)
t90% (d)
AUC0–∞ (pg•d/mL)
Estradiol benzoate in oil
0.65
971
1.2
3.9
2410
Estradiol valerate in oil
2.1
295
3.0
9.9
1886
Estradiol cypionate oil
4.3
155
6.7
22.3
2150
Estradiol cypionate suspension
1.2
241
5.1
16.9
2096
Estradiol enanthate in oil
6.5
160
4.6
15.1
2183
Polyestradiol phosphate a
18.0
34
28.4
94.2
2117
a Scaled instead to a single 32.5 mg injection (6.5 times higher dose than with the other esters).
The table below shows selected pharmacokinetic parameters for simulated curves at steady state with repeated administration of the included injectable estradiol preparations (Table 10). As with the previous table, estradiol undecylate was not included.
Table 10: Selected pharmacokinetic parameters for estradiol with injectable estradiol preparations with simulated repeated administration of 5 mg once every 7 days by intramuscular injection:
Estradiol preparation
Tmax (d)
Cmax (pg/mL)
Cmin (pg/mL)
Peak–trough diff. (pg/mL)
Peak–trough ratio
Cavg (pg/mL)
Estradiol benzoate in oil
0.64
990
29
962
35
344
Estradiol valerate in oil
1.9
384
142
242
2.7
269
Estradiol cypionate oil
3.1
339
262
77
1.3
307
Estradiol cypionate suspension
1.0
404
189
214
2.1
299
Estradiol enanthate in oil
4.0
329
288
41
1.1
312
Polyestradiol phosphate a
3.2
304
299
5
1.0
302
a Scaled instead to repeated injections of 32.5 mg every 7 days (6.5 times higher dose than with the other esters).
Terminal half-life (t1/2) is the time for the concentration of estradiol to decrease by 50% after pseudo-equilibrium of distribution has been reached—not the time required for half of an administered dose of the estradiol ester to be eliminated (Toutain & Bousquet-Mélou, 2004). It is calculated using only the terminal portion of a concentration–time curve, without the absorption or distribution phases influencing it (Toutain & Bousquet-Mélou, 2004). Due to flip–flop kinetics with depot injectables and the very short blood half-life of estradiol (~0.5–2 hours), what is being described by the terminal half-life in the case of depot estradiol injectables is not actually elimination of estradiol from blood but rather is the absorption of estradiol from the injection-site depot (Toutain & Bousquet-Mélou, 2004; Yáñez et al., 2011).
Discussion
Data Quality, Limitations, and Variability Between Studies
The accuracies of the curve fits for the different included injectable estradiol preparations are limited by the available data for these preparations. The quantity and quality of data are variable among these preparations. In some cases, such as with estradiol valerate in oil and estradiol cypionate in suspension, the data are overall quite good. In other instances, such as with estradiol cypionate in oil and estradiol enanthate in oil, the available data are more limited. There was undersampling of certain parts of the concentration–time curve with some preparations, for instance estradiol benzoate in oil (the early curve), estradiol enanthate in oil (much of the curve), and polyestradiol phosphate (the late curve). In the case of estradiol undecylate in oil, the available data for this preparation weren’t adequate to achieve a decent curve fit at all. The fit curves and calculated pharmacokinetic parameters of the included injectable estradiol preparations should be interpreted with the imperfect data in mind. For example, the curve shapes and pharmacokinetic parameters for the different preparations should not be taken as precise determinations in most cases but instead as rough estimates that would no doubt change with more and better data. Indeed, the fits and pharmacokinetic parameters were often noticeably sensitive to the influences of individual studies. Modeling decisions, such as the choice of pharmacokinetic model, or whether to fit directly to the combined processed data versus to the fits of individual studies, also yielded significantly different curve fits as well as calculated pharmacokinetic parameters.
Due to scarcity of data for several injectable estradiol preparations, the study selection criteria maximized data inclusion in order to allow for better curve fits at the risk of including potentially less reliable data. As examples, studies were included regardless of the status of the HPG axis of the participants, and Cmax data were included in the fitting if data were very limited. In the case of HPG axis state, studies with cycling women may result in greater error due to more variable levels of endogenous estradiol. Moreover, acute high levels of estradiol can induce a surge in luteinizing hormone levels after several days in gonadally intact women, and this may cause a delayed bump in estradiol levels (Wiki). One of the more overt instances of this can be seen in a study of estradiol benzoate in such women (Shaw, 1978 [Graph]). Many if not most of the included studies with estradiol benzoate involved women with intact HPG axes, whereas studies of this sort were uncommon with the other preparations. In the case of Cmax data, these data when Cmax corresponds to the mean of individual peaks are a different type of data than the peak of the mean curve of all individuals. Cmax levels can differ in both magnitude and timing compared to the mean curve peak (e.g., Oriowo et al., 1980 [Graph]; Rahimy, Ryan, & Hopkins, 1999). This is because for instance not all individuals peak at the same time and this variability in time to peak normally serves to dilute peak levels for the mean curve when compared to individual maximal concentrations. However, Cmax levels are in any case generally in the vicinity of the mean curve peak. While Cmax levels were excluded in the fitting for most injectable estradiol preparations, they were included in the case of estradiol enanthate. This was because the available mean and individual estradiol curve data were very limited for this specific preparation, and inclusion of Cmax data allowed for improved fitting in spite of its limitations. Lastly, some of the included data was once-monthly multi-dose, and research with once-monthly estradiol enanthate-containing combined injectable contraceptives has found that the time to peak levels may shift with repeated long-term use (Schiavon et al., 1988; Garza-Flores, 1994).
There was considerable variability between studies in terms of estradiol levels and concentration–time curve shapes with the same injectable estradiol preparation. The reasons for the large variability across studies are not fully clear. In any case, there are many potential factors that may contribute to this variability. These include preparation- and injection-related factors like formulation (e.g., oil vehicle, other components and excipients, concentration, particle size), injection volume, site of injection (e.g., buttocks, thigh, upper arm), injection technique (e.g., force of injection—and resulting depot droplet dimensions), and syringe dead space. They additionally include various subject- and research-related variables like differing blood-testing methodology, differing sample characteristics (e.g., age, weight, gender, ethnicity, physical activity, HPG axis state), and sampling error (Sinkula, 1978; Chien, 1981; Minto et al., 1997; Larsen & Larsen, 2009; Larsen et al., 2009; Florence, 2010; Larsen, Thing, & Larsen, 2012; Kalicharan, 2017). Older studies, which used potentially less accurate blood tests and tended to have smaller numbers of subjects, seemed to particularly add to the variability between studies. These studies may represent less reliable data than more recent research with larger sample sizes. The exclusion criteria helped to remove outliers for the different injectable estradiol preparations however. This meta-analysis does not take into account the potential factors underlying the variability between studies. To do so would be difficult, as in many cases information on these variables is not provided in individual studies and research quantifying their precise influences and relative importances is limited.
It is in any case known from other studies that different oil vehicles are absorbed at different rates from the injection site (Svendsen & Aaes‐Jørgensen, 1979; Schultz et al., 1998; Larsen et al., 2001) and can result in different concentration–time curve shapes (Ballard, 1978 [Excerpt]; Knudsen, Hansen, & Larsen, 1985). This is thought to be due to differences in oil lipophilicity and depot release rates. Viscosity of oils has also been hypothesized to potentially influence rate of depot escape (Schug, Donath, & Blume, 2012). However, research so far has not supported this hypothesis (Larsen & Larsen, 2009; Larsen, Thing, & Larsen, 2012). Oil vehicles can vary with injectable estradiol preparations even for the same estradiol ester. For instance, pharmaceutical estradiol valerate is formulated in sesame oil, castor oil, or sunflower oil depending on the preparation (Table). It is notable however that these three oils have similar lipophilicities (Table). On the other hand, homebrewed injectable estradiol preparations used by DIY transfeminine people often employ medium-chain triglyceride (MCT) oil as the oil vehicle. This oil (in the proprietary form of Viscoleo) has notably been found to be much more rapidly absorbed than conventional oils like sesame oil and castor oil in animals (Svendsen & Aaes‐Jørgensen, 1979; Schultz et al., 1998; Larsen et al., 2001). In addition, although based on very limited data, MCT oil has been found to give spikier and shorter-lasting depot injectable curves in humans (Knudsen, Hansen, & Larsen, 1985). As such, injectable estradiol preparations using MCT oil as the vehicle may have differing and less favorable concentration–time curve shapes than pharmaceutical injectable estradiol products. Other excipients, like benzyl alcohol, as well as factors like injection site and volume, have additionally been found to influence pharmacokinetic properties with depot injectables (Minto et al., 1997; Kalicharan, Schot, & Vromans, 2016). Excipients besides oil vehicle also vary by formulation (Table).
An implication of the variability between studies is that there is not a single estradiol concentration–time curve for a given injectable estradiol preparation but rather there are many, with these curves determined by variables such as formulation, dose/administration, and subject characteristics, among others. Hence, the curve fits determined in this meta-analysis represent only an estimation of the most typical and hence likely case, but the true curve for a preparation in a given context may be quite different.
Fitting all studies for a given injectable estradiol preparation individually first, and then fitting the fits of these studies, allowed for improved curve fits relative to directly fitting all of the combined processed original data for the preparation. The latter approach has limitations in that it has the effect of inherently weighting individual studies by quantity of time points (resulting in studies with greater time sampling having greater influence on the fit). Additionally, and more problematically, this approach can lead to distortions in curve shape due to different studies sampling different portions of the curve to differing extents in conjunction with systematic differences in curves between these studies. These are problems that fitting the fits of individual studies instead can solve. However, it is not possible to fit all individual studies, as some studies have limited time sampling and curve characterization which precludes fitting them appropriately. Cmax data are an example of this, which on their own cannot be fit properly. As such, it was not possible to fit the fits of the individual studies for all injectable estradiol preparations. Consequently, the fitting approach in this regard was not the same across esters, with some fit instead directly to the combined processed original data (e.g., estradiol enanthate, polyestradiol phosphate).
In spite of the various limitations of this work, aggregated analysis and modeling with injectable estradiol preparations has not previously been done. This informal meta-analysis provides among the most detailed insight into estradiol levels and curve shapes with these preparations available to date.
Durations and Curve Shapes
The curve shapes of non-polymeric injectable estradiol esters in oil relate strongly to lipophilicity. The more lipophilic the ester, the lower the peak levels and the more protracted the estradiol concentration–time curve. Accordingly, estradiol benzoate, one of the least lipophilic estradiol esters, has one of the spikiest curves and shortest durations, whereas more lipophilic estradiol esters, like estradiol cypionate in oil and estradiol enanthate, have comparatively flatter curves with delayed peaks and longer durations.
Duration of Estradiol Valerate
The estradiol concentration–time curve for injectable estradiol valerate in the well-known Oriowo et al. (1980) [Graph] study is notably spikier and shorter-lasting than the overall curve for estradiol valerate in this meta-analysis. On the other hand, the overall curve for injectable estradiol valerate in this meta-analysis was similar to (and considerably influenced by) the curves from several relatively recent and presumably better-quality studies of this injectable estradiol ester (e.g., Göretzlehner et al., 2002; Valle Alvarez, 2011; Schug, Donath, & Blume, 2012). It’s noteworthy that Oriowo et al. (1980) used a peanut oil-based formulation of estradiol valerate that differed from pharmaceutical injectable estradiol valerate preparations, which generally use sesame oil or castor oil as the carrier (as well as other excipients) (Table). This may have influenced the curve shape of estradiol valerate in Oriowo et al. (1980). The study also had a small sample size relative to the more recent studies (n=9 versus n=17, n=32, and n=24×2, respectively). Based on the newer and overall data, estradiol valerate appears to have a curve that is noticeably flatter and more prolonged than that suggested by Oriowo et al. (1980).
Duration of Estradiol Cypionate in Oil versus Estradiol Enanthate
Available estradiol concentration–time data for injectable estradiol cypionate in oil and estradiol enanthate in oil are more limited than with several of the other injectable estradiol preparations, and no direct comparisons of these two preparations exist at present. Based on some of the available literature on these injectable estradiol esters, most notably discussion by Oriowo et al. (1980) and a review of the pharmacokinetics of combined injectable contraceptives (Garza-Flores, 1994 [Graph]), it seemed that the duration of estradiol enanthate in oil was longer than that of estradiol cypionate in oil. However, this was based on limited research from separate and hence indirectly comparative studies of these esters. The estradiol cypionate in oil data from the relevant Garza-Flores (1994) figure was based on Oriowo et al. (1980) [Graph], and there are reasons to be cautious about relying on these data alone. The main concern is that curve shapes with the same injectable estradiol preparation can vary considerably across studies, as the present meta-analysis has shown. The reasons for this have yet to be fully clarified as already discussed, but among other factors may include varying formulations across studies of the same injectable estradiol ester. It is notable in this regard that Oriowo et al. (1980) used a formulation of estradiol cypionate that differs from conventional pharmaceutical estradiol cypionate in oil preparations—specifically, the study used a peanut oil-based formulation (with few other specifics) rather than the cottonseed oil-based preparation employed in marketed pharmaceutical formulations (Table). The study also had a somewhat small sample size (n=10) and may have had significant sampling error. Hence, single studies, perhaps particularly Oriowo et al. (1980), should be interpreted cautiously.
A small but interesting pharmacokinetic study which directly compared injectable testosterone cypionate (n=6) and testosterone enanthate (n=6) both in oil is relevant to the topic in question. This study found that equivalent doses of these testosterone esters using otherwise identical formulations produced virtually identical testosterone concentration–time curves (Schulte-Beerbühl & Nieschlag, 1980 [Graph]). The findings of this study are consistent with the fact that the lipophilicities of testosterone cypionate and testosterone enanthate (as measured by predicted log P) are very similar when directly compared (e.g., 5.1 vs. 5.11 with ALOGPS, 6.29 vs. 6.11 with ChemAxon logP, and 6.4 vs. 6.3 with XLogP3, respectively (Table). This of course is of importance as lipophilicity is thought to be the key factor determining the release kinetics of oil-based depot injectables (Sinkula, 1978; Shah, 2007; Larsen & Larsen, 2009; Larsen, Thing, & Larsen, 2012; Shahiwala, Mehta, & Momin, 2018). Analogously similar lipophilicities can be seen when comparing estradiol cypionate and estradiol enanthate, which employ the same ester moieties (e.g., predicted log P values of 6.47 vs. 6.45 with ALOGPS and 7.1 vs. 7.0 with XLogP3, respectively) (Table). Hence, on a theoretical level, injectable estradiol cypionate and estradiol enanthate, like injectable testosterone cypionate and testosterone enanthate, might be expected to produce very similar curves—at least provided all other variables, such as formulation, are held constant.
The present meta-analysis found that the overall estradiol curve for estradiol cypionate in oil was significantly less spikey and more prolonged than that observed in Oriowo et al. (1980). It is noteworthy in this regard that all of the other studies included for estradiol cypionate in oil specifically employed pharmaceutical Depo-Estradiol and that the overall curve for this preparation appears to be more consistent with its licensed injection interval for use in menopausal hormone therapy (1–5 mg once every 3–4 weeks) (Depo-Estradiol Label). Moreover, this meta-analysis found that injectable estradiol cypionate in oil and estradiol enanthate in oil had fairly similar and comparably flat and prolonged estradiol concentration–time curves. However, estradiol cypionate in oil appeared to peak earlier than estradiol enanthate, while estradiol enanthate was eliminated more rapidly than estradiol cypionate in oil in the terminal portion of the curve. In any case, the available concentration–time data for these preparations are limited, and the present work is not able to determine whether these estradiol esters have truly differing pharmacokinetic properties, as the apparent differences between the curves for these preparations may simply be due to statistical error. Taken together, estradiol cypionate in oil may have a less spikey and longer-lasting curve than that implied by Oriowo et al. (1980), and estradiol cypionate in oil and estradiol enanthate may have more similar curves than has been previously assumed.
Curve Shape of Estradiol Cypionate Suspension
While estradiol cypionate as an aqueous suspension is a relatively long-lasting injectable estradiol preparation similarly to estradiol cypionate in oil and estradiol enanthate in oil, it seems to differ in the shape of its estradiol concentration–time curve from these preparations. Estradiol cypionate as a suspension has a curve that appears to peak significantly earlier than estradiol cypionate in oil and other longer-acting oil-based injectable estradiol preparations. This might relate to the differing mechanisms of depot action and unique properties of injectable aqueous suspensions (Aly, 2019). In line with this notion, injectable medroxyprogesterone acetate suspension (Depo-Provera) also appears to peak rapidly despite having a very long duration (longer durations tending to be associated with delayed peaks in the case of oil-based depot injectables) (Graphs). Although aqueous suspensions generally last longer than oil solutions as injectables (Enever et al., 1983; Aly, 2019), this is not always the case, and estradiol cypionate suspension interestingly seems to be shorter-acting than estradiol cypionate in oil.
Estradiol Exposure and Potency
The average estradiol levels with the non-polymeric injectable estradiol esters when scaled to a dose and dosing interval of 5 mg every 7 days were around 300 pg/mL (~1,100 pmol/L). For comparison, in premenopausal cisgender women, estradiol production is on average about 200 μg/day (or 6 mg per month/cycle) and mean estradiol levels are around 100 pg/mL (~370 pmol/L) (Aly, 2019). After adjusting for the molecular weight of the ester, the estradiol levels for a given dose of non-polymeric injectable estradiol esters are in fairly close agreement with the estradiol levels for an equal quantity of estradiol produced endogenously by the ovaries in premenopausal cisgender women (very roughly around 1.2 mg estradiol per 7 days for injectable estradiol esters and 1.4 mg estradiol per 7 days for ovarian production to achieve average integrated estradiol levels of around 100 pg/mL). The preceding is in accordance with the fact that injectable estradiol valerate has been reported to have approximately 100% bioavailability (with this being less characterized but likely also the case for the other non-polymeric injectable estradiol esters) (Düsterberg & Nishino, 1982; Seibert & Günzel, 1994).
Although non-polymeric injectable estradiol esters have differing estradiol concentration–time curve shapes, they all appear to achieve fairly similar area-under-the-curve levels of estradiol when compared to one another. This is in accordance with the fact that differences in molecular weight and hence estradiol content with the different estradiol esters are fairly minor (all of the assessed non-polymeric esters range from 62 to 76% of that of estradiol in terms of estradiol content, and all but estradiol undecylate are in the range of 69 to 76%) (Table). The appearance of differences in area-under-the-curve levels of estradiol in the present meta-analysis is probably just due to statistical error, and true differences cannot be established by this meta-analysis. An implication of the similar area-under-the-curve estradiol levels with the different non-polymeric injectable estradiol esters is that these preparations can all be expected to deliver a roughly comparable amount of estradiol for the same dose.
On the other hand, the polymeric ester polyestradiol phosphate appears to produce around 6- to 7-fold lower area-under-the-curve and average estradiol levels than non-polymeric estradiol esters. This suggests that the estradiol in polyestradiol phosphate is not 100% bioavailable, and is supported by the fact that this ester is used clinically at substantially higher dosages than other injectable estradiol esters (40–320 mg/month), even for the same indications such as menopausal hormone therapy and treatment of prostate cancer (Wiki; Estradurin Labels). This does not seem to have been previously described in the literature, and the reasons for it are unknown. It seems possible that polyestradiol phosphate may be partially excreted before it can be cleaved into estradiol and thereby rendered partly inactive, in turn necessitating the use of higher doses to achieve the same estradiol levels and therapeutic effect.
Although two given injectable estradiol preparations may produce equivalent total estradiol levels, this does not necessarily mean that they will always have the same estrogenic potency (i.e., strength of effect at a given dose). It is plausible that spikier estradiol concentration–time curves, like with estradiol benzoate, may have overall lower estrogenic potency than more steady curves, like with estradiol enanthate. This is because estrogen receptors for a given tissue should become saturated at a certain point due to the finite quantity of available receptors in the tissue. As a result, high peak estradiol levels with spikier curves may effectively be “wasted” to varying extents in different tissues. On the other hand, more spikey estradiol curves, due to higher peak estradiol levels, might have greater influence on tissues that require high estradiol levels for effect such as the liver (and by extension on coagulation and associated health risks) (Aly, 2020). However, these possibilities are speculative and theoretical. Although some literature exists that is relevant to this issue (e.g., Parkes, 1937; Bradbury, Long, & Durham, 1953), there is very little research in this area. Consequently, it is not currently possible to take into account time-related variations in estradiol levels or differing estradiol curve shapes when assessing the comparative estrogenic potency between injectable estradiol preparations (or between other estradiol forms/routes). It is also noteworthy that these variations depend on injection interval and may be reduced with shorter injection intervals that maintain steadier estradiol levels, which must also be considered.
Variability Between Individuals
There is substantial variation in total estradiol levels and curve shapes between people with the same injectable estradiol preparation. Indicators of interindividual variability such as standard deviation or 95% range have not been included in this meta-analysis at this time due to the large amount of additional time and work this would require (e.g., additional extraction of error bars from all studies and analysis). In any case, individual studies that were included show this marked interindividual variation (e.g., Oriowo et al., 1980; Derra, 1981 [Graph]; Aedo et al., 1985 [Graphs]; Sang et al., 1987 [Graphs]; Rahimy & Ryan, 1999 [Graph]; Valle Alvarez, 2011 [Graph]; Schug, Donath, & Blume, 2012 [Graphs]). Highly variable estradiol levels are already well-established with oral and transdermal estradiol (Kuhl, 2005; Wiki). Less variability might be expected with non-polymeric injectable estradiol esters since these preparations appear to have approximately complete bioavailability. However, it seems that even with injectable forms of estradiol, the variability between people is still quite substantial. An implication of this is that the appropriate dose and dosing interval of an injectable estradiol formulation for a given person will vary considerably. This emphasizes the importance of blood work to ensure that injectable estradiol preparations are neither overdosed—which can increase health risks such as blood clots (Aly, 2020)—nor underdosed—which may result in suboptimal testosterone suppression and therapeutic efficacy.
Insights for Clinical Guidelines and Dosing Recommendations
Clinical guidelines for transgender health (see also Aly (2020)) provide recommendations on doses and dosing intervals of injectable estradiol valerate in oil and estradiol cypionate in oil (Table 11). Dosing recommendations are not given for other injectable estradiol preparations, which are much less commonly used in transgender medicine. The recommended doses for estradiol valerate and estradiol cypionate vary widely depending on the guidelines, whereas the recommended intervals are consistently once every 1 to 2 weeks. The doses for estradiol valerate range from 2 to 20 mg/week or 5 to 80 mg/2 weeks and the doses for estradiol cypionate range from <1 to 10 mg/week or <2 to 80 mg/2 weeks. For reference, the Endocrine Society guidelines and the University of California, San Francisco (UCSF) guidelines are the most major clinical guidelines for transgender hormone therapy at present (Aly, 2020). The Endocrine Society guidelines recommend 5 to 30 mg/2 weeks or 2 to 10 mg/week for either estradiol valerate or estradiol cypionate (Hembree et al., 2017). Conversely, the UCSF guidelines recommend <20 to 40 mg/2 weeks for estradiol valerate and <2 to 5 mg/2 weeks for estradiol cypionate (with the option to divide dose into weekly injections if cyclical side effects occur) (Deutsch, 2016a).
Table 11: Recommended doses and injection intervals of injectable estradiol preparations (specifically estradiol valerate and estradiol cypionate) in transgender medicine clinical guidelinesa:
a Several other guidelines recommend doses and intervals that appear to be taken directly from the Endocrine Society or UCSF guidelines and thus are not listed here but can be found elsewhere (Aly, 2020).
A number of concerns arise when the doses and intervals of injectable estradiol valerate and estradiol cypionate recommended by the major transgender clinical guidelines are considered in the context of the present informal meta-analysis and when they are compared between guidelines. Based on the present work, dosages of injectable preparations recommended by the major transgender clinical guidelines appear to result in estradiol exposure that is markedly higher than that with the recommended dosages for other routes and forms of estradiol (e.g., oral or transdermal). Whereas a dosage of 5 mg/week of any non-polymeric injectable estradiol ester appears to give average estradiol levels of around 300 pg/mL (~1,100 pmol/L), which are already supraphysiological, doses of injectable estradiol valerate or estradiol cypionate recommended by guidelines are as high as 15 to 20 mg per week. The average estradiol concentrations that would be expected to result from such doses per this meta-analysis (e.g., ~600–1,200 pg/mL or 2,200–4,400 pmol/L at 10–20 mg/week) (Figure 10) would vastly exceed the ranges for estradiol levels in transfeminine people advised by the same guidelines (generally about 50–200 pg/mL or ~180–730 pmol/L) (Table). This is not merely theoretical; for example, a study that used 40 mg/week estradiol valerate by intramuscular injection in cisgender women with estrogen deficiency to produce “pseudopregnancy” reported measured estradiol levels of about 2,500 pg/mL (~9,200 pmol/L) at 3 months and 3,100 pg/mL (~11,400 pmol/L) at 6 months of treatment (Ulrich, Pfeifer, & Lauritzen, 1994). Moreover, highly supraphysiological estradiol levels with guideline-based injectable estradiol doses are not unexpected when normal production of estradiol in premenopausal cisgender women is considered (~1.4 mg per week or 6 mg per month/cycle giving mean estradiol levels of ~100 pg/mL or 370 pmol/L) (Aly, 2019). Clinical safety data on high doses of injectable estradiol esters like estradiol valerate and estradiol cypionate are lacking at present, but excessive estrogenic exposure is known to increase the risk of health complications such as blood clots (Aly, 2020). The very high doses of these preparations that are recommended by guidelines should raise considerable reservations about their safety.
Figure 10: Simulated estradiol levels with injectable estradiol valerate at the doses and interval (5–40 mg/2 weeks) preferentially recommended by current major transgender care guidelines. Steady-state estradiol levels are reached by about the second or third injection with this injection interval and levels do not further accumulate. An alternative version of this figure with half-doses at a once-weekly interval (i.e., 2.5–20 mg/week) is also provided (Graph).
The present author elsewhere has listed doses of injectable estradiol preparations that are roughly comparable in terms of total estradiol exposure to doses for other estradiol forms and routes used in transfeminine people (Aly, 2020). These doses range from about 1 to 6 mg per week for “low dose” to “very high dose” therapy with non-polymeric injectable estradiol esters (Graph). This dose range for injectable estradiol is likely to be more appropriate for use in transfeminine people than current recommendations by many guidelines. Although high estradiol levels can be useful in transfeminine hormone therapy when antiandrogens are not used due to their greater efficacy than physiological levels in terms of testosterone suppression, only modestly supraphysiological estradiol levels (e.g., ~200–300 pg/mL or 730–1,100 pmol/L) appear to be required for strong testosterone suppression (Aly, 2019; Langley et al., 2021; Aly, 2020). In relation to this, doses of injectable estradiol need not be excessive.
Some guidelines, such as the Endocrine Society guidelines, recommend the same doses and intervals for both estradiol valerate and estradiol cypionate, whereas other guidelines, such as the UCSF guidelines, recommend different doses for these two injectable estradiol esters. Concerningly, the doses for estradiol valerate and estradiol cypionate recommended by the UCSF guidelines differ by roughly an order of magnitude (<20 to 40 mg/2 weeks for estradiol valerate and <2 to 5 mg/2 weeks for estradiol cypionate). These estradiol esters appear to produce similar average estradiol levels (e.g., around 300 pg/mL or 1,100 pmol/L at a dosage of 5 mg/week) and have concentration–time curve shapes that are not extremely different, with estradiol cypionate being only somewhat flatter and more prolonged than estradiol valerate. As such, it would appear that similar doses should be appropriate for these esters. This is supported by the fact that the same doses of estradiol valerate and estradiol cypionate are used in combined injectable contraceptives in cisgender women (both 5 mg once per month) and that these doses were carefully determined during an intensive clinical development programme for these preparations (Garza-Flores, 1994; Newton, d’Arcangues, & Hall, 1994; Sang, 1994; Toppozada, 1994). This programme notably included dose-ranging and direct-comparison studies. Based on the present analysis, the current recommendations by the UCSF guidelines may result in marked overdosage in the case of estradiol valerate and potential underdosage in the case of estradiol cypionate.
Transgender health guidelines recommend an injection interval for estradiol valerate and estradiol cypionate in oil of once every 1 to 2 weeks. Although an injection interval of 2 weeks seems technically feasible in the case of both of these preparations, such an interval would appear to result in substantial fluctuations in estradiol levels, with high peak levels and low troughs. This is particularly true in the case of the shorter-acting estradiol valerate (Figures 10, 11). Considering the wide fluctuations and unknown effects of this variability, as well as the fact that testosterone suppression when applicable may depend on sustained higher estradiol levels, it may be advisable that a once-weekly interval be preferentially recommended for these preparations. This would achieve steadier estradiol levels and would reduce potential problems due to high or low estradiol levels (Figure 11). Alternatively, a shorter interval of once every 5 days may be used with estradiol valerate to further reduce the variability in estradiol levels that occurs with this preparation (Figure 11). On the other hand, an injection interval of once every 10 days to 2 weeks may be practical and allowable in the case of the longer-acting estradiol cypionate in oil (as well as estradiol enanthate) (Figure 11)—provided that the injection cycles are well-tolerated and testosterone suppression remains adequate. When selecting different injection intervals, doses should be scaled by the interval to maintain equivalent total estradiol exposure (e.g., 3.5 mg/5 days, 5 mg/7 days, 7 mg/10 days, or 10 mg/14 days for high-dose non-polymeric injectable estradiol esters).
Figure 11: Simulated estradiol levels with a high dosage of injectable estradiol valerate or estradiol cypionate in oil at different injection intervals (doses scaled by interval to be equivalent in total estradiol exposure).
With the preceding concerns about the doses and intervals of injectable estradiol preparations recommended by transgender care guidelines considered, the question of how these recommendations were determined arises. Unfortunately, current guidelines do not generally describe how they arrived at their recommendations nor do they usually cite sources to support them. It is notable that the UCSF guidelines recommend doses and intervals for injectable estradiol preparations that are nearly identical to those advised by Christian Hamburger and Harry Benjamin in the late 1960s in the first medical textbook on transgender people (Hamburger & Benjamin, 1969). These authors recommended a dose of 10–40 mg/2 weeks for estradiol valerate and of 2–5 mg/2 weeks for estradiol cypionate (although Benjamin additionally stated that after 4–8 months, the same doses could be used at a longer injection interval of once every 4 weeks). These recommendations were notably made before estradiol blood tests became practicably available and were prior to the advent of modern pharmacokinetic studies. Hence, the recommendations for at least these guidelines appear to be based mainly on past expert opinion and long-standing historical precedent rather than on pharmacokinetic or clinical data. The same is likely to also be true for most other guidelines. High doses with certain injectable estradiol preparations (namely estradiol valerate) were probably originally employed for the purpose of achieving longer durations and more convenient injection intervals. This was notably prior to the risks of excessive estrogenic exposure like blood clots becoming known, and these doses may simply have never been revised.
Among the surveyed guidelines for transgender hormone therapy, only the UCSF guidelines (Deutsch, 2016b) and the Sherbourne Health/Rainbow Health Ontario guidelines (Bourns, 2019) referenced pharmacokinetic literature in their discussion of injectable estradiol. The specific publications cited by these guidelines were Düsterberg & Nishino (1982), Sierra-Ramírez et al. (2011), and Thurman et al. (2013). Although it is favorable to see guidelines considering published pharmacokinetic data for informing use of these preparations, there are a few concerns about the studies that were cited. Düsterberg & Nishino (1982) in its study of injectable estradiol valerate had a very small sample size (n=2), and this study was excluded as an outlier in the present meta-analysis due to unusually high estradiol levels. The findings of Düsterberg & Nishino (1982) also do not seem to have actually been used to guide dosing recommendations in the case of the UCSF guidelines, since if this were the case, the recommended doses should have been much lower. On the other hand, Bourns (2019) cited the same study and recommended injectable estradiol valerate at doses of 3–4 mg/week or 6–8 mg/2 weeks. These doses are well below those recommended by other transgender care guidelines and appear to be more appropriate for use in transfeminine people in light of the present meta-analysis. Sierra-Ramírez et al. (2011) and Thurman et al. (2013), although better-quality studies than Düsterberg & Nishino (1982), described injectable estradiol cypionate suspension rather than estradiol cypionate in oil. The oil-based version of estradiol cypionate is the form normally used in transfeminine hormone therapy, and there are important differences between these estradiol cypionate preparations such that pharmacokinetic studies for the suspension can’t necessarily be generalized to the oil solution. These preparations do in any case produce similar total estradiol levels however and hence doses should be comparable for them.
This meta-analysis is only informal and unpublished research. Nonetheless, based on the results of this work and the preceding discussion, current dosing recommendations for injectable estradiol preparations by most transgender clinical guidelines appear to be highly excessive and likely unsafe, with injection intervals that may additionally be too widely spaced. Transgender care guidelines should consider reassessing these recommendations, and the transgender medical community should make an effort to better characterize the pharmacokinetics and optimal dosing schemes of injectable estradiol preparations in transfeminine people in the future. Since clinical data on these preparations are scarce and will probably remain so in the near-term, use of published pharmacokinetic data may be further considered for guiding dosing recommendations for injectable estradiol. As identified and catalogued by this meta-analysis, there is a wealth of existing data that could be used to better inform transgender care guidelines in terms of the use of injectable estradiol preparations in transfeminine people.
Interactive Web Simulator
This informal meta-analysis of estradiol concentration–time data with injectable estradiol preparations was conducted for the purpose of deriving accurate and representative estradiol curves for incorporation into a web-based injectable estradiol simulator intended for use by transfeminine people and their clinicians. This web app is able to simulate both single-injection curves and repeated-injection curves with these preparations. An informational page for this simulator can be found at the following location:
There are various possibilities for further work on this project in the future. For example, assessment of interindividual variability for estradiol levels with injectable estradiol preparations could be included in the meta-analysis. As another example, it would be fairly straightforward and valuable to expand the meta-analysis as well as simulator to other hormonal preparations such as injectable testosterone preparations and other estradiol routes and forms like oral estradiol, sublingual estradiol, and estradiol pellets. Pharmacokinetic literature for some of these preparations has already been collected by this author. However, these future possibilities would require much additional time and effort to complete.
Special Thanks
A special thank you to Violet and Lila for their indispensable input and guidance on modeling topics during the work on this project. An additional thanks to Violet for deriving a special three-compartment pharmacokinetic model that was used in this work. Please also check out Violet’s own projects Tilia—an effort to empower trans people with tools to manage their hormonal transitions—and TransKit—a work-in-progress pharmacokinetic simulation library specifically tailored for transgender hormone therapy. Lastly, thank you to all the peer reviewers who carefully reviewed this article prior to it being posted.
Updates
Update 1: WPATH SOC8 Guidelines
In September 2022, the World Professional Association for Transgender Health (WPATH) Standards of Care for the Health of Transgender and Gender Diverse People Version 8 (SOC8) were published and made recommendations on transgender hormone therapy for the first time (Coleman et al., 2022). These guidelines are among the most highly regarded and consulted transgender care guidelines. In terms of the recommended doses of hormonal medications for transgender people, the WPATH SOC8 appear to have largely copied the Endocrine Society’s 2017 guidelines on transgender hormone therapy (Hembree et al., 2017). More specifically, in the case of injectable estradiol preparations for transfeminine people, doses of 5–30 mg/2 weeks or 2–10 mg/week estradiol valerate or estradiol cypionate were recommended. There was no discussion of injectable estradiol in the guidelines besides the preceding doses and intervals being included in a table, and no literature citations were included to support these doses. As described in the present work, these recommendations include doses and intervals that appear to be highly excessive, too widely spaced, and are likely unsafe. As such, major transgender care guidelines unfortunately continue to make uncited recommendations for injectable estradiol that are out of step with insights available from abundant published pharmacokinetic data. These recommendations are likely inadvisable, with the possibility of substantial health risks.
Update 2: Literature Mentions
The following publications in the literature have cited or mentioned Transfeminine Science’s injectable estradiol simulator and/or meta-analysis since the project was published in mid-2021:
Hughes et al. (2022)
Hughes, J. H., Woo, K. H., Keizer, R. J., & Goswami, S. (2022). Clinical Decision Support for Precision Dosing: Opportunities for Enhanced Equity and Inclusion in Health Care. Clinical Pharmacology & Therapeutics, 113(3), 565–574. [DOI:10.1002/cpt.2799]:
Lastly, we recommend that developers of [clinical decision support software (CDSS)] for dosing take an iterative and participatory approach to designing systems. By involving stakeholders in the design process, they will develop solutions that best suit users’ needs and are more likely to be adopted and used correctly. This participatory approach should involve interviews and usability testing with clinicians. Formal usability testing and analysis with real end users can improve the quality and usefulness of a system.88 Though patients themselves are not typically the end users of CDSS, their expertise (especially that of marginalized groups and organized patient advocacy organizations) can also inform CDSS developers. As an example, transgender people have compiled their own resources to understanding dosing regimens in the absence of clear clinical guidelines.89 Developers of CDSS could provide a great deal of value to these patient populations, and improve their software’s utility, by working with them to understand their needs from a dosing tool.
89. Aly, W. An interactive web simulator for estradiol levels with injectable estradiol esters. Transfeminine Science <https://transfemscience.org/articles/injectable-e2-simulator-release/> (2021) Accessed November 1, 2022.
Jaafar et al. (2022)
Jaafar, S., Torres-Leguizamon, M., Duplessy, C., & Stambolis-Ruhstorfer, M. (2022). Hormonothérapie injectable et réduction des risques: pratiques, difficultés, santé des personnes trans en France. [Hormone replacement therapy injections and harm reduction: practices, difficulties, and transgender people’s health in France.] Sante Publique, 34(HS2), 109–122. [Google Scholar] [PubMed] [DOI:10.3917/spub.hs2.0109] [Translated]:
With regard to feminizing [substitutive hormone therapy (HS)], there are no specialty injectables based on estrogens in the French pharmacopoeia. This makes it impossible to set up estrogen monotherapies which require high dosages that are more difficult to obtain with specialties with other galenic forms [5]. Faced with this lack of care, some trans women and transfeminine people obtain estradiol-based injectable solutions on the Internet or through other sources [6]. […]
5. Aly. An informal meta-analysis of estradiol curves with injectable estradiol preparations [Internet]. Transfem Sci. 2021 July 16. [Visited on 29/12/2022]. Online : https://transfemscience.org/articles/injectable-e2-meta-analysis/.
Linet (2023)
Linet, T. (2023). Prise en charge endocrinologique d’une personne trans. [Endocrinological care of a trans person.] In Faucher, P., Hassoun, D., & Linet, T. (Eds.). Santé sexuelle et reproductive des personnes LGBT [Sexual and Reproductive Health of LGBT People] (pp. 109–124). Issy-les-Moulineaux, France: Elsevier Masson. [Google Books] [URL] [WorldCat] [Excerpt] [Translated]:
Choice of estrogen.
Estradiol is generally the most prescribed estrogen. It is given orally or sublingually in transfeminine people with no significant cardiovascular risk factors. For others, the percutaneous form (patches, gel) is recommended.
The starting dose is 2 mg of estradiol orally with a step increase of 2 mg every 2 to 3 months until the optimal dose is reached [1]. For the patches, the initial dosage and the increments are 50 or 100 μg, and for the gel 2.5 g. This means that the optimal dose is generally 6 to 8 mg per day for the oral route, 3 to 4 mg for the sublingual route, and 300 to 400 μg for the patches (see table 11.1).
It may happen in consultation that the person does not wish to use the prescribed estrogens and wishes to continue the self-prescription of injectable estrogens. It is then possible to evaluate with them the most suitable dosage using the Transfem Science Injection Simulator (https://transfemscience.org/misc/injectable-e2-simulator/). Risk prevention related to injections (needles) can be done. Associations can help the person find 25 G needles of 40 mm useful this type of treatment.
Rothman et al. (2024)
Rothman, M. S., Ariel, D., Kelley, C., Hamnvik, O. R., Abramowitz, J., Irwig, M. S., Soe, K., Davidge-Pitts, C., Misakian, A. L., Safer, J. D., & Iwamoto, S. J. (2024). The Use of Injectable Estradiol in Transgender and Gender Diverse Adults: A Scoping Review of Dose and Serum Estradiol Levels. Endocrine Practice, 30(9), 870–878. [DOI:10.1016/j.eprac.2024.05.008]:
In recent years, we have noted trends in our clinical practices with TGD adults requesting injectable estradiol, particularly in the United States. The reasons given can vary; it may be due to ease of weekly or every two weeks administration, fatigue of taking daily oral medications and skin reactions to or cost of transdermal preparations. There have been discussions as to the roles of estrone/estradiol ratios in feminization and whether injectable estradiol might lead to more favorable results, however research has not supported a role for estrone in optimizing feminizing outcomes [13]. There is also a belief that higher levels can be attained with injections and may lead to faster and more complete feminization; however, there is a lack of data in the literature to support these conclusions. Such conversations occurring on reddit.com and even some hormone provider websites, are perhaps related to the historical use of high dose injectable estradiol noted above [14]. However, there is a paucity of data to guide clinicians on what dose, type and at what interval estradiol esters should be injected and when levels should be measured to ensure physiologic range estradiol levels. In fact, recent reports and clinical observations have raised concerns that the dosing suggested in guidelines may result in supraphysiological estradiol levels and that higher doses and levels may put patients at elevated risk of thromboembolic events [15-18]. This scoping review examines the available data on levels achieved with various dosages of estradiol injections in TGD adults. We also report on testosterone suppression, route (i.e., SC vs. IM), and type of estradiol ester as well as timing of blood draw relative to dose, where available.
Acknowledgment
[…] [We] thank Aly from Transfemscience for community representation and correspondence.
Toffoli Ribeiro, C., Gois, Í., da Rosa Borges, M., Ferreira, L. G. A., Brandão Vasco, M., Ferreira, J. G., Maia, T. C., & Dias-da-Silva, M. R. (2024). Assessment of parenteral estradiol and dihydroxyprogesterone use among other feminizing regimens for transgender women: insights on satisfaction with breast development from community-based healthcare services. Annals of Medicine, 56(1), 2406458. [DOI:10.1080/07853890.2024.2406458]:
Utilizing a previously published meta-analysis method of estradiol concentration-time data from publicly available information on cisgender women who had used EEn or EEn/DHPA [17], we reanalyzed and integrated data from various studies. […]
[…] The V3C Fitter and Desmos tools, accessible online at https://alyw234237.github.io/injectable-e2-simulator/v3c-fitter/ and https://www.desmos.com/calculator/ndgvp2avhj?lang=pt-BR respectively, were utilized for fitting the three-compartment pharmacokinetic model. […]
Pharmacokinetics of injectable estradiol enanthate
[…] The boxplot graph (Figure 5) illustrates that the median estradiol levels in trans women using EEn/DHPA fell within this population’s expected average range values (100–200pg/mL).
Figure 5. Meta-analysis of estradiol concentration-time data from cisgender women under EEn alone or EEn/DHPA. Fitted data curves from various studies individually and combined into a single-dose curve over 30 days were generated based on an informal meta-analysis of published estradiol concentration-time data from cisgender women under EEn or EEn/DHPA [17]. […]
References
[17] Aly. 2021. An informal meta-analysis of estradiol curves with injectable estradiol preparations. Transfeminine Sci. https:// transfemscience.org/articles/injectable-e2-meta-analysis/
Update 3: Herndon et al. (2023)
In March 2023, the following study on injectable estradiol in transfeminine people was published online:
Herndon, J. S., Maheshwari, A. K., Nippoldt, T. B., Carlson, S. J., Davidge-Pitts, C. J., & Chang, A. Y. (2023). Comparison of Subcutaneous and Intramuscular Estradiol Regimens as part of Gender-Affirming Hormone Therapy. Endocrine Practice, 29(5), 356–361. [DOI:10.1016/j.eprac.2023.02.006]
The study was a retrospective analysis of individualized injectable estradiol in adult transfeminine people who received hormone therapy at the Mayo Clinic. Doses of injectable estradiol were adjusted by clinical providers based on estradiol levels, testosterone suppression, and feminization goals, and subsequently these clinical data were retrospectively studied by Mayo Clinic researchers. The primary aim of the study was to compare injectable estradiol by intramuscular versus subcutaneous routes. However, other general considerations for injectable estradiol, such as dosing, estradiol levels, testosterone suppression, type of injectable estradiol ester (estradiol valerate vs. estradiol cypionate), and estradiol monotherapy versus concomitant use of antiandrogens, were also assessed. The paper noted that the study was the largest to assess injectable estradiol in transfeminine people to date and was the first to directly compare intramuscular and subcutaneous injectable estradiol routes in transfeminine people.
Injectable estradiol doses were adjusted to achieve estradiol and testosterone levels within therapeutic ranges defined by the Endocrine Society 2017 guidelines (>100 pg/mL [367 pg/mL] for estradiol and <50 ng/dL [<1.7 nmol/L] for testosterone). Estradiol levels were measured exclusively using liquid chromatography–tandem mass spectrometry (LC–MS/MS), while the assay method for measuring testosterone levels was not specified. In terms of when in the injection cycle estradiol levels were measured, the authors stated the following: (1) “Timing of estradiol blood draw in relation to injection was not protocolized” and (2) “In our practice, although estradiol concentrations were generally checked midway through the injection cycle, we were unable to document with certainty the timing of the estradiol lab testing which may have influenced the results and/or the outliers”. Only the most recent blood test for each individual was analyzed, with the results of earlier tests discarded. Doses were analyzed in per-week amounts, regardless of dosing frequency (either once weekly or once every two weeks).
There were a total of 130 transfeminine people on injectable estradiol who were analyzed in the study. Of these individuals, 56 received intramuscular (i.m.) injections and 74 received subcutaneous (s.c.) injections. The median duration of therapy for injectable estradiol was 3.0 years for both routes. The vast majority of people used weekly injections (91.1% for i.m., 98.6% for s.c.), whereas the small remainder (n=6) injected once every 2 weeks. Similarly, the great majority used injectable estradiol valerate (89.3% for i.m., 86.5% for s.c.), while a small subset (n=16) used injectable estradiol cypionate. The authors did not state the injectable vehicles, but they can be confidently assumed to have both been in oil. The treatment-individualized doses per week of injectable estradiol were median 4 mg (interquartile range (IQR) 3–5.15 mg; range 1–8 mg) for the i.m. route and median 3.75 mg (IQR 3–4 mg; range 1–8 mg) for the s.c. route, with the differences in doses between groups being slightly but significantly different (p = 0.005). For the small number of people on two-week injection cycles, the doses for the combined i.m. and s.c. groups were median 8.5 mg (range 6–16 mg) every 2 weeks. Estradiol levels with injectable estradiol were median 189.5 pg/mL (IQR 126.8–252.5 or 122.5–257 pg/mL; range ~33–575 pg/mL] for i.m. and median 196 pg/mL (IQR 125.3–298.5 pg/mL; range ~23–581 pg/mL) for s.c., with the differences between groups not being significantly different (p = 0.70). The percentages of individuals with estradiol levels in target range (>100 pg/mL) were 78.6% for i.m. and 82.4% for s.c. The estradiol doses and levels of individual patients for each route were also provided in the paper (Graph). It can be seen that more individuals clustered into the higher range of doses with i.m. than with s.c. injections.
In the case of estradiol valerate versus estradiol cypionate, dose per week for i.m. with estradiol valerate was median 4 mg (IQR 3–5.45 mg) and with estradiol cypionate was median 4 mg (IQR 2.25–5 mg). In the case of s.c., dose per week with estradiol valerate was median 4 mg (IQR 3–4 mg) and with estradiol cypionate was median 3 mg (IQR 2–3 mg). The doses between estradiol valerate and estradiol cypionate were not significantly different in the case of i.m. (p = 0.51), but were significantly different in the case of s.c. (p = 0.025). Estradiol levels with the two different injectable estradiol esters were not provided.
On multiple regression analysis, injectable estradiol dose was significantly positively associated with estradiol levels (p < 0.001) following adjustment for several variables (injection route, body mass index (BMI), antiandrogen use, gonadectomy status). Each 1 mg per week in dose was associated with estradiol levels that were increased by (estimate ± standard error) 57.42 ± 10.46 pg/mL. No other variable, including notably BMI, was significantly associated with estradiol levels. According to the authors, the dose-dependently higher estradiol levels with injectable estradiol suggested the need to start at lower doses that are gradually increased in conjunction with close monitoring of hormone levels.
Testosterone levels in those with gonads were 11 ng/dL (IQR 0–19.8 ng/dL) for i.m. and 11 ng/dL (0–20 ng/dL) for s.c., with levels not significantly different between groups (p = 0.92). Adequate testosterone suppression (<50 ng/dL) in those with gonads was achieved in 84.6% with i.m. and 86.6% with s.c. In the small subset of individuals on injections every two weeks (n=6), 100% of individuals achieved target estradiol and testosterone levels. A majority of individuals on injectable estradiol in the study concomitantly used an antiandrogen, with this usually being spironolactone or finasteride. In a minority of individuals, injectable estradiol monotherapy, without concomitant use of an antiandrogen, was employed and hormone levels were measured (n=17). In this subgroup, estradiol levels were median 220 pg/mL (IQR 180–264 pg/mL) at a dose per week of median 4 mg (IQR 3–6 mg), with estradiol levels noticeably higher than in the overall group. In terms of hormone levels for those on injectable estradiol monotherapy, 100% achieved therapeutic estradiol levels (>100 pg/mL) and 88.2% achieved target testosterone levels (<50 ng/dL). The authors noted that most individuals on injectable estradiol monotherapy were able to adequately suppress testosterone, but that higher doses and levels of estradiol may be needed for testosterone suppression in this context.
Herndon et al. (2023) noted that existing recommendations for injectable estradiol in transfeminine people suggest doses of 2 to 10 mg per week or 5 to 30 mg every 2 weeks, referencing the Endocrine Society 2017 guidelines (Hembree et al., 2017) and UCSF 2016 guidelines (Deutsch, 2016a). They also noted that the UCSF 2016 guidelines recommended lower doses of estradiol cypionate than estradiol valerate, which they attributed to pharmacokinetic differences between the esters (citing Oriowo et al. (1980) for this claim). However, the authors noted that the differences between estradiol valerate and estradiol cypionate doses they observed were small, and questioned the clinical relevance of the differences. The authors also tactfully critiqued dosing recommendations by existing guidelines, and suggested their own data to guide dosing instead, with the following relevant excerpts:
Prior studies used for development of guidelines for parenteral doses are suboptimal given their small sample sizes or pre-specificized [gender-affirming hormone therapy (GAHT)] protocols with no adjustment of estradiol doses or no information on hormone concentrations achieved. [Discussion of Deutsch, Bhakri, & Kubicek (2015) and Mueller et al. (2011) …]
Overall, the studies used to support the current dosing recommendation guidelines for parenteral estradiol dosing are limited, incomplete with regards to hormone concentrations achieved, and do not provide SC as an available option. The doses of estradiol used in this study (with either SC or IM approach), were successful in achieving serum estradiol concentrations at the cisgender female range. Most importantly, as compared to current available guidelines and consensus statements [1, 2], these doses of estradiol valerate are less than half of what is recommended for both initial and maintenance dosing and achieved suppression of testosterone.
Lower doses of parenteral injections than previously described in clinical practice guidelines achieved therapeutic estradiol concentrations. Our data can serve as a dosing guide for initial and maintenance use of parenteral estradiol, which is different than what has been previously described.
Herndon et al. (2023) concluded that injectable estradiol by both i.m. and s.c. routes is effective in achieving therapeutic estradiol levels in transfeminine people. They noted that there were not meaningful differences between i.m. and s.c. in terms of dose, although i.m. may require slightly higher doses than s.c. to achieve therapeutic estradiol levels. The authors stated that doses of injectable estradiol to achieve therapeutic estradiol levels in transfeminine people were lower than previously recommended by guidelines and other publications. They concluded that clinical use of injectable estradiol in transfeminine people should include discussion of both i.m. and s.c. routes and invidiualization by patient. Finally, they called for more clinical studies on injectable estradiol in transfeminine people to evaluate clinical outcomes, feminization, and additional risks and benefits of this route compared to other routes.
The findings of Herndon et al. (2023) are pleasingly consistent with the results of the present meta-analysis. Based on the findings of this meta-analysis, assuming a linear relationship between dose and estradiol levels, estradiol levels with non-polymeric injectable estradiol esters, like estradiol valerate and estradiol cypionate in oil via intramuscular injection, increase by around 60 pg/mL on average for each 1 mg per week in dose (with Herndon et al. (2023) finding a value of 57 pg/mL per 1 mg using a multiple linear regression model). In relation to this, mean integrated estradiol levels of around 250 pg/mL on average would be expected at a dosage of 4 mg once per week. Accordingly, Herndon et al. (2023) found median estradiol levels of 190 to 196 pg/mL at per-week median doses of 3.75 to 4 mg. Similarly, the present work recommended injectable estradiol doses with non-polymeric esters of 1 to 6 mg per week (to achieve mean integrated estradiol levels of roughly 50–300 pg/mL), which is comparable to the range of about 2 to 6 mg per week used in most transfeminine people in Herndon et al. (2023) (to achieve estradiol levels of at least 100 pg/mL, along with adequate testosterone suppression). Additionally, it was noted in this meta-analysis—based on clinical research in cisgender men with prostate cancer—that only modestly supraphysiological estradiol levels, of no more than approximately 200 to 300 pg/mL, are likely to be needed for strong testosterone suppression in transfeminine people. This has likewise been confirmed with solid clinical data in transfeminine people by Herndon et al. (2023), with 88% of those on injectable estradiol monotherapy having testosterone levels of <50 ng/dL at a median injectable estradiol dose of 4 mg/week and with median estradiol levels of 220 pg/mL. It is the opinion of the present author that Herndon et al. (2023) is a very important and formative study, with clinical implications that go far beyond merely supporting the s.c. use of injectable estradiol. The study represents the first major step in the published literature to correcting the dosing and intervals of injectable estradiol in transgender care guidelines and in transgender health generally. I commend the researchers for their work.
Update 4: Rothman et al. (2024a) and Rothman et al. (2024b)
In February 2024, the following short review on injectable estradiol dosing in transfeminine people by Micol Rothman and colleagues was published online:
Rothman, M. S., Hamnvik, O. P. R., Davidge-Pitts, C., Safer, J. D., Ariel, D., Tangpricha, V., Abramowitz, J., Soe, K., Sarvaideo, J., Kelley, C., Irwig, M. S., & Iwamoto, S. J. (2024). Revisiting Injectable Estrogen Dosing Recommendations for Gender-Affirming Hormone Therapy. Transgender Health, 9(6), 463–465. [DOI:10.1089/trgh.2023.0209]
Here is the abstract of the paper:
Injectable estrogens are options for gender-affirming hormone therapy per guidelines, which suggest intramuscular dosages of 5–30 mg every 2 weeks or 2–10 mg weekly with estradiol cypionate or valerate interchangeably. Data among transgender and gender-diverse patients are limited due to local unavailability and concerns around laboratory assay variability and estradiol (E2) level fluctuation. We note a concerning trend where patients are prescribed high-dose injections based on the guidelines leading to serum E2 levels well above the range recommended in the same guidelines. Our review indicates that 5 mg weekly or lower should be prescribed when initiating injectable estrogens to avoid supraphysiologic E2 levels.
Then, in May 2024, the following longer and more comprehensive review on injectable estradiol dosing in transfeminine people by Rothman and most of the same other academics was published online:
Rothman, M. S., Ariel, D., Kelley, C., Hamnvik, O. R., Abramowitz, J., Irwig, M. S., Soe, K., Davidge-Pitts, C., Misakian, A. L., Safer, J. D., & Iwamoto, S. J. (2024). The Use of Injectable Estradiol in Transgender and Gender Diverse Adults: A Scoping Review of Dose and Serum Estradiol Levels. Endocrine Practice, 30(9), 870–878. [DOI:10.1016/j.eprac.2024.05.008]
Here is the abstract of this paper:
Objective: Feminizing gender-affirming hormone therapy is the mainstay of treatment for many transgender and gender diverse people. Injectable estradiol preparations are recommended by the World Professional Association for Transgender Health Standards of Care 8 and the Endocrine Society guidelines. Many patients prefer this route of administration, but few studies have rigorously assessed optimal dosing or route.
Methods: We performed a scoping review of the available data on estradiol levels achieved with various dosages of estradiol injections in transgender and gender diverse adults on feminizing gender-affirming hormone therapy. We also report on testosterone suppression, route (ie, subcutaneous vs intramuscular), and type of injectable estradiol ester as well as timing of blood draw relative to the most recent dose, where available.
Results: The data we reviewed suggest that the current guidelines, which recommend starting doses 2 to 10 mg weekly or 5 to 30 mg every 2 weeks of estradiol cypionate or valerate, are too high and likely lead to patients having supraphysiologic levels across much of their injection cycle.
Conclusions: The optimal starting dose for injectable estradiol remains unclear and whether it should differ for cypionate and valerate. Based on the data available, we suggest that clinicians start injectable estradiol cypionate or valerate via subcutaneous or intramuscular injections at a dose ≤5 mg weekly and then titrate accordingly to keep levels within guideline-recommended range. Future studies should assess timing of injections and subsequent levels more precisely across the injection cycle and between esters.
This paper notably also cited the present Transfeminine Science article as raising concerns about guideline-based dosing for injectable estradiol and potential health complications from these doses.
Aside from Micol Rothman herself, these reviews were also authored by other well-known experts in transgender health. For instance, two of the coauthors, Joshua Safer and Michael Irwig, were authors for the WPATH SOC8 hormone therapy chapter (WPATH SOC8 Full Contributor List). Additionally, Safer was one of the authors for the Endocrine Society’s transgender hormone therapy guidelines (Hembree et al., 2017). As such, it would appear that transgender medicine has finally started to seriously correct injectable estradiol dosing. This is a very important development. Now, the appropriate dosing and intervals of injectable estradiol will need to be more precisely established and the corrections will need to make their way into updated transgender hormone therapy guidelines and general clinical practice.
A letter to the editor commented on and concorded with Rothman and colleagues’ second literature review:
Patel, K. T., & Tangpricha, V. (2024). Parenteral Estradiol for Transgender Women: Time to adjust the dose. Endocrine Practice, 30(9), 893–894. [DOI:10.1016/j.eprac.2024.07.005]
Update 5: Kariyawasam et al. (2024)
In March 2024, the following study of estradiol levels with different routes of estradiol in transfeminine people, including injectable estradiol, was published:
Kariyawasam, N. M., Ahmad, T., Sarma, S., & Fung, R. (2024). Comparison of Estrone/Estradiol Ratio and Levels in Transfeminine Individuals on Different Routes of Estradiol. Transgender Health, ahead of print. [DOI:10.1089/trgh.2023.0138]
The study stratified injectable estradiol doses into different dosing levels, accounted for timing of blood draws, and compared injectable estradiol to other estradiol routes. The other routes included oral estradiol, sublingual estradiol, and transdermal estradiol. The form of injectable estradiol used was estradiol valerate in dose groups including ≤4 mg/week (“low-dose”), >4 mg/week to ≤8 mg/week (“medium-dose”), and >8 mg/week (“high-dose”). In the study, this injectable estradiol regimen resulted in supraphysiological estradiol levels in the medium- to high-dose groups (>4 mg/week) and dramatically higher estradiol levels than with the other estradiol routes (Data). Median estradiol levels were reported in a subsequent paper as follows: “Figure 2 from the paper shows estradiol levels across the 3 groups. Although exact numbers are not given in this figure, we learned through correspondence with the authors that the low dose injection group [n=8] had a median level of 202.7 ± SD 232.6 pg/mL, the medium group [n=22] 465.2 ± SD 466.3 pg/mL, and the high group [n=3] 574.4 ± SD147.3 pg/mL (converted from SI units)” (Rothman et al., 2024b). Although the sample sizes for the different dose groups were small, this study, along with Herndon et al. (2023), provides some of the best clinical data on estradiol levels with injectable estradiol in transfeminine people that have so far been published.
Update 6: Patel et al. (2024)
In June 2024, the following open-access review discussing injectable estradiol in transfeminine people and calling for updated transgender health guidelines was published:
Patel, R., Korenman, S., Weimer, A., & Grock, S. (2024). A Call for Updates to Hormone Therapy Guidelines for Gender-Diverse Adults Assigned Male at Birth. Cureus, 16(6), e62262. [DOI:10.7759/cureus.62262] [PDF]
The following quote is the relevant excerpt on injectable estradiol from the review:
The current guideline-based dosing recommendations for estradiol vary considerably, which is problematic for clinicians and patients who rely on guidelines to initiate treatment. Most notably, the conversion rates between parenteral estradiol valerate and estradiol cypionate vary drastically between the UCSF Guidelines for the Primary and Gender-Affirming Care of Transgender and Gender Nonbinary People (UCSF Guidelines) and The Endocrine Society Clinical Practice Guidelines for Endocrine Treatment of Gender-Dysphoric/Gender-Incongruent Persons (the Endocrine Society Guidelines). The UCSF Guidelines indicate the conversion between estradiol valerate and cypionate to be as high as a 4:1 ratio [2], while the Endocrine Society Guidelines provide no dosing differentiations [1]. Herndon and colleagues demonstrated that the conversion between estradiol cypionate and estradiol valerate is closer to 1:1 [4]. Further equivalence studies are needed to clarify ideal dosing conversions.
The Endocrine Society Guidelines recommend titrating estradiol to 100-200 pg/mL [1]. The UCSF Guidelines recommend 2-4 mg daily as the starting dose for oral estradiol and 5 mg weekly for parenteral estradiol valerate [2]. The Endocrine Society Guidelines suggest oral estradiol 2-6 mg daily and parenteral estradiol 2- 10 mg weekly [1]. However, Chantrapanichkul et al. found that intramuscular injections of estradiol valerate greater than 5 mg weekly led to mean estradiol concentrations well above 200 pg/mL, while 4-5 mg of oral estradiol daily only led to minimum desired concentrations [5]. Similarly, Herndon et al. found that subcutaneous estradiol at a median dose of 3.75 mg per week led to a median estradiol level of 196 pg/mL [4]. Thus, current guideline-based dosing may lead providers to choose doses of injectable estradiol that would result in supratherapeutic serum estradiol levels. In light of these recent publications, it is clear that guideline-based dosing for estradiol needs updating. In our clinical experience, parenteral estradiol valerate at doses of 2-4 mg weekly typically leads to physiologic estradiol levels. Estradiol cypionate should likely be dosed in a 1:1 ratio with estradiol valerate until future data are obtained.
Lastly, while estradiol valerate and cypionate are only FDA-approved for intramuscular administration, many patients prefer subcutaneous administration. There are small studies that suggest the pharmacokinetics of intramuscular and subcutaneous estradiol are similar [4]. While the UCSF Guidelines comment on the use of subcutaneous estradiol, other guidelines should be updated to include this option for patients [2].
Update 7: Toffoli Ribeiro et al. (2024)
Toffoli Ribeiro, C., Gois, Í., da Rosa Borges, M., Ferreira, L. G. A., Brandão Vasco, M., Ferreira, J. G., Maia, T. C., & Dias-da-Silva, M. R. (2024). Assessment of parenteral estradiol and dihydroxyprogesterone use among other feminizing regimens for transgender women: insights on satisfaction with breast development from community-based healthcare services. Annals of Medicine, 56(1), 2406458. [DOI:10.1080/07853890.2024.2406458]:
This study examines the effects of a commonly used injectable hormone combination, specifically estradiol enanthate with dihydroxyprogesterone acetophenide (EEn/DHPA), […] Our research focused on a retrospective longitudinal study involving a large cohort of transwomen evaluated between 2020 and 2022, comprising 101 participants. We assessed serum levels of estradiol (E2), testosterone (T), luteinizing hormone (LH), and follicle-stimulating hormone (FSH), comparing the EEn/DHPA hormonal regimen with other combined estrogen-progestogen (CEP) therapies. […] Our findings indicated that participants using the EEn/DHPA regimen exhibited significantly higher serum E2 levels (mean: 186 pg/mL ± 32 pg/mL) than those using other therapies (62 ± 7 pg/mL), along with lower FSH levels, but no significant differences in T and LH levels. […] These results suggest that an injectable, low-cost EEn/DHPA administered every three weeks could serve as an alternative feminizing regimen, particularly considering the extensive long-term experience of the local transgender community. Further longitudinal studies on the efficacy of feminizing-body effects and endovascular risks of various parenteral CEP types are warranted to improve primary healthcare provision for transgender persons.
Introduction
Injectable combined estrogens with progestogens (CEP) have long been widely used in Brazil and other Latin American countries, predominantly among ciswomen as an injectable contraceptive and by Brazilian transgender women and travestis as GAHT [8]. Despite the absence of recognition by the Endocrine Society as an alternative hormonal regimen due to concerns regarding thrombogenicity and challenges in routine monitoring through blood testing, the prevalent use of CEP necessitates evaluating its regimen recommendations. This has led our research to delve deeper into understanding CEP regimens, considering the experiences of travestis amidst distinct sociocultural lifestyles and limited access to public endocrinological care services [15,16]. Hence, our objective is to elucidate our observations in monitoring trans individuals utilizing CEP regimens by evaluating hormone levels […] within a cohort of transwomen employing the most common injectable CEP, namely estradiol enanthate with dihydroxyprogesterone acetophenide (EEn/DHPA) and comparing these observations with other GAHT regimens.
Subjects and methods
Estradiol enanthate pharmacokinetics curve
Utilizing a previously published meta-analysis method of estradiol concentration-time data from publicly available information on cisgender women who had used EEn or EEn/DHPA [17], we reanalyzed and integrated data from various studies. A unified single-dose curve for 30 days was created. We employed least squares regression for studies with four or more concentration-time data points (solid lines). We manually adjusted other studies with three data points to fit into a single-dose curve.
Each study’s data were adjusted for baseline estradiol levels or endogenous estradiol production and then normalized by 10 mg. The V3C Fitter and Desmos tools, accessible online at https://alyw234237.github.io/injectable-e2-simulator/v3c-fitter/ and https://www.desmos.com/calculator/ndgvp2avhj?lang=pt-BR respectively, were utilized for fitting the three-compartment pharmacokinetic model. Estradiol levels from transgender women on EEn/DHPA in this study were presented using a box plot graph featuring percentiles at 10, 25, 50, 75, and 90.
Results
Hormonal levels during the follow-up of feminizing regimens
Scatter plot graphs depicted the measurement of sex hormones (Figure 2). Serum estradiol levels in the EEn/ DHPA group (mean: 186.4pg/mL ± 32.8pg/mL) were significantly higher than those in the group using other therapies (62.2±6.9pg/mL) (Figure 2(A)). Within the EEn/DHPA group, serum FSH levels were significantly lower compared to the other group (Others) (Figure 2(B)). However, no significant difference was found between the groups concerning testosterone (Figure 2(C)) and LH (Figure 2(D)) levels.
Pharmacokinetics of injectable estradiol enanthate
Serum estradiol levels in trans women using EEn/DHPA reached the target levels for this population during hormone therapy, a trend not observed in participants using other feminizing hormone therapies (Table 1). The boxplot graph (Figure 5) illustrates that the median estradiol levels in trans women using EEn/DHPA fell within this population’s expected average range values (100–200pg/mL).
Figure 5. Meta-analysis of estradiol concentration-time data from cisgender women under EEn alone or EEn/DHPA. Fitted data curves from various studies individually and combined into a single-dose curve over 30 days were generated based on an informal meta-analysis of published estradiol concentration-time data from cisgender women under EEn or EEn/DHPA [17]. For studies with four or more concentration-time data points (solid lines) and the fit of combined data (thick black line), least squares regression to a three-compartment pharmacokinetic model was employed. A single-dose curve was manually adjusted for studies with three data points (dashed lines). Data from each study were adjusted for endogenous estradiol production via baseline or trough estradiol levels subtraction and normalized by 10mg. The graph illustrates estradiol levels from the transwoman cohort in a boxplot. The shaded area represents the optimal target range for estradiol levels in transwomen under hormone therapy. The boxplot graph displays the percentiles 10, 25, 50, 75, and 90 for estradiol levels of transwomen under EEn/DHPA in this study (N=53).
Discussion
Our study represents a pioneering contribution to the literature by demonstrating that Brazilian trans women undergoing EEn/DHPA therapy achieved estradiol levels comparable to those observed during the follicular phase in cisgender women. […]
Our study further noted that DHPA demonstrates comparable efficacy to cyproterone or other anti-androgens in achieving optimal LH pituitary suppression and reducing testosterone levels. EEn/ DHPA, an affordable injectable contraceptive widely accessible in South American countries, presents a promising avenue for attaining target hormone levels among transfeminine individuals.
Additionally, our investigation, which reviewed pharmacokinetic data, supports the potential implementation of EEn/DHPA in a 21-day regimen to sustain optimal estradiol levels. While alternative medications exist to inhibit testosterone production and action, their availability varies based on regional healthcare provider systems. […]
EEn/DHPA, commonly used as a long-lasting injectable contraceptive [21–23], has found application in feminizing hormone therapy for transfeminine people, notably in travestis in Brazil [7,8,24,25]. […]
In conclusion, our long-term cohort study suggests that administering parenteral estradiol enanthate with dihydroxyprogesterone acetophenide every three weeks could serve as a practical option for feminizing hormone regimens in transgender women. Nonetheless, adopting an individualized approach that takes into account each individual’s goals, response to prior hormone therapies, and medical history is crucial. This personalized approach is central to improving healthcare provision and ensuring optimal outcomes in bodily changes. By continuing to explore and refine hormone therapy regimens, we can better support the health and well-being of transgender individuals on their gender-affirming journey.
References
[17] Aly. 2021. An informal meta-analysis of estradiol curves with injectable estradiol preparations. Transfeminine Sci. https:// transfemscience.org/articles/injectable-e2-meta-analysis/
Update 8: Misakian et al. (2025)
Misakian, A. L., Kelley, C. E., Sullivan, E. A., Chang, J. J., Singh, G., Kokosa, S., Avila, J., Cooper, H., Liang, J. W., Botzheim, B., Quint, M., Jeevananthan, A., Chi, E., Harmer, M., Hiatt, L., Kowalewski, M., Steinberg, B., Tausinga, T., Tanner, H., Ho, T. F., … Ariel, D. (2025). Injectable Estradiol Use in Transgender and Gender-Diverse Individuals throughout the United States. The Journal of Clinical Endocrinology & Metabolism, dgaf015. [DOI:10.1210/clinem/dgaf015]:
Context: Guidelines for use of injectable estradiol esters (valerate [EV] and cypionate [EC]) among transgender and gender-diverse (TGD) individuals designated male at birth vary considerably, with many providers noting supraphysiologic serum estradiol concentrations based on current dosing recommendations.
Objectives: This work aimed to 1. determine the dose of injectable estradiol (subcutaneous [SC] and intramuscular [IM]) needed to reach guideline-recommended estradiol concentrations for TGD adults using EC/EV; 2. describe the relationship between estradiol concentration relative to timing/dose of last estradiol injection and other covariates; and 3. determine dosing differences between IM/SC EV/EC.
Methods: A cross-sectional retrospective study was conducted across 6 US medical centers including TGD adults on same-dose injectable estradiol for more than 75 days, with confirmed timing of estradiol concentration relative to last injection, from January 1, 2019 to December 31, 2023. Descriptive statistics were used to describe patient characteristics and weighted linear mixed models to evaluate relationship between various covariates and estradiol concentration.
Results: Data from 562 patients were included. Among those injecting every 7 days who reached the guideline-recommended estradiol concentration (n = 131, 27.5%), the median estradiol dose was 4.0 mg (interquartile range, 3.0-5.0 mg). Among all patients, the majority reached supraphysiologic estradiol concentrations (>200 pg/mL [>734 pmol/L]) while dose and timing in the injection cycle were significant covariates for the estradiol concentration. There were no significant dosing differences between IM/SC EV/EC.
Conclusion: Injectable estradiol esters effectively reach guideline-recommended estradiol concentrations but at lower doses than previously recommended. Estradiol concentrations are best interpreted relative to timing of last injection. Route of administration and type of ester do not significantly affect estradiol concentrations.
[…]
And a letter to the editor commenting on the paper:
Milano, C., & Harper, J. (2025). Comments on Injectable Estradiol Use in Transgender and Gender-Diverse Individuals in the US. The Journal of Clinical Endocrinology & Metabolism, dgaf134. [DOI:10.1210/clinem/dgaf134]
Update 9: Slack et al. (2025)
Slack, D. J., Di Via Ioschpe, A., Saturno, M., Kihuwa-Mani, S., Amakiri, U. O., Guerra, D., Karim, S., & Safer, J. D. (2025). Examining the Influence of the Route of Administration and Dose of Estradiol on Serum Estradiol and Testosterone Levels in Feminizing Gender-Affirming Hormone Therapy. Endocrine Practice, 31(1), 19–27. [DOI:10.1016/j.eprac.2024.10.002]:
Introduction: […] This study investigates the effect of route of administration (ROA) and dose of estradiol on estradiol (E2) and testosterone (T) levels in transfeminine individuals.
Methods: We conducted a chart review of 573 patients with an active prescription for estradiol for feminizing GAHT and serum hormone levels available.
Results: […] Intramuscular estradiol was associated with lower T and higher E2 compared to oral and transdermal ROAs (P < .001), with many achieving target hormone levels even at low doses.
Conclusions: […] The intramuscular ROA appears to be the most potent delivery of estradiol with impact on serum hormone levels with doses on the low end of guideline-suggested ranges.
[…]
Update 10: Carlson et al. (2025)
Carlson, S. M., Dominguez, C., Jeevananthan, A., & Crowley, M. J. (2025). Follow-Up Estradiol Levels Based on Regimen Formulation With Guideline-Concordant Gender-Affirming Hormone Therapy. Journal of the Endocrine Society, 9(3), bvae205. [DOI:10.1210/jendso/bvae205]:
Context: Endocrine Society guidelines for dosing of feminizing gender-affirming hormone therapy (GAHT) have remained essentially unchanged since 2009. The Endocrine Society recommends periodic monitoring of serum estradiol levels, with the goal of maintaining levels in the premenopausal cisgender female range (100-200 pg/mL). However, it is not clear whether guideline-concordant dosing consistently produces guideline-recommended levels across common estradiol formulation types (oral pills, parenteral injections, transdermal patches).
Objective: All transgender and nonbinary patients receiving estradiol-based GAHT between October 2015 and March 2023 were reviewed at a single center, with the goal of determining the frequency with which guideline-concordant dosing with different estradiol formulations led to guideline-recommended estradiol levels.
Methods: Demographics, GAHT regimen, and estradiol levels were obtained via chart review, and data were analyzed descriptively.
Results: The analytic population included n = 35 individuals, including n = 9 prescribed oral estradiol pills, n = 11 prescribed parenteral injections, and n = 15 prescribed transdermal patches. With guideline-concordant doses of oral estradiol (mean 2.8 mg daily), the mean follow-up level was 168 pg/mL; 32% of follow-up levels were subtherapeutic and 14% were supratherapeutic. With guideline-concordant doses of parenteral estradiol (mean 5.8 mg weekly), the mean midpoint follow-up level was 342 pg/mL; 91% of midpoint follow-up levels were supratherapeutic. With guideline-concordant doses of transdermal estradiol (mean 0.09 mg/day), the mean follow-up level was 81.5 pg/mL; 70% of follow-up levels were subtherapeutic.
Conclusion: Supratherapeutic follow-up estradiol levels were common with guideline-concordant parenteral estradiol doses, as were subtherapeutic follow-up levels with guideline-concordant transdermal doses. These findings may suggest the need for revision of guideline-recommended estradiol doses for these formulations
[…]
Update 11: Kanin et al. (2025)
Kanin, M., Slack, M., Patel, R., Chen, K. T., Jackson, N., Williams, K. C., & Grock, S. (2025). Injectable Estradiol Dosing Regimens in Transgender and Nonbinary Adults Listed as Male at Birth. Journal of the Endocrine Society, bvaf004. [DOI:10.1210/jendso/bvaf004]:
Context: Many transgender and nonbinary (TGNB) individuals assigned male at birth (AMAB) seek hormone therapy to achieve physical and emotional changes. Standard therapy includes estradiol, with or without an antiandrogen. Our clinical observations suggest that currently recommended injectable estradiol dosing may lead to supratherapeutic estradiol levels.
Objective: We sought to evaluate whether lower-than-recommended doses of injectable estradiol were effective in achieving serum estradiol and testosterone goals.
Methods: We conducted a retrospective cohort study to evaluate injectable estradiol dosing in treatment-naive AMAB individuals initiating hormone therapy. Data from a single provider at an academic center from January 2017 to March 2023 were analyzed. A total of 29 patients were eligible for inclusion. The primary variables of estradiol dosage, serum estradiol, and testosterone levels were analyzed over 15 months.
Results: The average estradiol dose decreased from 4.3 to 3.7 mg weekly (P < .001) during the study period with a final on-treatment estradiol level of 248 pg/mL. All individuals achieved a testosterone level of less than 50 ng/dL during the study period. The average initial on-treatment testosterone level was not significantly different from average final on-treatment measurement of 24.0 mg/dL (P = .95). […]
Conclusion: Lower doses of injectable estradiol can achieve therapeutic estradiol levels with excellent testosterone suppression. […]
[…]
This study had been previously published as a conference abstract:
Kanin, M., Slack, M., Patel, R., Chen, K. T., Jackson, N., Williams, K., & Grock, S. (2024). 8309 Injectable Estradiol Dosing Regimens; A Retrospective Review of Hormone Therapy for Gender-Diverse Adults Assigned Male at Birth. Journal of the Endocrine Society, 8(Suppl 1), bvae163-1706. [DOI:10.1210/jendso/bvae163.1706]
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+An Informal Meta-Analysis of Estradiol Curves with Injectable Estradiol Preparations - Transfeminine Science
An Informal Meta-Analysis of Estradiol Curves with Injectable Estradiol Preparations
By Aly | First published July 16, 2021 | Last modified May 8, 2025
Abstract / TL;DR
Injectable estradiol preparations such as estradiol valerate and estradiol cypionate in oil are frequently used as estrogens in transfeminine hormone therapy. However, there is little characterization of these preparations in transfeminine people and dosing recommendations by transgender health guidelines appear to be based on expert opinion rather than on clinical data. To help shed light on the properties of injectable estradiol and to better inform dosing considerations in transfeminine people, an informal meta-analysis of available clinical data on estradiol concentration–time curves with major injectable estradiol formulations was conducted. The included preparations were injectable estradiol benzoate in oil, estradiol valerate in oil, estradiol cypionate both in oil and as a suspension, estradiol enanthate in oil, estradiol undecylate in oil, and polyestradiol phosphate. The literature was searched for clinical concentration–time data with these injectable estradiol esters and these data were collected and analyzed. Meta-analysis consisted of data for each injectable estradiol preparation being processed and fit with pharmacokinetic models. Selected pharmacokinetic parameters were additionally determined and reported. The results of this work were discussed with regard to characteristics of injectable estradiol preparations like curve shapes, durations, estrogenic exposure, and variability between people and studies. Recommendations for injectable estradiol preparations by transgender health guidelines were also explored in light of the present results. Current guidelines recommend doses of these preparations that appear to be highly excessive with injection intervals that are too widely spaced. Based on the findings of the present meta-analysis, recommendations by guidelines should be reassessed. Finally, the fitted curves in this work were incorporated into an interactive web-based injectable estradiol simulator intended for use by transfeminine people and their medical providers to help guide therapeutic decisions.
Introduction
Estradiol is the main estrogen used in transfeminine hormone therapy and is available in a variety of different forms for use by different routes of administration. The most commonly employed forms are oral, sublingual, transdermal, and injectable preparations. Injectable estradiol preparations have been discontinued in many countries and hence are unavailable for use in transfeminine hormone therapy in many parts of the world, for instance in most of Europe (Glintborg et al., 2021). However, they are still used by many transfeminine people particularly in the United States and in the do-it-yourself (DIY) community. The most commonly used forms include estradiol valerate, estradiol cypionate, and estradiol enanthate all in oil. Injectable estradiol preparations have certain advantages over other estradiol forms that make them a popular choice for use in transfeminine hormone therapy. These include often lower cost, capacity to easily achieve higher estradiol levels that can be useful for testosterone suppression, less frequent administration, and theoretically reduced health risks relative to oral estradiol at equivalent doses due to the lack of the first pass with this route (Aly, 2020). The higher estradiol levels with injections are particularly useful for estradiol monotherapy, in which an antiandrogen is not used.
Clinically used injectable estradiol preparations are formulated not as estradiol but as estradiol esters. When injected into muscle or fat in oil solutions or crystalline aqueous suspensions, these estradiol esters form depots at the injection site from which they are slowly released. Subsequent to release, estradiol esters are rapidly metabolized into estradiol and hence act as prodrugs. When estradiol itself is given by intramuscular injection in an aqueous solution or oil solution, it is rapidly absorbed and has a very short duration. Due to having lipophilic esters, most clinically used injectable estradiol esters are more fat-soluble than estradiol (as measured by oil–water partition coefficient (P)) (Table). When these esters are administered as oil solutions by intramuscular or subcutaneous injection, their increased lipophilicity causes them to be released from the injection-site depot more slowly than estradiol and to therefore have longer durations. In the case of fatty acid esters, the longer the chain length of the ester—as in e.g. estradiol valerate (5 carbons) vs. estradiol enanthate (7 carbons) vs. estradiol undecylate (10 carbons)—the greater the fat solubility, the slower the rate of release from the depot, and the longer the time to peak levels and duration (Edkins, 1959; Sinkula, 1978; Chien, 1981; Kuhl, 2005; Kalicharan, 2017; Vhora et al., 2019). The durations of both injectable oil solutions and aqueous suspensions depend on the ester and its particular physicochemical properties, but the characteristics of these preparations are different and they work in distinct ways to produce their depot effects (Enever et al., 1983; Aly, 2019). The durations of oil solutions are dependent on the lipophilicity of the ester as well as oil vehicle, whereas the durations of aqueous suspensions depend on the properties of the ester crystal lattice as well as crystal sizes (Chien, 1981; Enever et al., 1983; Aly, 2019). The polymeric estradiol ester polyestradiol phosphate is more hydrophilic (water-soluble) than estradiol and works differently than other injectable estradiol preparations. Ιt is composed of many estradiol molecules linked together via phosphate esters (on average 13 molecules of estradiol per one molecule of polyestradiol phosphate) and has a prolonged duration due to slow cleavage into estradiol following injection. Estradiol esters are able to substantially prolong the duration of estradiol when used as injectables and these preparations have durations ranging from days to months depending on the ester and how it is formulated (Table).
In order to aid understanding of concentration–time profiles with injectable estradiol preparations, I’ve developed an interactive web-based injectable estradiol simulator for transfeminine people and their medical providers. During work on this simulator, it became apparent that there is substantial variability in estradiol levels and curve shapes between different studies even with the same injectable estradiol ester. The injectable estradiol simulator was originally designed to simulate curves from only a single well-known pharmacokinetic study that directly compared estradiol benzoate, estradiol valerate, and estradiol cypionate in oil (Oriowo et al., 1980 [Graph]). However, due to the considerable differences in estradiol levels and curves across studies, it was decided that relying on only one study for such a project would be untenable. Instead, for the simulations to be reasonably accurate to the available data, many studies would need to be incorporated. Including additional studies would also allow for inclusion of other injectable estradiol esters in the simulator. As a result, the present work—an informal meta-analysis of estradiol curves with injectable estradiol formulations—was conducted for the simulator project.
Methods
A literature search was performed to identify studies reporting clinical estradiol concentration–time data with major injectable estradiol formulations (Table 1). All of these preparations have been used in transfeminine hormone therapy at one time or another in different parts of the world, although only estradiol valerate in oil and estradiol cypionate in oil are widely used today. Some of the injectable preparations included have notably been discontinued. Acceptable data for the search included mean and individual estradiol concentration data and Cmax estradiol levels (mean peak estradiol levels of individual subjects at time Tmax). Databases like PubMed, Google Scholar, and WorldCat were searched using relevant keywords (e.g., estradiol ester names and variations thereof as well as major brand names). Publications with relevant information were catalogued for data collection. Only single-dose data and multi-dose data that allowed estradiol levels to return to baseline between doses (as in e.g. repeated once-monthly combined injectable contraceptives) were included. Studies were included regardless of the hypothalamic–pituitary–gonadal axis (HPG axis) status of the participants. The study selection criteria aimed to maximize data inclusion due to scarcity of data for several preparations. If however there were many studies for a specific preparation, studies with only 1 or 2 subjects were generally skipped due to the limited additional value that they would provide. When data were in figures in papers—as was generally the case—they were extracted from the graphs using WebPlotDigitizer.
Table 1: Major injectable estradiol formulations (ordered roughly from shortest- to longest-acting):
Following their collection, data were processed, aggregated, and modeled. Data were adjusted for endogenous estradiol production and were normalized by dose. Adjustment for endogenous estradiol production was generally done via subtraction of baseline estradiol levels. In a number of cases however, subtraction of trough estradiol levels or of estradiol levels from a control group was required instead. Data were also weighted by sample size. In a handful of instances, certain missing information (e.g., time to peak levels, baseline levels, subject body weights) was filled in with reasonable assumptions to help maximize data inclusion. Data were processed in the form of mean estradiol curve data rather than individual-subject data (except for rare n=1 studies). The combined processed data from all studies for each injectable estradiol preparation were fit via least squares regression to one-, two-, and three-compartmentpharmacokinetic models with first-order absorption and elimination that were obtained from the literature and other sources (e.g., Colburn, 1981; Wagner, 1993; Fisher & Shafer, 2007; Lixoft, 2008; Abuhelwa, Foster, & Upton, 2015; Certara, 2020). These models fit most curves from individual studies very well. Fitting the combined curve fits of all individual studies (as opposed to fitting all of the combined processed data directly) was additionally evaluated for each injectable estradiol preparation, and if it was feasible for the preparation and allowed for better fitting results, was employed instead. Fitting directly to the combined processed data has the effect of weighting individual studies by quantity of time points, whereas fitting the combined curve fits of studies eliminates this. The Akaike information criterion (AIC) was used to help guide model selection for fitting of the preparations. Curve fitting was performed using the Python library Lmfit with the Levenberg–Marquardt algorithm. Cmax concentrations are a different form of data than mean curve estradiol concentration–time data, and for this reason, were not included in the fitting unless data were very limited for a given injectable estradiol preparation. Outlying data were also excluded from fitting in a number of instances and this allowed for improved curve fits with more uniform area-under-the-curve levels. The main criterion used for excluding curves was fit area-under-the-curve levels that deviated considerably from what was typical for the injectable estradiol preparations (generally less than about 50% of the average or greater than about 150% of the average).
A selection of pharmacokinetic parameters were calculated for each injectable estradiol preparation using the single-dose fit curves and compartmental pharmacokinetic analyses. These parameters included maximal or peak concentrations of estradiol after a single dose scaled to 5 mg (Cmax), time to maximal concentrations of estradiol after a single dose (Tmax), total area-under-the-curve concentrations of estradiol after a single dose (AUC0–∞), terminal elimination half-life after a single dose (t1/2), and the terminal 90% life after a single dose (t90%) (calculated as t1/2 × 3.322). In addition, selected pharmacokinetic parameters were calculated for simulated repeated administration of each injectable preparation at steady state with a dose and dose interval of 5 mg once every 7 days using the single-dose fit curves and compartmental pharmacokinetic analyses. These parameters included time to peak concentrations of estradiol (Tmax), peak and trough concentrations of estradiol (Cmax and Cmin, respectively), peak–trough difference (PTD; Cmax – Cmin), peak–trough ratio (PTR; Cmax ÷ Cmin), and integratedmean concentrations of estradiol (Cavg). Simulation of repeated administration was performed by stacking estradiol levels for multiple injections. Cmax and Tmax were defined and calculated in general as peak estradiol level and time to peak level of the fit mean curve as opposed to the mean peak level and mean time to peak level of individual subjects. This is because the latter would not be possible to compute as most studies reported only estradiol mean curve data. Pharmacokinetic parameters were calculated using relevant pharmacokinetic equations and, as a sanity check, were compared against those computed by PKSolver, a Microsoft Excel pharmacokinetics add-in program (Zhang et al., 2010).
Results
The figures in the subsequent sections show the original data from studies adjusted for endogenous estradiol levels and normalized to a common dose as well as the curve fits to the data (or alternatively the curve fits of the fits of the data depending on the preparation) for the included injectable estradiol preparations. Estradiol benzoate, estradiol cypionate in oil, and estradiol cypionate suspension were fit to the fits of all individual studies for these preparations, whereas estradiol enanthate, estradiol undecylate, and polyestradiol phosphate were fit directly to the combined processed data for these esters. In the case of estradiol valerate, the two fitting approaches gave nearly identical curves, and so fitting the combined processed original data was done for simplicity for this preparation. Cmax studies were excluded in the fitting for all preparations except estradiol enanthate, for which available estradiol concentration–time data were otherwise very limited. The data for the injectable estradiol preparations were generally fit best by a three-compartment pharmacokinetic model (Desmos). As a result, and for consistency, this model was used in the fitting of all preparations.
Estradiol Benzoate
Injectable estradiol benzoate has been extensively used in the past in scientific research, most notably in studies elucidating the function and dynamics of the HPG axis. One such use of estradiol benzoate has been the estrogen provocation test, a diagnostic test of HPG axis function. Due to its use in research, substantial estradiol concentration–time data with injectable estradiol benzoate exists. A total of 26 publications and concentration–time data for 355 individual injections were identified (Table 2).
Table 2: Studies of injectable estradiol benzoate (Spreadsheet; Plotly):
a Total number of injections, not total number of subjects.
A number of studies were excluded from fitting due to much higher or lower area-under-the-curve levels than average. A couple of studies were omitted from the meta-analysis as they only reported total estrogen levels rather than estradiol levels with estradiol benzoate (Akande, 1974; Weiss, Nachtigall, & Ganguly, 1976). Two studies were omitted due partly to being very old and using very early and inaccurate blood tests (Varangot & Cedard, 1957; Ittrich & Pots, 1965 [Graph]). The processed original data and fit of fits curve for estradiol benzoate are shown in Figure 1.
Figure 1: Published estradiol concentration–time curves and fit of fit curves (thick black or white line) with a single intramuscular injection of estradiol benzoate in oil solution over a period of 7 days. Each curve was adjusted for endogenous estradiol levels, normalized to a dose of 5 mg, and fit with a compartmental pharmacokinetic model. Following this, the combined fit curves of the individual studies were fit using the same pharmacokinetic model. The original data from the studies for estradiol benzoate are also provided elsewhere (Spreadsheet; Plotly).
Estradiol Valerate
Studies with curve data on injectable estradiol valerate come from its use in menopausal hormone therapy and other therapeutic indications for estrogens, its use in combined injectable contraceptives, and use in scientific research. A total of 28 publications and concentration–time data for 309 individual injections were identified for estradiol valerate (Table 3).
Table 3: Studies of injectable estradiol valerate (Spreadsheet; Plotly):
Study
na
Subjects
Dose
Reference(s)
S7175
12
Premenopausal women with menstrual migraine (n=10) and amenorrheic/postmenopausal women with history of menstrual migraine (n=2)
Normally cycling transmasculine people not on hormone therapy (n=31), transfeminine people not on hormone therapy (n=14), and gonadally intact transfeminine people on oral estrogen therapy (n=9)
a Total number of injections, not total number of subjects.
A few of these studies were excluded from fitting due generally to much higher or lower area-under-the-curve levels than average or due to being Cmax data. One study was omitted as it only reported estrone levels rather than estradiol levels (Ibrahim, 1996). Another study was not included due to being in pregnant women with concomitant pregnancy termination (Garner & Armstrong, 1977). One last study was omitted due partly to being very old and using very early and inaccurate blood tests (Ittrich & Pots, 1965 [Graph]). The processed original data and fit curve for estradiol valerate are shown in Figure 2.
Figure 2: Published estradiol concentration–time curves and fit curve (thick black or white line) with a single intramuscular injection of estradiol valerate in oil solution over a period of 30 days. Curves were adjusted for endogenous estradiol levels, normalized to a dose of 10 mg, and fit with a compartmental pharmacokinetic model. Fitting of the combined fits of individual studies for this preparation was explored but gave a nearly identical overall curve, so the overall fit curve for the combined processed original data was used for simplicity for this preparation. The original data from the studies for estradiol valerate are also provided elsewhere (Spreadsheet; Plotly).
Estradiol Cypionate Oil
Estradiol cypionate in oil is used in menopausal hormone therapy and for other estrogen indications. However, its use has been more limited relative to other injectable estradiol preparations, like estradiol valerate. Only a handful of studies with relevant data were identified for estradiol cypionate in oil. This included 4 publications and estradiol concentration–time data for 49 individual injections (Table 4).
Table 4: Studies of injectable estradiol cypionate in oil (Spreadsheet; Plotly):
a Total number of injections, not total number of subjects.
No curves were excluded from fitting in the case of this preparation. The processed original data and fit of fit curves for estradiol cypionate in oil are shown in Figure 3.
Figure 3: Published estradiol concentration–time curves and fit of fit curves (thick black or white line) with a single intramuscular injection of estradiol cypionate in oil solution over a period of 30 days. Each curve was adjusted for endogenous estradiol levels, normalized to a dose of 5 mg, and fit with a compartmental pharmacokinetic model. Following this, the combined fit curves of the individual studies were fit using the same pharmacokinetic model. The original data from the studies for estradiol cypionate in oil are also provided elsewhere (Spreadsheet; Plotly).
Estradiol Cypionate Suspension
Estradiol cypionate suspension has been used exclusively in combined injectable contraceptives. For this reason, many relatively high quality pharmacokinetic studies with this injectable preparation have been conducted. A total of 9 publications and estradiol concentration–time data for 131 individual injections were identified for estradiol cypionate suspension (Table 5).
Table 5: Studies of injectable estradiol cypionate suspension (Spreadsheet; Plotly):
a Total number of injections, not total number of subjects. b By subcutaneous injection rather than intramuscular injection.
One of these studies used subcutaneous injection instead of the usual intramuscular injection but the resulting curve was very similar to the curve for intramuscular injection in the same study (Sierra-Ramírez et al., 2011 [Graph]). Several Cmax studies were excluded from fitting for this preparation. One pharmacokinetic study only measured estradiol cypionate levels rather than estradiol levels and hence was not included (Martins et al., 2019 [Graph]). The processed original data and fit of fit curves for estradiol cypionate suspension are shown in Figure 4.
Figure 4: Published estradiol concentration–time curves and fit of fits curve (thick black or white line) with a single intramuscular (or in one case subcutaneous) injection of a microcrystalline aqueous suspension of estradiol cypionate over a period of 30 days. Each curve was adjusted for endogenous estradiol levels, normalized to a dose of 5 mg, and fit with a compartmental pharmacokinetic model. Following this, the combined fit curves of the individual studies were fit using the same pharmacokinetic model. The original data from the studies for estradiol cypionate suspension are also provided elsewhere (Spreadsheet; Plotly).
Estradiol Enanthate
Estradiol enanthate has been used exclusively in combined injectable contraceptives. Several pharmacokinetic studies have been conducted with it because of this. A total of 7 publications and concentration–time data for 270 individual injections were identified for estradiol enanthate (Table 6).
Table 6: Studies of injectable estradiol enanthate (Spreadsheet; Plotly):
a Total number of injections, not total number of subjects.
Of the available data, 216 of the injections were from a single study and mainly included only Cmax levels. Wiemeyer et al. (1986) was excluded from fitting due to having unusually high area-under-the-curve levels with a small sample size (n=3). Because of the scarcity of estradiol concentration–time data available for estradiol enanthate, Cmax studies were included in the fitting for this preparation. The processed original data and fit curve for estradiol enanthate are shown in Figure 5.
Figure 5: Published estradiol concentration–time curves and fit curve (thick black or white line) with a single intramuscular injection of estradiol enanthate in oil solution over a period of 30 days. Curves were adjusted for endogenous estradiol levels, normalized to a dose of 10 mg, and fit with a compartmental pharmacokinetic model. The original data from the studies for estradiol enanthate are also provided elsewhere (Spreadsheet; Plotly).
Estradiol Undecylate
Estradiol undecylate was formerly used in the treatment of prostate cancer and in menopausal hormone therapy as well as for other estrogen therapeutic indications. However, it was discontinued many years ago and is no longer used today. Nonetheless, estradiol undecylate is of significant historical interest as an injectable estradiol preparation. A total of 4 publications and estradiol concentration–time data for 7 individual injections were identified for estradiol undecylate (Table 7).
Table 7: Studies of injectable estradiol undecylate (Spreadsheet; Plotly):
a Total number of injections, not total number of subjects.
Unfortunately, the identified data were of very low quality, with small sample sizes and considerable variations in estradiol levels. Moreover, estradiol undecylate is a very long-acting injectable estradiol ester with a duration measured in months, and the follow up in these studies only went to about 2 weeks post-injection. For these reasons, it was not possible to fit the data for estradiol undecylate in a reasonably accurate way—as suggested by area-under-the-curve estradiol levels that were only around one-third those of the other non-polymeric injectable estradiol esters. Limited multi-dose hormone concentration–time data also exist for estradiol undecylate, but these data could not be incorporated (Jacobi & Altwein, 1979 [Graph]; Jacobi et al., 1980 [Graph]; Derra, 1981 [Graph]). The processed original data and fit curve for estradiol undecylate are shown in Figure 6.
Figure 6: Published estradiol concentration–time curves and fit curve (thick black or white line) with a single intramuscular injection of estradiol undecylate in oil solution over a period of 90 days. Curves were adjusted for endogenous estradiol levels, normalized to a dose of 50 mg, and fit with a compartmental pharmacokinetic model. The original data from the studies for estradiol undecylate are also provided elsewhere (Spreadsheet; Plotly).
Polyestradiol Phosphate
Polyestradiol phosphate has been used primarily in the treatment of prostate cancer but has also been used for estrogen therapeutic indications like treatment of breast cancer and menopausal hormone therapy. While this injectable estradiol preparation has been used widely in the past, it appears to have recently been discontinued. All of the identified studies with estradiol concentration–time data on polyestradiol phosphate were in men with prostate cancer. A total of 11 publications and concentration–time data for 114 individual injections were identified for polyestradiol phosphate (Table 8).
Table 8: Studies of injectable polyestradiol phosphate (Spreadsheet; Plotly):
a Total number of injections, not total number of subjects.
A few older and strongly outlying studies were excluded from the fitting. The processed original data and fit curve for polyestradiol phosphate are shown in Figure 7.
Figure 7: Published estradiol concentration–time curves and fit curve (thick black or white line) with a single intramuscular injection of an aqueous solution of polyestradiol phosphate over a period of 90 days. The graph was clipped to maximum estradiol levels of 600 pg/mL (~2,200 pmol/L) for better viewability. Curves were adjusted for endogenous estradiol levels, normalized to a dose of 160 mg, and fit with a compartmental pharmacokinetic model. The original data from the studies for polyestradiol phosphate are also provided elsewhere (Spreadsheet; Plotly).
Other Injectable Estradiol Preparations
A number of clinical studies with estradiol concentration–time data for other injectable estradiol preparations were also identified during literature search:
Estradiol (unesterified) in an “aqueous” preparation (type of aqueous preparation unspecified but probably a microcrystalline aqueous suspension) (Jones et al., 1978 [Graph])
These preparations were not included in the present meta-analysis due to their relative obscurity and the limited data available for them. In addition, there were concerns about fitting the used pharmacokinetic models to the formulations with multiple estradiol components and to the microsphere formulations.
No estradiol concentration–time data were identified for certain other injectable estradiol forms of interest, like unesterified estradiol in aqueous solution, estradiol benzoate as a microcrystalline aqueous suspension (Agofollin Depot; Ovocyclin M), or estradiol benzoate butyrate/dihydroxyprogesterone acetophenide in oil (Redimen, Soluna, Unijab) (another lesser-known combined injectable contraceptive).
All Injectable Estradiol Preparations Together
Figure 8 shows the curve fits for all of the injectable estradiol preparations scaled to a single dose of 5 mg (or equivalent) together in the same figure. The dose for polyestradiol phosphate was scaled to be about 6.5 times higher than the other injectable estradiol preparations in order to make it roughly equivalent to them in terms of total estradiol exposure. This was because polyestradiol phosphate was found to produce much lower area-under-the-curve estradiol levels than the other injectable estradiol preparations (see the Discussion section). Estradiol undecylate was not included in Figure 8 as a decent fit curve could not be obtained for it due to the very limited data available for this preparation.
Figure 8: Curve fits of published estradiol concentration–time data with different injectable estradiol preparations by intramuscular injection scaled to equivalent doses and plotted over a period of 20 days in a single combined graph. Polyestradiol phosphate is scaled to a 6.5-fold higher dose that is roughly equivalent to that for the other esters as it gave total estradiol levels that were around 6 or 7 times lower than the other esters at the same dose. An alternative version of this figure without estradiol benzoate and with the x-axis spanning 30 days is also provided (Graph).
Figure 9 shows simulated curves at steady state for repeated administration of all of the injectable estradiol preparations scaled to a dose of 5 mg (or equivalent) once every 7 days. As with the previous figure, the dose for polyestradiol phosphate was scaled to be about 6.5 times higher than the other injectable estradiol preparations and estradiol undecylate was not included in the figure.
Figure 9: Simulated curves at steady state for repeated administration of different injectable estradiol preparations by intramuscular injection scaled to equivalent doses and plotted over three injection cycles. This simulation was based on the fit curves of the published single-dose estradiol concentration–time data reported in this meta-analysis. Polyestradiol phosphate is scaled to a 6.5-fold higher dose that is roughly equivalent to that for the other esters as it gave total estradiol levels that were around 6 or 7 times lower than the other esters at the same dose. An alternative version of this figure without estradiol benzoate is also provided (Graph).
For more simulated estradiol concentration–time curves with repeated injections of these injectable estradiol preparations, please see the accompanying interactive web simulator.
Selected Pharmacokinetic Parameters
The table below shows selected pharmacokinetic parameters for the fit curves of the included injectable estradiol preparations (Table 9). Estradiol undecylate was not included in the table due to the lack of data needed to achieve a decent curve fit for this preparation and the uncertainty of its parameters.
Table 9: Selected pharmacokinetic parameters for estradiol with injectable estradiol preparations following a single 5 mg dose by intramuscular injection:
Estradiol preparation
Tmax (d)
Cmax (pg/mL)
t1/2 (d)
t90% (d)
AUC0–∞ (pg•d/mL)
Estradiol benzoate in oil
0.65
971
1.2
3.9
2410
Estradiol valerate in oil
2.1
295
3.0
9.9
1886
Estradiol cypionate oil
4.3
155
6.7
22.3
2150
Estradiol cypionate suspension
1.2
241
5.1
16.9
2096
Estradiol enanthate in oil
6.5
160
4.6
15.1
2183
Polyestradiol phosphate a
18.0
34
28.4
94.2
2117
a Scaled instead to a single 32.5 mg injection (6.5 times higher dose than with the other esters).
The table below shows selected pharmacokinetic parameters for simulated curves at steady state with repeated administration of the included injectable estradiol preparations (Table 10). As with the previous table, estradiol undecylate was not included.
Table 10: Selected pharmacokinetic parameters for estradiol with injectable estradiol preparations with simulated repeated administration of 5 mg once every 7 days by intramuscular injection:
Estradiol preparation
Tmax (d)
Cmax (pg/mL)
Cmin (pg/mL)
Peak–trough diff. (pg/mL)
Peak–trough ratio
Cavg (pg/mL)
Estradiol benzoate in oil
0.64
990
29
962
35
344
Estradiol valerate in oil
1.9
384
142
242
2.7
269
Estradiol cypionate oil
3.1
339
262
77
1.3
307
Estradiol cypionate suspension
1.0
404
189
214
2.1
299
Estradiol enanthate in oil
4.0
329
288
41
1.1
312
Polyestradiol phosphate a
3.2
304
299
5
1.0
302
a Scaled instead to repeated injections of 32.5 mg every 7 days (6.5 times higher dose than with the other esters).
Terminal half-life (t1/2) is the time for the concentration of estradiol to decrease by 50% after pseudo-equilibrium of distribution has been reached—not the time required for half of an administered dose of the estradiol ester to be eliminated (Toutain & Bousquet-Mélou, 2004). It is calculated using only the terminal portion of a concentration–time curve, without the absorption or distribution phases influencing it (Toutain & Bousquet-Mélou, 2004). Due to flip–flop kinetics with depot injectables and the very short blood half-life of estradiol (~0.5–2 hours), what is being described by the terminal half-life in the case of depot estradiol injectables is not actually elimination of estradiol from blood but rather is the absorption of estradiol from the injection-site depot (Toutain & Bousquet-Mélou, 2004; Yáñez et al., 2011).
Discussion
Data Quality, Limitations, and Variability Between Studies
The accuracies of the curve fits for the different included injectable estradiol preparations are limited by the available data for these preparations. The quantity and quality of data are variable among these preparations. In some cases, such as with estradiol valerate in oil and estradiol cypionate in suspension, the data are overall quite good. In other instances, such as with estradiol cypionate in oil and estradiol enanthate in oil, the available data are more limited. There was undersampling of certain parts of the concentration–time curve with some preparations, for instance estradiol benzoate in oil (the early curve), estradiol enanthate in oil (much of the curve), and polyestradiol phosphate (the late curve). In the case of estradiol undecylate in oil, the available data for this preparation weren’t adequate to achieve a decent curve fit at all. The fit curves and calculated pharmacokinetic parameters of the included injectable estradiol preparations should be interpreted with the imperfect data in mind. For example, the curve shapes and pharmacokinetic parameters for the different preparations should not be taken as precise determinations in most cases but instead as rough estimates that would no doubt change with more and better data. Indeed, the fits and pharmacokinetic parameters were often noticeably sensitive to the influences of individual studies. Modeling decisions, such as the choice of pharmacokinetic model, or whether to fit directly to the combined processed data versus to the fits of individual studies, also yielded significantly different curve fits as well as calculated pharmacokinetic parameters.
Due to scarcity of data for several injectable estradiol preparations, the study selection criteria maximized data inclusion in order to allow for better curve fits at the risk of including potentially less reliable data. As examples, studies were included regardless of the status of the HPG axis of the participants, and Cmax data were included in the fitting if data were very limited. In the case of HPG axis state, studies with cycling women may result in greater error due to more variable levels of endogenous estradiol. Moreover, acute high levels of estradiol can induce a surge in luteinizing hormone levels after several days in gonadally intact women, and this may cause a delayed bump in estradiol levels (Wiki). One of the more overt instances of this can be seen in a study of estradiol benzoate in such women (Shaw, 1978 [Graph]). Many if not most of the included studies with estradiol benzoate involved women with intact HPG axes, whereas studies of this sort were uncommon with the other preparations. In the case of Cmax data, these data when Cmax corresponds to the mean of individual peaks are a different type of data than the peak of the mean curve of all individuals. Cmax levels can differ in both magnitude and timing compared to the mean curve peak (e.g., Oriowo et al., 1980 [Graph]; Rahimy, Ryan, & Hopkins, 1999). This is because for instance not all individuals peak at the same time and this variability in time to peak normally serves to dilute peak levels for the mean curve when compared to individual maximal concentrations. However, Cmax levels are in any case generally in the vicinity of the mean curve peak. While Cmax levels were excluded in the fitting for most injectable estradiol preparations, they were included in the case of estradiol enanthate. This was because the available mean and individual estradiol curve data were very limited for this specific preparation, and inclusion of Cmax data allowed for improved fitting in spite of its limitations. Lastly, some of the included data was once-monthly multi-dose, and research with once-monthly estradiol enanthate-containing combined injectable contraceptives has found that the time to peak levels may shift with repeated long-term use (Schiavon et al., 1988; Garza-Flores, 1994).
There was considerable variability between studies in terms of estradiol levels and concentration–time curve shapes with the same injectable estradiol preparation. The reasons for the large variability across studies are not fully clear. In any case, there are many potential factors that may contribute to this variability. These include preparation- and injection-related factors like formulation (e.g., oil vehicle, other components and excipients, concentration, particle size), injection volume, site of injection (e.g., buttocks, thigh, upper arm), injection technique (e.g., force of injection—and resulting depot droplet dimensions), and syringe dead space. They additionally include various subject- and research-related variables like differing blood-testing methodology, differing sample characteristics (e.g., age, weight, gender, ethnicity, physical activity, HPG axis state), and sampling error (Sinkula, 1978; Chien, 1981; Minto et al., 1997; Larsen & Larsen, 2009; Larsen et al., 2009; Florence, 2010; Larsen, Thing, & Larsen, 2012; Kalicharan, 2017). Older studies, which used potentially less accurate blood tests and tended to have smaller numbers of subjects, seemed to particularly add to the variability between studies. These studies may represent less reliable data than more recent research with larger sample sizes. The exclusion criteria helped to remove outliers for the different injectable estradiol preparations however. This meta-analysis does not take into account the potential factors underlying the variability between studies. To do so would be difficult, as in many cases information on these variables is not provided in individual studies and research quantifying their precise influences and relative importances is limited.
It is in any case known from other studies that different oil vehicles are absorbed at different rates from the injection site (Svendsen & Aaes‐Jørgensen, 1979; Schultz et al., 1998; Larsen et al., 2001) and can result in different concentration–time curve shapes (Ballard, 1978 [Excerpt]; Knudsen, Hansen, & Larsen, 1985). This is thought to be due to differences in oil lipophilicity and depot release rates. Viscosity of oils has also been hypothesized to potentially influence rate of depot escape (Schug, Donath, & Blume, 2012). However, research so far has not supported this hypothesis (Larsen & Larsen, 2009; Larsen, Thing, & Larsen, 2012). Oil vehicles can vary with injectable estradiol preparations even for the same estradiol ester. For instance, pharmaceutical estradiol valerate is formulated in sesame oil, castor oil, or sunflower oil depending on the preparation (Table). It is notable however that these three oils have similar lipophilicities (Table). On the other hand, homebrewed injectable estradiol preparations used by DIY transfeminine people often employ medium-chain triglyceride (MCT) oil as the oil vehicle. This oil (in the proprietary form of Viscoleo) has notably been found to be much more rapidly absorbed than conventional oils like sesame oil and castor oil in animals (Svendsen & Aaes‐Jørgensen, 1979; Schultz et al., 1998; Larsen et al., 2001). In addition, although based on very limited data, MCT oil has been found to give spikier and shorter-lasting depot injectable curves in humans (Knudsen, Hansen, & Larsen, 1985). As such, injectable estradiol preparations using MCT oil as the vehicle may have differing and less favorable concentration–time curve shapes than pharmaceutical injectable estradiol products. Other excipients, like benzyl alcohol, as well as factors like injection site and volume, have additionally been found to influence pharmacokinetic properties with depot injectables (Minto et al., 1997; Kalicharan, Schot, & Vromans, 2016). Excipients besides oil vehicle also vary by formulation (Table).
An implication of the variability between studies is that there is not a single estradiol concentration–time curve for a given injectable estradiol preparation but rather there are many, with these curves determined by variables such as formulation, dose/administration, and subject characteristics, among others. Hence, the curve fits determined in this meta-analysis represent only an estimation of the most typical and hence likely case, but the true curve for a preparation in a given context may be quite different.
Fitting all studies for a given injectable estradiol preparation individually first, and then fitting the fits of these studies, allowed for improved curve fits relative to directly fitting all of the combined processed original data for the preparation. The latter approach has limitations in that it has the effect of inherently weighting individual studies by quantity of time points (resulting in studies with greater time sampling having greater influence on the fit). Additionally, and more problematically, this approach can lead to distortions in curve shape due to different studies sampling different portions of the curve to differing extents in conjunction with systematic differences in curves between these studies. These are problems that fitting the fits of individual studies instead can solve. However, it is not possible to fit all individual studies, as some studies have limited time sampling and curve characterization which precludes fitting them appropriately. Cmax data are an example of this, which on their own cannot be fit properly. As such, it was not possible to fit the fits of the individual studies for all injectable estradiol preparations. Consequently, the fitting approach in this regard was not the same across esters, with some fit instead directly to the combined processed original data (e.g., estradiol enanthate, polyestradiol phosphate).
In spite of the various limitations of this work, aggregated analysis and modeling with injectable estradiol preparations has not previously been done. This informal meta-analysis provides among the most detailed insight into estradiol levels and curve shapes with these preparations available to date.
Durations and Curve Shapes
The curve shapes of non-polymeric injectable estradiol esters in oil relate strongly to lipophilicity. The more lipophilic the ester, the lower the peak levels and the more protracted the estradiol concentration–time curve. Accordingly, estradiol benzoate, one of the least lipophilic estradiol esters, has one of the spikiest curves and shortest durations, whereas more lipophilic estradiol esters, like estradiol cypionate in oil and estradiol enanthate, have comparatively flatter curves with delayed peaks and longer durations.
Duration of Estradiol Valerate
The estradiol concentration–time curve for injectable estradiol valerate in the well-known Oriowo et al. (1980) [Graph] study is notably spikier and shorter-lasting than the overall curve for estradiol valerate in this meta-analysis. On the other hand, the overall curve for injectable estradiol valerate in this meta-analysis was similar to (and considerably influenced by) the curves from several relatively recent and presumably better-quality studies of this injectable estradiol ester (e.g., Göretzlehner et al., 2002; Valle Alvarez, 2011; Schug, Donath, & Blume, 2012). It’s noteworthy that Oriowo et al. (1980) used a peanut oil-based formulation of estradiol valerate that differed from pharmaceutical injectable estradiol valerate preparations, which generally use sesame oil or castor oil as the carrier (as well as other excipients) (Table). This may have influenced the curve shape of estradiol valerate in Oriowo et al. (1980). The study also had a small sample size relative to the more recent studies (n=9 versus n=17, n=32, and n=24×2, respectively). Based on the newer and overall data, estradiol valerate appears to have a curve that is noticeably flatter and more prolonged than that suggested by Oriowo et al. (1980).
Duration of Estradiol Cypionate in Oil versus Estradiol Enanthate
Available estradiol concentration–time data for injectable estradiol cypionate in oil and estradiol enanthate in oil are more limited than with several of the other injectable estradiol preparations, and no direct comparisons of these two preparations exist at present. Based on some of the available literature on these injectable estradiol esters, most notably discussion by Oriowo et al. (1980) and a review of the pharmacokinetics of combined injectable contraceptives (Garza-Flores, 1994 [Graph]), it seemed that the duration of estradiol enanthate in oil was longer than that of estradiol cypionate in oil. However, this was based on limited research from separate and hence indirectly comparative studies of these esters. The estradiol cypionate in oil data from the relevant Garza-Flores (1994) figure was based on Oriowo et al. (1980) [Graph], and there are reasons to be cautious about relying on these data alone. The main concern is that curve shapes with the same injectable estradiol preparation can vary considerably across studies, as the present meta-analysis has shown. The reasons for this have yet to be fully clarified as already discussed, but among other factors may include varying formulations across studies of the same injectable estradiol ester. It is notable in this regard that Oriowo et al. (1980) used a formulation of estradiol cypionate that differs from conventional pharmaceutical estradiol cypionate in oil preparations—specifically, the study used a peanut oil-based formulation (with few other specifics) rather than the cottonseed oil-based preparation employed in marketed pharmaceutical formulations (Table). The study also had a somewhat small sample size (n=10) and may have had significant sampling error. Hence, single studies, perhaps particularly Oriowo et al. (1980), should be interpreted cautiously.
A small but interesting pharmacokinetic study which directly compared injectable testosterone cypionate (n=6) and testosterone enanthate (n=6) both in oil is relevant to the topic in question. This study found that equivalent doses of these testosterone esters using otherwise identical formulations produced virtually identical testosterone concentration–time curves (Schulte-Beerbühl & Nieschlag, 1980 [Graph]). The findings of this study are consistent with the fact that the lipophilicities of testosterone cypionate and testosterone enanthate (as measured by predicted log P) are very similar when directly compared (e.g., 5.1 vs. 5.11 with ALOGPS, 6.29 vs. 6.11 with ChemAxon logP, and 6.4 vs. 6.3 with XLogP3, respectively (Table). This of course is of importance as lipophilicity is thought to be the key factor determining the release kinetics of oil-based depot injectables (Sinkula, 1978; Shah, 2007; Larsen & Larsen, 2009; Larsen, Thing, & Larsen, 2012; Shahiwala, Mehta, & Momin, 2018). Analogously similar lipophilicities can be seen when comparing estradiol cypionate and estradiol enanthate, which employ the same ester moieties (e.g., predicted log P values of 6.47 vs. 6.45 with ALOGPS and 7.1 vs. 7.0 with XLogP3, respectively) (Table). Hence, on a theoretical level, injectable estradiol cypionate and estradiol enanthate, like injectable testosterone cypionate and testosterone enanthate, might be expected to produce very similar curves—at least provided all other variables, such as formulation, are held constant.
The present meta-analysis found that the overall estradiol curve for estradiol cypionate in oil was significantly less spikey and more prolonged than that observed in Oriowo et al. (1980). It is noteworthy in this regard that all of the other studies included for estradiol cypionate in oil specifically employed pharmaceutical Depo-Estradiol and that the overall curve for this preparation appears to be more consistent with its licensed injection interval for use in menopausal hormone therapy (1–5 mg once every 3–4 weeks) (Depo-Estradiol Label). Moreover, this meta-analysis found that injectable estradiol cypionate in oil and estradiol enanthate in oil had fairly similar and comparably flat and prolonged estradiol concentration–time curves. However, estradiol cypionate in oil appeared to peak earlier than estradiol enanthate, while estradiol enanthate was eliminated more rapidly than estradiol cypionate in oil in the terminal portion of the curve. In any case, the available concentration–time data for these preparations are limited, and the present work is not able to determine whether these estradiol esters have truly differing pharmacokinetic properties, as the apparent differences between the curves for these preparations may simply be due to statistical error. Taken together, estradiol cypionate in oil may have a less spikey and longer-lasting curve than that implied by Oriowo et al. (1980), and estradiol cypionate in oil and estradiol enanthate may have more similar curves than has been previously assumed.
Curve Shape of Estradiol Cypionate Suspension
While estradiol cypionate as an aqueous suspension is a relatively long-lasting injectable estradiol preparation similarly to estradiol cypionate in oil and estradiol enanthate in oil, it seems to differ in the shape of its estradiol concentration–time curve from these preparations. Estradiol cypionate as a suspension has a curve that appears to peak significantly earlier than estradiol cypionate in oil and other longer-acting oil-based injectable estradiol preparations. This might relate to the differing mechanisms of depot action and unique properties of injectable aqueous suspensions (Aly, 2019). In line with this notion, injectable medroxyprogesterone acetate suspension (Depo-Provera) also appears to peak rapidly despite having a very long duration (longer durations tending to be associated with delayed peaks in the case of oil-based depot injectables) (Graphs). Although aqueous suspensions generally last longer than oil solutions as injectables (Enever et al., 1983; Aly, 2019), this is not always the case, and estradiol cypionate suspension interestingly seems to be shorter-acting than estradiol cypionate in oil.
Estradiol Exposure and Potency
The average estradiol levels with the non-polymeric injectable estradiol esters when scaled to a dose and dosing interval of 5 mg every 7 days were around 300 pg/mL (~1,100 pmol/L). For comparison, in premenopausal cisgender women, estradiol production is on average about 200 μg/day (or 6 mg per month/cycle) and mean estradiol levels are around 100 pg/mL (~370 pmol/L) (Aly, 2019). After adjusting for the molecular weight of the ester, the estradiol levels for a given dose of non-polymeric injectable estradiol esters are in fairly close agreement with the estradiol levels for an equal quantity of estradiol produced endogenously by the ovaries in premenopausal cisgender women (very roughly around 1.2 mg estradiol per 7 days for injectable estradiol esters and 1.4 mg estradiol per 7 days for ovarian production to achieve average integrated estradiol levels of around 100 pg/mL). The preceding is in accordance with the fact that injectable estradiol valerate has been reported to have approximately 100% bioavailability (with this being less characterized but likely also the case for the other non-polymeric injectable estradiol esters) (Düsterberg & Nishino, 1982; Seibert & Günzel, 1994).
Although non-polymeric injectable estradiol esters have differing estradiol concentration–time curve shapes, they all appear to achieve fairly similar area-under-the-curve levels of estradiol when compared to one another. This is in accordance with the fact that differences in molecular weight and hence estradiol content with the different estradiol esters are fairly minor (all of the assessed non-polymeric esters range from 62 to 76% of that of estradiol in terms of estradiol content, and all but estradiol undecylate are in the range of 69 to 76%) (Table). The appearance of differences in area-under-the-curve levels of estradiol in the present meta-analysis is probably just due to statistical error, and true differences cannot be established by this meta-analysis. An implication of the similar area-under-the-curve estradiol levels with the different non-polymeric injectable estradiol esters is that these preparations can all be expected to deliver a roughly comparable amount of estradiol for the same dose.
On the other hand, the polymeric ester polyestradiol phosphate appears to produce around 6- to 7-fold lower area-under-the-curve and average estradiol levels than non-polymeric estradiol esters. This suggests that the estradiol in polyestradiol phosphate is not 100% bioavailable, and is supported by the fact that this ester is used clinically at substantially higher dosages than other injectable estradiol esters (40–320 mg/month), even for the same indications such as menopausal hormone therapy and treatment of prostate cancer (Wiki; Estradurin Labels). This does not seem to have been previously described in the literature, and the reasons for it are unknown. It seems possible that polyestradiol phosphate may be partially excreted before it can be cleaved into estradiol and thereby rendered partly inactive, in turn necessitating the use of higher doses to achieve the same estradiol levels and therapeutic effect.
Although two given injectable estradiol preparations may produce equivalent total estradiol levels, this does not necessarily mean that they will always have the same estrogenic potency (i.e., strength of effect at a given dose). It is plausible that spikier estradiol concentration–time curves, like with estradiol benzoate, may have overall lower estrogenic potency than more steady curves, like with estradiol enanthate. This is because estrogen receptors for a given tissue should become saturated at a certain point due to the finite quantity of available receptors in the tissue. As a result, high peak estradiol levels with spikier curves may effectively be “wasted” to varying extents in different tissues. On the other hand, more spikey estradiol curves, due to higher peak estradiol levels, might have greater influence on tissues that require high estradiol levels for effect such as the liver (and by extension on coagulation and associated health risks) (Aly, 2020). However, these possibilities are speculative and theoretical. Although some literature exists that is relevant to this issue (e.g., Parkes, 1937; Bradbury, Long, & Durham, 1953), there is very little research in this area. Consequently, it is not currently possible to take into account time-related variations in estradiol levels or differing estradiol curve shapes when assessing the comparative estrogenic potency between injectable estradiol preparations (or between other estradiol forms/routes). It is also noteworthy that these variations depend on injection interval and may be reduced with shorter injection intervals that maintain steadier estradiol levels, which must also be considered.
Variability Between Individuals
There is substantial variation in total estradiol levels and curve shapes between people with the same injectable estradiol preparation. Indicators of interindividual variability such as standard deviation or 95% range have not been included in this meta-analysis at this time due to the large amount of additional time and work this would require (e.g., additional extraction of error bars from all studies and analysis). In any case, individual studies that were included show this marked interindividual variation (e.g., Oriowo et al., 1980; Derra, 1981 [Graph]; Aedo et al., 1985 [Graphs]; Sang et al., 1987 [Graphs]; Rahimy & Ryan, 1999 [Graph]; Valle Alvarez, 2011 [Graph]; Schug, Donath, & Blume, 2012 [Graphs]). Highly variable estradiol levels are already well-established with oral and transdermal estradiol (Kuhl, 2005; Wiki). Less variability might be expected with non-polymeric injectable estradiol esters since these preparations appear to have approximately complete bioavailability. However, it seems that even with injectable forms of estradiol, the variability between people is still quite substantial. An implication of this is that the appropriate dose and dosing interval of an injectable estradiol formulation for a given person will vary considerably. This emphasizes the importance of blood work to ensure that injectable estradiol preparations are neither overdosed—which can increase health risks such as blood clots (Aly, 2020)—nor underdosed—which may result in suboptimal testosterone suppression and therapeutic efficacy.
Insights for Clinical Guidelines and Dosing Recommendations
Clinical guidelines for transgender health (see also Aly (2020)) provide recommendations on doses and dosing intervals of injectable estradiol valerate in oil and estradiol cypionate in oil (Table 11). Dosing recommendations are not given for other injectable estradiol preparations, which are much less commonly used in transgender medicine. The recommended doses for estradiol valerate and estradiol cypionate vary widely depending on the guidelines, whereas the recommended intervals are consistently once every 1 to 2 weeks. The doses for estradiol valerate range from 2 to 20 mg/week or 5 to 80 mg/2 weeks and the doses for estradiol cypionate range from <1 to 10 mg/week or <2 to 80 mg/2 weeks. For reference, the Endocrine Society guidelines and the University of California, San Francisco (UCSF) guidelines are the most major clinical guidelines for transgender hormone therapy at present (Aly, 2020). The Endocrine Society guidelines recommend 5 to 30 mg/2 weeks or 2 to 10 mg/week for either estradiol valerate or estradiol cypionate (Hembree et al., 2017). Conversely, the UCSF guidelines recommend <20 to 40 mg/2 weeks for estradiol valerate and <2 to 5 mg/2 weeks for estradiol cypionate (with the option to divide dose into weekly injections if cyclical side effects occur) (Deutsch, 2016a).
Table 11: Recommended doses and injection intervals of injectable estradiol preparations (specifically estradiol valerate and estradiol cypionate) in transgender medicine clinical guidelinesa:
a Several other guidelines recommend doses and intervals that appear to be taken directly from the Endocrine Society or UCSF guidelines and thus are not listed here but can be found elsewhere (Aly, 2020).
A number of concerns arise when the doses and intervals of injectable estradiol valerate and estradiol cypionate recommended by the major transgender clinical guidelines are considered in the context of the present informal meta-analysis and when they are compared between guidelines. Based on the present work, dosages of injectable preparations recommended by the major transgender clinical guidelines appear to result in estradiol exposure that is markedly higher than that with the recommended dosages for other routes and forms of estradiol (e.g., oral or transdermal). Whereas a dosage of 5 mg/week of any non-polymeric injectable estradiol ester appears to give average estradiol levels of around 300 pg/mL (~1,100 pmol/L), which are already supraphysiological, doses of injectable estradiol valerate or estradiol cypionate recommended by guidelines are as high as 15 to 20 mg per week. The average estradiol concentrations that would be expected to result from such doses per this meta-analysis (e.g., ~600–1,200 pg/mL or 2,200–4,400 pmol/L at 10–20 mg/week) (Figure 10) would vastly exceed the ranges for estradiol levels in transfeminine people advised by the same guidelines (generally about 50–200 pg/mL or ~180–730 pmol/L) (Table). This is not merely theoretical; for example, a study that used 40 mg/week estradiol valerate by intramuscular injection in cisgender women with estrogen deficiency to produce “pseudopregnancy” reported measured estradiol levels of about 2,500 pg/mL (~9,200 pmol/L) at 3 months and 3,100 pg/mL (~11,400 pmol/L) at 6 months of treatment (Ulrich, Pfeifer, & Lauritzen, 1994). Moreover, highly supraphysiological estradiol levels with guideline-based injectable estradiol doses are not unexpected when normal production of estradiol in premenopausal cisgender women is considered (~1.4 mg per week or 6 mg per month/cycle giving mean estradiol levels of ~100 pg/mL or 370 pmol/L) (Aly, 2019). Clinical safety data on high doses of injectable estradiol esters like estradiol valerate and estradiol cypionate are lacking at present, but excessive estrogenic exposure is known to increase the risk of health complications such as blood clots (Aly, 2020). The very high doses of these preparations that are recommended by guidelines should raise considerable reservations about their safety.
Figure 10: Simulated estradiol levels with injectable estradiol valerate at the doses and interval (5–40 mg/2 weeks) preferentially recommended by current major transgender care guidelines. Steady-state estradiol levels are reached by about the second or third injection with this injection interval and levels do not further accumulate. An alternative version of this figure with half-doses at a once-weekly interval (i.e., 2.5–20 mg/week) is also provided (Graph).
The present author elsewhere has listed doses of injectable estradiol preparations that are roughly comparable in terms of total estradiol exposure to doses for other estradiol forms and routes used in transfeminine people (Aly, 2020). These doses range from about 1 to 6 mg per week for “low dose” to “very high dose” therapy with non-polymeric injectable estradiol esters (Graph). This dose range for injectable estradiol is likely to be more appropriate for use in transfeminine people than current recommendations by many guidelines. Although high estradiol levels can be useful in transfeminine hormone therapy when antiandrogens are not used due to their greater efficacy than physiological levels in terms of testosterone suppression, only modestly supraphysiological estradiol levels (e.g., ~200–300 pg/mL or 730–1,100 pmol/L) appear to be required for strong testosterone suppression (Aly, 2019; Langley et al., 2021; Aly, 2020). In relation to this, doses of injectable estradiol need not be excessive.
Some guidelines, such as the Endocrine Society guidelines, recommend the same doses and intervals for both estradiol valerate and estradiol cypionate, whereas other guidelines, such as the UCSF guidelines, recommend different doses for these two injectable estradiol esters. Concerningly, the doses for estradiol valerate and estradiol cypionate recommended by the UCSF guidelines differ by roughly an order of magnitude (<20 to 40 mg/2 weeks for estradiol valerate and <2 to 5 mg/2 weeks for estradiol cypionate). These estradiol esters appear to produce similar average estradiol levels (e.g., around 300 pg/mL or 1,100 pmol/L at a dosage of 5 mg/week) and have concentration–time curve shapes that are not extremely different, with estradiol cypionate being only somewhat flatter and more prolonged than estradiol valerate. As such, it would appear that similar doses should be appropriate for these esters. This is supported by the fact that the same doses of estradiol valerate and estradiol cypionate are used in combined injectable contraceptives in cisgender women (both 5 mg once per month) and that these doses were carefully determined during an intensive clinical development programme for these preparations (Garza-Flores, 1994; Newton, d’Arcangues, & Hall, 1994; Sang, 1994; Toppozada, 1994). This programme notably included dose-ranging and direct-comparison studies. Based on the present analysis, the current recommendations by the UCSF guidelines may result in marked overdosage in the case of estradiol valerate and potential underdosage in the case of estradiol cypionate.
Transgender health guidelines recommend an injection interval for estradiol valerate and estradiol cypionate in oil of once every 1 to 2 weeks. Although an injection interval of 2 weeks seems technically feasible in the case of both of these preparations, such an interval would appear to result in substantial fluctuations in estradiol levels, with high peak levels and low troughs. This is particularly true in the case of the shorter-acting estradiol valerate (Figures 10, 11). Considering the wide fluctuations and unknown effects of this variability, as well as the fact that testosterone suppression when applicable may depend on sustained higher estradiol levels, it may be advisable that a once-weekly interval be preferentially recommended for these preparations. This would achieve steadier estradiol levels and would reduce potential problems due to high or low estradiol levels (Figure 11). Alternatively, a shorter interval of once every 5 days may be used with estradiol valerate to further reduce the variability in estradiol levels that occurs with this preparation (Figure 11). On the other hand, an injection interval of once every 10 days to 2 weeks may be practical and allowable in the case of the longer-acting estradiol cypionate in oil (as well as estradiol enanthate) (Figure 11)—provided that the injection cycles are well-tolerated and testosterone suppression remains adequate. When selecting different injection intervals, doses should be scaled by the interval to maintain equivalent total estradiol exposure (e.g., 3.5 mg/5 days, 5 mg/7 days, 7 mg/10 days, or 10 mg/14 days for high-dose non-polymeric injectable estradiol esters).
Figure 11: Simulated estradiol levels with a high dosage of injectable estradiol valerate or estradiol cypionate in oil at different injection intervals (doses scaled by interval to be equivalent in total estradiol exposure).
With the preceding concerns about the doses and intervals of injectable estradiol preparations recommended by transgender care guidelines considered, the question of how these recommendations were determined arises. Unfortunately, current guidelines do not generally describe how they arrived at their recommendations nor do they usually cite sources to support them. It is notable that the UCSF guidelines recommend doses and intervals for injectable estradiol preparations that are nearly identical to those advised by Christian Hamburger and Harry Benjamin in the late 1960s in the first medical textbook on transgender people (Hamburger & Benjamin, 1969). These authors recommended a dose of 10–40 mg/2 weeks for estradiol valerate and of 2–5 mg/2 weeks for estradiol cypionate (although Benjamin additionally stated that after 4–8 months, the same doses could be used at a longer injection interval of once every 4 weeks). These recommendations were notably made before estradiol blood tests became practicably available and were prior to the advent of modern pharmacokinetic studies. Hence, the recommendations for at least these guidelines appear to be based mainly on past expert opinion and long-standing historical precedent rather than on pharmacokinetic or clinical data. The same is likely to also be true for most other guidelines. High doses with certain injectable estradiol preparations (namely estradiol valerate) were probably originally employed for the purpose of achieving longer durations and more convenient injection intervals. This was notably prior to the risks of excessive estrogenic exposure like blood clots becoming known, and these doses may simply have never been revised.
Among the surveyed guidelines for transgender hormone therapy, only the UCSF guidelines (Deutsch, 2016b) and the Sherbourne Health/Rainbow Health Ontario guidelines (Bourns, 2019) referenced pharmacokinetic literature in their discussion of injectable estradiol. The specific publications cited by these guidelines were Düsterberg & Nishino (1982), Sierra-Ramírez et al. (2011), and Thurman et al. (2013). Although it is favorable to see guidelines considering published pharmacokinetic data for informing use of these preparations, there are a few concerns about the studies that were cited. Düsterberg & Nishino (1982) in its study of injectable estradiol valerate had a very small sample size (n=2), and this study was excluded as an outlier in the present meta-analysis due to unusually high estradiol levels. The findings of Düsterberg & Nishino (1982) also do not seem to have actually been used to guide dosing recommendations in the case of the UCSF guidelines, since if this were the case, the recommended doses should have been much lower. On the other hand, Bourns (2019) cited the same study and recommended injectable estradiol valerate at doses of 3–4 mg/week or 6–8 mg/2 weeks. These doses are well below those recommended by other transgender care guidelines and appear to be more appropriate for use in transfeminine people in light of the present meta-analysis. Sierra-Ramírez et al. (2011) and Thurman et al. (2013), although better-quality studies than Düsterberg & Nishino (1982), described injectable estradiol cypionate suspension rather than estradiol cypionate in oil. The oil-based version of estradiol cypionate is the form normally used in transfeminine hormone therapy, and there are important differences between these estradiol cypionate preparations such that pharmacokinetic studies for the suspension can’t necessarily be generalized to the oil solution. These preparations do in any case produce similar total estradiol levels however and hence doses should be comparable for them.
This meta-analysis is only informal and unpublished research. Nonetheless, based on the results of this work and the preceding discussion, current dosing recommendations for injectable estradiol preparations by most transgender clinical guidelines appear to be highly excessive and likely unsafe, with injection intervals that may additionally be too widely spaced. Transgender care guidelines should consider reassessing these recommendations, and the transgender medical community should make an effort to better characterize the pharmacokinetics and optimal dosing schemes of injectable estradiol preparations in transfeminine people in the future. Since clinical data on these preparations are scarce and will probably remain so in the near-term, use of published pharmacokinetic data may be further considered for guiding dosing recommendations for injectable estradiol. As identified and catalogued by this meta-analysis, there is a wealth of existing data that could be used to better inform transgender care guidelines in terms of the use of injectable estradiol preparations in transfeminine people.
Interactive Web Simulator
This informal meta-analysis of estradiol concentration–time data with injectable estradiol preparations was conducted for the purpose of deriving accurate and representative estradiol curves for incorporation into a web-based injectable estradiol simulator intended for use by transfeminine people and their clinicians. This web app is able to simulate both single-injection curves and repeated-injection curves with these preparations. An informational page for this simulator can be found at the following location:
There are various possibilities for further work on this project in the future. For example, assessment of interindividual variability for estradiol levels with injectable estradiol preparations could be included in the meta-analysis. As another example, it would be fairly straightforward and valuable to expand the meta-analysis as well as simulator to other hormonal preparations such as injectable testosterone preparations and other estradiol routes and forms like oral estradiol, sublingual estradiol, and estradiol pellets. Pharmacokinetic literature for some of these preparations has already been collected by this author. However, these future possibilities would require much additional time and effort to complete.
Special Thanks
A special thank you to Violet and Lila for their indispensable input and guidance on modeling topics during the work on this project. An additional thanks to Violet for deriving a special three-compartment pharmacokinetic model that was used in this work. Please also check out Violet’s own projects Tilia—an effort to empower trans people with tools to manage their hormonal transitions—and TransKit—a work-in-progress pharmacokinetic simulation library specifically tailored for transgender hormone therapy. Lastly, thank you to all the peer reviewers who carefully reviewed this article prior to it being posted.
Updates
Update 1: WPATH SOC8 Guidelines
In September 2022, the World Professional Association for Transgender Health (WPATH) Standards of Care for the Health of Transgender and Gender Diverse People Version 8 (SOC8) were published and made recommendations on transgender hormone therapy for the first time (Coleman et al., 2022). These guidelines are among the most highly regarded and consulted transgender care guidelines. In terms of the recommended doses of hormonal medications for transgender people, the WPATH SOC8 appear to have largely copied the Endocrine Society’s 2017 guidelines on transgender hormone therapy (Hembree et al., 2017). More specifically, in the case of injectable estradiol preparations for transfeminine people, doses of 5–30 mg/2 weeks or 2–10 mg/week estradiol valerate or estradiol cypionate were recommended. There was no discussion of injectable estradiol in the guidelines besides the preceding doses and intervals being included in a table, and no literature citations were included to support these doses. As described in the present work, these recommendations include doses and intervals that appear to be highly excessive, too widely spaced, and are likely unsafe. As such, major transgender care guidelines unfortunately continue to make uncited recommendations for injectable estradiol that are out of step with insights available from abundant published pharmacokinetic data. These recommendations are likely inadvisable, with the possibility of substantial health risks.
Update 2: Literature Mentions
The following publications in the literature have cited or mentioned Transfeminine Science’s injectable estradiol simulator and/or meta-analysis since the project was published in mid-2021:
Hughes et al. (2022)
Hughes, J. H., Woo, K. H., Keizer, R. J., & Goswami, S. (2022). Clinical Decision Support for Precision Dosing: Opportunities for Enhanced Equity and Inclusion in Health Care. Clinical Pharmacology & Therapeutics, 113(3), 565–574. [DOI:10.1002/cpt.2799]:
Lastly, we recommend that developers of [clinical decision support software (CDSS)] for dosing take an iterative and participatory approach to designing systems. By involving stakeholders in the design process, they will develop solutions that best suit users’ needs and are more likely to be adopted and used correctly. This participatory approach should involve interviews and usability testing with clinicians. Formal usability testing and analysis with real end users can improve the quality and usefulness of a system.88 Though patients themselves are not typically the end users of CDSS, their expertise (especially that of marginalized groups and organized patient advocacy organizations) can also inform CDSS developers. As an example, transgender people have compiled their own resources to understanding dosing regimens in the absence of clear clinical guidelines.89 Developers of CDSS could provide a great deal of value to these patient populations, and improve their software’s utility, by working with them to understand their needs from a dosing tool.
89. Aly, W. An interactive web simulator for estradiol levels with injectable estradiol esters. Transfeminine Science <https://transfemscience.org/articles/injectable-e2-simulator-release/> (2021) Accessed November 1, 2022.
Jaafar et al. (2022)
Jaafar, S., Torres-Leguizamon, M., Duplessy, C., & Stambolis-Ruhstorfer, M. (2022). Hormonothérapie injectable et réduction des risques: pratiques, difficultés, santé des personnes trans en France. [Hormone replacement therapy injections and harm reduction: practices, difficulties, and transgender people’s health in France.] Sante Publique, 34(HS2), 109–122. [Google Scholar] [PubMed] [DOI:10.3917/spub.hs2.0109] [Translated]:
With regard to feminizing [substitutive hormone therapy (HS)], there are no specialty injectables based on estrogens in the French pharmacopoeia. This makes it impossible to set up estrogen monotherapies which require high dosages that are more difficult to obtain with specialties with other galenic forms [5]. Faced with this lack of care, some trans women and transfeminine people obtain estradiol-based injectable solutions on the Internet or through other sources [6]. […]
5. Aly. An informal meta-analysis of estradiol curves with injectable estradiol preparations [Internet]. Transfem Sci. 2021 July 16. [Visited on 29/12/2022]. Online : https://transfemscience.org/articles/injectable-e2-meta-analysis/.
Linet (2023)
Linet, T. (2023). Prise en charge endocrinologique d’une personne trans. [Endocrinological care of a trans person.] In Faucher, P., Hassoun, D., & Linet, T. (Eds.). Santé sexuelle et reproductive des personnes LGBT [Sexual and Reproductive Health of LGBT People] (pp. 109–124). Issy-les-Moulineaux, France: Elsevier Masson. [Google Books] [URL] [WorldCat] [Excerpt] [Translated]:
Choice of estrogen.
Estradiol is generally the most prescribed estrogen. It is given orally or sublingually in transfeminine people with no significant cardiovascular risk factors. For others, the percutaneous form (patches, gel) is recommended.
The starting dose is 2 mg of estradiol orally with a step increase of 2 mg every 2 to 3 months until the optimal dose is reached [1]. For the patches, the initial dosage and the increments are 50 or 100 μg, and for the gel 2.5 g. This means that the optimal dose is generally 6 to 8 mg per day for the oral route, 3 to 4 mg for the sublingual route, and 300 to 400 μg for the patches (see table 11.1).
It may happen in consultation that the person does not wish to use the prescribed estrogens and wishes to continue the self-prescription of injectable estrogens. It is then possible to evaluate with them the most suitable dosage using the Transfem Science Injection Simulator (https://transfemscience.org/misc/injectable-e2-simulator/). Risk prevention related to injections (needles) can be done. Associations can help the person find 25 G needles of 40 mm useful this type of treatment.
Rothman et al. (2024)
Rothman, M. S., Ariel, D., Kelley, C., Hamnvik, O. R., Abramowitz, J., Irwig, M. S., Soe, K., Davidge-Pitts, C., Misakian, A. L., Safer, J. D., & Iwamoto, S. J. (2024). The Use of Injectable Estradiol in Transgender and Gender Diverse Adults: A Scoping Review of Dose and Serum Estradiol Levels. Endocrine Practice, 30(9), 870–878. [DOI:10.1016/j.eprac.2024.05.008]:
In recent years, we have noted trends in our clinical practices with TGD adults requesting injectable estradiol, particularly in the United States. The reasons given can vary; it may be due to ease of weekly or every two weeks administration, fatigue of taking daily oral medications and skin reactions to or cost of transdermal preparations. There have been discussions as to the roles of estrone/estradiol ratios in feminization and whether injectable estradiol might lead to more favorable results, however research has not supported a role for estrone in optimizing feminizing outcomes [13]. There is also a belief that higher levels can be attained with injections and may lead to faster and more complete feminization; however, there is a lack of data in the literature to support these conclusions. Such conversations occurring on reddit.com and even some hormone provider websites, are perhaps related to the historical use of high dose injectable estradiol noted above [14]. However, there is a paucity of data to guide clinicians on what dose, type and at what interval estradiol esters should be injected and when levels should be measured to ensure physiologic range estradiol levels. In fact, recent reports and clinical observations have raised concerns that the dosing suggested in guidelines may result in supraphysiological estradiol levels and that higher doses and levels may put patients at elevated risk of thromboembolic events [15-18]. This scoping review examines the available data on levels achieved with various dosages of estradiol injections in TGD adults. We also report on testosterone suppression, route (i.e., SC vs. IM), and type of estradiol ester as well as timing of blood draw relative to dose, where available.
Acknowledgment
[…] [We] thank Aly from Transfemscience for community representation and correspondence.
Toffoli Ribeiro, C., Gois, Í., da Rosa Borges, M., Ferreira, L. G. A., Brandão Vasco, M., Ferreira, J. G., Maia, T. C., & Dias-da-Silva, M. R. (2024). Assessment of parenteral estradiol and dihydroxyprogesterone use among other feminizing regimens for transgender women: insights on satisfaction with breast development from community-based healthcare services. Annals of Medicine, 56(1), 2406458. [DOI:10.1080/07853890.2024.2406458]:
Utilizing a previously published meta-analysis method of estradiol concentration-time data from publicly available information on cisgender women who had used EEn or EEn/DHPA [17], we reanalyzed and integrated data from various studies. […]
[…] The V3C Fitter and Desmos tools, accessible online at https://alyw234237.github.io/injectable-e2-simulator/v3c-fitter/ and https://www.desmos.com/calculator/ndgvp2avhj?lang=pt-BR respectively, were utilized for fitting the three-compartment pharmacokinetic model. […]
Pharmacokinetics of injectable estradiol enanthate
[…] The boxplot graph (Figure 5) illustrates that the median estradiol levels in trans women using EEn/DHPA fell within this population’s expected average range values (100–200pg/mL).
Figure 5. Meta-analysis of estradiol concentration-time data from cisgender women under EEn alone or EEn/DHPA. Fitted data curves from various studies individually and combined into a single-dose curve over 30 days were generated based on an informal meta-analysis of published estradiol concentration-time data from cisgender women under EEn or EEn/DHPA [17]. […]
References
[17] Aly. 2021. An informal meta-analysis of estradiol curves with injectable estradiol preparations. Transfeminine Sci. https:// transfemscience.org/articles/injectable-e2-meta-analysis/
Update 3: Herndon et al. (2023)
In March 2023, the following study on injectable estradiol in transfeminine people was published online:
Herndon, J. S., Maheshwari, A. K., Nippoldt, T. B., Carlson, S. J., Davidge-Pitts, C. J., & Chang, A. Y. (2023). Comparison of Subcutaneous and Intramuscular Estradiol Regimens as part of Gender-Affirming Hormone Therapy. Endocrine Practice, 29(5), 356–361. [DOI:10.1016/j.eprac.2023.02.006]
The study was a retrospective analysis of individualized injectable estradiol in adult transfeminine people who received hormone therapy at the Mayo Clinic. Doses of injectable estradiol were adjusted by clinical providers based on estradiol levels, testosterone suppression, and feminization goals, and subsequently these clinical data were retrospectively studied by Mayo Clinic researchers. The primary aim of the study was to compare injectable estradiol by intramuscular versus subcutaneous routes. However, other general considerations for injectable estradiol, such as dosing, estradiol levels, testosterone suppression, type of injectable estradiol ester (estradiol valerate vs. estradiol cypionate), and estradiol monotherapy versus concomitant use of antiandrogens, were also assessed. The paper noted that the study was the largest to assess injectable estradiol in transfeminine people to date and was the first to directly compare intramuscular and subcutaneous injectable estradiol routes in transfeminine people.
Injectable estradiol doses were adjusted to achieve estradiol and testosterone levels within therapeutic ranges defined by the Endocrine Society 2017 guidelines (>100 pg/mL [367 pg/mL] for estradiol and <50 ng/dL [<1.7 nmol/L] for testosterone). Estradiol levels were measured exclusively using liquid chromatography–tandem mass spectrometry (LC–MS/MS), while the assay method for measuring testosterone levels was not specified. In terms of when in the injection cycle estradiol levels were measured, the authors stated the following: (1) “Timing of estradiol blood draw in relation to injection was not protocolized” and (2) “In our practice, although estradiol concentrations were generally checked midway through the injection cycle, we were unable to document with certainty the timing of the estradiol lab testing which may have influenced the results and/or the outliers”. Only the most recent blood test for each individual was analyzed, with the results of earlier tests discarded. Doses were analyzed in per-week amounts, regardless of dosing frequency (either once weekly or once every two weeks).
There were a total of 130 transfeminine people on injectable estradiol who were analyzed in the study. Of these individuals, 56 received intramuscular (i.m.) injections and 74 received subcutaneous (s.c.) injections. The median duration of therapy for injectable estradiol was 3.0 years for both routes. The vast majority of people used weekly injections (91.1% for i.m., 98.6% for s.c.), whereas the small remainder (n=6) injected once every 2 weeks. Similarly, the great majority used injectable estradiol valerate (89.3% for i.m., 86.5% for s.c.), while a small subset (n=16) used injectable estradiol cypionate. The authors did not state the injectable vehicles, but they can be confidently assumed to have both been in oil. The treatment-individualized doses per week of injectable estradiol were median 4 mg (interquartile range (IQR) 3–5.15 mg; range 1–8 mg) for the i.m. route and median 3.75 mg (IQR 3–4 mg; range 1–8 mg) for the s.c. route, with the differences in doses between groups being slightly but significantly different (p = 0.005). For the small number of people on two-week injection cycles, the doses for the combined i.m. and s.c. groups were median 8.5 mg (range 6–16 mg) every 2 weeks. Estradiol levels with injectable estradiol were median 189.5 pg/mL (IQR 126.8–252.5 or 122.5–257 pg/mL; range ~33–575 pg/mL] for i.m. and median 196 pg/mL (IQR 125.3–298.5 pg/mL; range ~23–581 pg/mL) for s.c., with the differences between groups not being significantly different (p = 0.70). The percentages of individuals with estradiol levels in target range (>100 pg/mL) were 78.6% for i.m. and 82.4% for s.c. The estradiol doses and levels of individual patients for each route were also provided in the paper (Graph). It can be seen that more individuals clustered into the higher range of doses with i.m. than with s.c. injections.
In the case of estradiol valerate versus estradiol cypionate, dose per week for i.m. with estradiol valerate was median 4 mg (IQR 3–5.45 mg) and with estradiol cypionate was median 4 mg (IQR 2.25–5 mg). In the case of s.c., dose per week with estradiol valerate was median 4 mg (IQR 3–4 mg) and with estradiol cypionate was median 3 mg (IQR 2–3 mg). The doses between estradiol valerate and estradiol cypionate were not significantly different in the case of i.m. (p = 0.51), but were significantly different in the case of s.c. (p = 0.025). Estradiol levels with the two different injectable estradiol esters were not provided.
On multiple regression analysis, injectable estradiol dose was significantly positively associated with estradiol levels (p < 0.001) following adjustment for several variables (injection route, body mass index (BMI), antiandrogen use, gonadectomy status). Each 1 mg per week in dose was associated with estradiol levels that were increased by (estimate ± standard error) 57.42 ± 10.46 pg/mL. No other variable, including notably BMI, was significantly associated with estradiol levels. According to the authors, the dose-dependently higher estradiol levels with injectable estradiol suggested the need to start at lower doses that are gradually increased in conjunction with close monitoring of hormone levels.
Testosterone levels in those with gonads were 11 ng/dL (IQR 0–19.8 ng/dL) for i.m. and 11 ng/dL (0–20 ng/dL) for s.c., with levels not significantly different between groups (p = 0.92). Adequate testosterone suppression (<50 ng/dL) in those with gonads was achieved in 84.6% with i.m. and 86.6% with s.c. In the small subset of individuals on injections every two weeks (n=6), 100% of individuals achieved target estradiol and testosterone levels. A majority of individuals on injectable estradiol in the study concomitantly used an antiandrogen, with this usually being spironolactone or finasteride. In a minority of individuals, injectable estradiol monotherapy, without concomitant use of an antiandrogen, was employed and hormone levels were measured (n=17). In this subgroup, estradiol levels were median 220 pg/mL (IQR 180–264 pg/mL) at a dose per week of median 4 mg (IQR 3–6 mg), with estradiol levels noticeably higher than in the overall group. In terms of hormone levels for those on injectable estradiol monotherapy, 100% achieved therapeutic estradiol levels (>100 pg/mL) and 88.2% achieved target testosterone levels (<50 ng/dL). The authors noted that most individuals on injectable estradiol monotherapy were able to adequately suppress testosterone, but that higher doses and levels of estradiol may be needed for testosterone suppression in this context.
Herndon et al. (2023) noted that existing recommendations for injectable estradiol in transfeminine people suggest doses of 2 to 10 mg per week or 5 to 30 mg every 2 weeks, referencing the Endocrine Society 2017 guidelines (Hembree et al., 2017) and UCSF 2016 guidelines (Deutsch, 2016a). They also noted that the UCSF 2016 guidelines recommended lower doses of estradiol cypionate than estradiol valerate, which they attributed to pharmacokinetic differences between the esters (citing Oriowo et al. (1980) for this claim). However, the authors noted that the differences between estradiol valerate and estradiol cypionate doses they observed were small, and questioned the clinical relevance of the differences. The authors also tactfully critiqued dosing recommendations by existing guidelines, and suggested their own data to guide dosing instead, with the following relevant excerpts:
Prior studies used for development of guidelines for parenteral doses are suboptimal given their small sample sizes or pre-specificized [gender-affirming hormone therapy (GAHT)] protocols with no adjustment of estradiol doses or no information on hormone concentrations achieved. [Discussion of Deutsch, Bhakri, & Kubicek (2015) and Mueller et al. (2011) …]
Overall, the studies used to support the current dosing recommendation guidelines for parenteral estradiol dosing are limited, incomplete with regards to hormone concentrations achieved, and do not provide SC as an available option. The doses of estradiol used in this study (with either SC or IM approach), were successful in achieving serum estradiol concentrations at the cisgender female range. Most importantly, as compared to current available guidelines and consensus statements [1, 2], these doses of estradiol valerate are less than half of what is recommended for both initial and maintenance dosing and achieved suppression of testosterone.
Lower doses of parenteral injections than previously described in clinical practice guidelines achieved therapeutic estradiol concentrations. Our data can serve as a dosing guide for initial and maintenance use of parenteral estradiol, which is different than what has been previously described.
Herndon et al. (2023) concluded that injectable estradiol by both i.m. and s.c. routes is effective in achieving therapeutic estradiol levels in transfeminine people. They noted that there were not meaningful differences between i.m. and s.c. in terms of dose, although i.m. may require slightly higher doses than s.c. to achieve therapeutic estradiol levels. The authors stated that doses of injectable estradiol to achieve therapeutic estradiol levels in transfeminine people were lower than previously recommended by guidelines and other publications. They concluded that clinical use of injectable estradiol in transfeminine people should include discussion of both i.m. and s.c. routes and invidiualization by patient. Finally, they called for more clinical studies on injectable estradiol in transfeminine people to evaluate clinical outcomes, feminization, and additional risks and benefits of this route compared to other routes.
The findings of Herndon et al. (2023) are pleasingly consistent with the results of the present meta-analysis. Based on the findings of this meta-analysis, assuming a linear relationship between dose and estradiol levels, estradiol levels with non-polymeric injectable estradiol esters, like estradiol valerate and estradiol cypionate in oil via intramuscular injection, increase by around 60 pg/mL on average for each 1 mg per week in dose (with Herndon et al. (2023) finding a value of 57 pg/mL per 1 mg using a multiple linear regression model). In relation to this, mean integrated estradiol levels of around 250 pg/mL on average would be expected at a dosage of 4 mg once per week. Accordingly, Herndon et al. (2023) found median estradiol levels of 190 to 196 pg/mL at per-week median doses of 3.75 to 4 mg. Similarly, the present work recommended injectable estradiol doses with non-polymeric esters of 1 to 6 mg per week (to achieve mean integrated estradiol levels of roughly 50–300 pg/mL), which is comparable to the range of about 2 to 6 mg per week used in most transfeminine people in Herndon et al. (2023) (to achieve estradiol levels of at least 100 pg/mL, along with adequate testosterone suppression). Additionally, it was noted in this meta-analysis—based on clinical research in cisgender men with prostate cancer—that only modestly supraphysiological estradiol levels, of no more than approximately 200 to 300 pg/mL, are likely to be needed for strong testosterone suppression in transfeminine people. This has likewise been confirmed with solid clinical data in transfeminine people by Herndon et al. (2023), with 88% of those on injectable estradiol monotherapy having testosterone levels of <50 ng/dL at a median injectable estradiol dose of 4 mg/week and with median estradiol levels of 220 pg/mL. It is the opinion of the present author that Herndon et al. (2023) is a very important and formative study, with clinical implications that go far beyond merely supporting the s.c. use of injectable estradiol. The study represents the first major step in the published literature to correcting the dosing and intervals of injectable estradiol in transgender care guidelines and in transgender health generally. I commend the researchers for their work.
Update 4: Rothman et al. (2024a) and Rothman et al. (2024b)
In February 2024, the following short review on injectable estradiol dosing in transfeminine people by Micol Rothman and colleagues was published online:
Rothman, M. S., Hamnvik, O. P. R., Davidge-Pitts, C., Safer, J. D., Ariel, D., Tangpricha, V., Abramowitz, J., Soe, K., Sarvaideo, J., Kelley, C., Irwig, M. S., & Iwamoto, S. J. (2024). Revisiting Injectable Estrogen Dosing Recommendations for Gender-Affirming Hormone Therapy. Transgender Health, 9(6), 463–465. [DOI:10.1089/trgh.2023.0209]
Here is the abstract of the paper:
Injectable estrogens are options for gender-affirming hormone therapy per guidelines, which suggest intramuscular dosages of 5–30 mg every 2 weeks or 2–10 mg weekly with estradiol cypionate or valerate interchangeably. Data among transgender and gender-diverse patients are limited due to local unavailability and concerns around laboratory assay variability and estradiol (E2) level fluctuation. We note a concerning trend where patients are prescribed high-dose injections based on the guidelines leading to serum E2 levels well above the range recommended in the same guidelines. Our review indicates that 5 mg weekly or lower should be prescribed when initiating injectable estrogens to avoid supraphysiologic E2 levels.
Then, in May 2024, the following longer and more comprehensive review on injectable estradiol dosing in transfeminine people by Rothman and most of the same other academics was published online:
Rothman, M. S., Ariel, D., Kelley, C., Hamnvik, O. R., Abramowitz, J., Irwig, M. S., Soe, K., Davidge-Pitts, C., Misakian, A. L., Safer, J. D., & Iwamoto, S. J. (2024). The Use of Injectable Estradiol in Transgender and Gender Diverse Adults: A Scoping Review of Dose and Serum Estradiol Levels. Endocrine Practice, 30(9), 870–878. [DOI:10.1016/j.eprac.2024.05.008]
Here is the abstract of this paper:
Objective: Feminizing gender-affirming hormone therapy is the mainstay of treatment for many transgender and gender diverse people. Injectable estradiol preparations are recommended by the World Professional Association for Transgender Health Standards of Care 8 and the Endocrine Society guidelines. Many patients prefer this route of administration, but few studies have rigorously assessed optimal dosing or route.
Methods: We performed a scoping review of the available data on estradiol levels achieved with various dosages of estradiol injections in transgender and gender diverse adults on feminizing gender-affirming hormone therapy. We also report on testosterone suppression, route (ie, subcutaneous vs intramuscular), and type of injectable estradiol ester as well as timing of blood draw relative to the most recent dose, where available.
Results: The data we reviewed suggest that the current guidelines, which recommend starting doses 2 to 10 mg weekly or 5 to 30 mg every 2 weeks of estradiol cypionate or valerate, are too high and likely lead to patients having supraphysiologic levels across much of their injection cycle.
Conclusions: The optimal starting dose for injectable estradiol remains unclear and whether it should differ for cypionate and valerate. Based on the data available, we suggest that clinicians start injectable estradiol cypionate or valerate via subcutaneous or intramuscular injections at a dose ≤5 mg weekly and then titrate accordingly to keep levels within guideline-recommended range. Future studies should assess timing of injections and subsequent levels more precisely across the injection cycle and between esters.
This paper notably also cited the present Transfeminine Science article as raising concerns about guideline-based dosing for injectable estradiol and potential health complications from these doses.
Aside from Micol Rothman herself, these reviews were also authored by other well-known experts in transgender health. For instance, two of the coauthors, Joshua Safer and Michael Irwig, were authors for the WPATH SOC8 hormone therapy chapter (WPATH SOC8 Full Contributor List). Additionally, Safer was one of the authors for the Endocrine Society’s transgender hormone therapy guidelines (Hembree et al., 2017). As such, it would appear that transgender medicine has finally started to seriously correct injectable estradiol dosing. This is a very important development. Now, the appropriate dosing and intervals of injectable estradiol will need to be more precisely established and the corrections will need to make their way into updated transgender hormone therapy guidelines and general clinical practice.
A letter to the editor commented on and concorded with Rothman and colleagues’ second literature review:
Patel, K. T., & Tangpricha, V. (2024). Parenteral Estradiol for Transgender Women: Time to adjust the dose. Endocrine Practice, 30(9), 893–894. [DOI:10.1016/j.eprac.2024.07.005]
Update 5: Kariyawasam et al. (2024)
In March 2024, the following study of estradiol levels with different routes of estradiol in transfeminine people, including injectable estradiol, was published:
Kariyawasam, N. M., Ahmad, T., Sarma, S., & Fung, R. (2024). Comparison of Estrone/Estradiol Ratio and Levels in Transfeminine Individuals on Different Routes of Estradiol. Transgender Health, ahead of print. [DOI:10.1089/trgh.2023.0138]
The study stratified injectable estradiol doses into different dosing levels, accounted for timing of blood draws, and compared injectable estradiol to other estradiol routes. The other routes included oral estradiol, sublingual estradiol, and transdermal estradiol. The form of injectable estradiol used was estradiol valerate in dose groups including ≤4 mg/week (“low-dose”), >4 mg/week to ≤8 mg/week (“medium-dose”), and >8 mg/week (“high-dose”). In the study, this injectable estradiol regimen resulted in supraphysiological estradiol levels in the medium- to high-dose groups (>4 mg/week) and dramatically higher estradiol levels than with the other estradiol routes (Data). Median estradiol levels were reported in a subsequent paper as follows: “Figure 2 from the paper shows estradiol levels across the 3 groups. Although exact numbers are not given in this figure, we learned through correspondence with the authors that the low dose injection group [n=8] had a median level of 202.7 ± SD 232.6 pg/mL, the medium group [n=22] 465.2 ± SD 466.3 pg/mL, and the high group [n=3] 574.4 ± SD147.3 pg/mL (converted from SI units)” (Rothman et al., 2024b). Although the sample sizes for the different dose groups were small, this study, along with Herndon et al. (2023), provides some of the best clinical data on estradiol levels with injectable estradiol in transfeminine people that have so far been published.
Update 6: Patel et al. (2024)
In June 2024, the following open-access review discussing injectable estradiol in transfeminine people and calling for updated transgender health guidelines was published:
Patel, R., Korenman, S., Weimer, A., & Grock, S. (2024). A Call for Updates to Hormone Therapy Guidelines for Gender-Diverse Adults Assigned Male at Birth. Cureus, 16(6), e62262. [DOI:10.7759/cureus.62262] [PDF]
The following quote is the relevant excerpt on injectable estradiol from the review:
The current guideline-based dosing recommendations for estradiol vary considerably, which is problematic for clinicians and patients who rely on guidelines to initiate treatment. Most notably, the conversion rates between parenteral estradiol valerate and estradiol cypionate vary drastically between the UCSF Guidelines for the Primary and Gender-Affirming Care of Transgender and Gender Nonbinary People (UCSF Guidelines) and The Endocrine Society Clinical Practice Guidelines for Endocrine Treatment of Gender-Dysphoric/Gender-Incongruent Persons (the Endocrine Society Guidelines). The UCSF Guidelines indicate the conversion between estradiol valerate and cypionate to be as high as a 4:1 ratio [2], while the Endocrine Society Guidelines provide no dosing differentiations [1]. Herndon and colleagues demonstrated that the conversion between estradiol cypionate and estradiol valerate is closer to 1:1 [4]. Further equivalence studies are needed to clarify ideal dosing conversions.
The Endocrine Society Guidelines recommend titrating estradiol to 100-200 pg/mL [1]. The UCSF Guidelines recommend 2-4 mg daily as the starting dose for oral estradiol and 5 mg weekly for parenteral estradiol valerate [2]. The Endocrine Society Guidelines suggest oral estradiol 2-6 mg daily and parenteral estradiol 2- 10 mg weekly [1]. However, Chantrapanichkul et al. found that intramuscular injections of estradiol valerate greater than 5 mg weekly led to mean estradiol concentrations well above 200 pg/mL, while 4-5 mg of oral estradiol daily only led to minimum desired concentrations [5]. Similarly, Herndon et al. found that subcutaneous estradiol at a median dose of 3.75 mg per week led to a median estradiol level of 196 pg/mL [4]. Thus, current guideline-based dosing may lead providers to choose doses of injectable estradiol that would result in supratherapeutic serum estradiol levels. In light of these recent publications, it is clear that guideline-based dosing for estradiol needs updating. In our clinical experience, parenteral estradiol valerate at doses of 2-4 mg weekly typically leads to physiologic estradiol levels. Estradiol cypionate should likely be dosed in a 1:1 ratio with estradiol valerate until future data are obtained.
Lastly, while estradiol valerate and cypionate are only FDA-approved for intramuscular administration, many patients prefer subcutaneous administration. There are small studies that suggest the pharmacokinetics of intramuscular and subcutaneous estradiol are similar [4]. While the UCSF Guidelines comment on the use of subcutaneous estradiol, other guidelines should be updated to include this option for patients [2].
Update 7: Toffoli Ribeiro et al. (2024)
Toffoli Ribeiro, C., Gois, Í., da Rosa Borges, M., Ferreira, L. G. A., Brandão Vasco, M., Ferreira, J. G., Maia, T. C., & Dias-da-Silva, M. R. (2024). Assessment of parenteral estradiol and dihydroxyprogesterone use among other feminizing regimens for transgender women: insights on satisfaction with breast development from community-based healthcare services. Annals of Medicine, 56(1), 2406458. [DOI:10.1080/07853890.2024.2406458]:
This study examines the effects of a commonly used injectable hormone combination, specifically estradiol enanthate with dihydroxyprogesterone acetophenide (EEn/DHPA), […] Our research focused on a retrospective longitudinal study involving a large cohort of transwomen evaluated between 2020 and 2022, comprising 101 participants. We assessed serum levels of estradiol (E2), testosterone (T), luteinizing hormone (LH), and follicle-stimulating hormone (FSH), comparing the EEn/DHPA hormonal regimen with other combined estrogen-progestogen (CEP) therapies. […] Our findings indicated that participants using the EEn/DHPA regimen exhibited significantly higher serum E2 levels (mean: 186 pg/mL ± 32 pg/mL) than those using other therapies (62 ± 7 pg/mL), along with lower FSH levels, but no significant differences in T and LH levels. […] These results suggest that an injectable, low-cost EEn/DHPA administered every three weeks could serve as an alternative feminizing regimen, particularly considering the extensive long-term experience of the local transgender community. Further longitudinal studies on the efficacy of feminizing-body effects and endovascular risks of various parenteral CEP types are warranted to improve primary healthcare provision for transgender persons.
Introduction
Injectable combined estrogens with progestogens (CEP) have long been widely used in Brazil and other Latin American countries, predominantly among ciswomen as an injectable contraceptive and by Brazilian transgender women and travestis as GAHT [8]. Despite the absence of recognition by the Endocrine Society as an alternative hormonal regimen due to concerns regarding thrombogenicity and challenges in routine monitoring through blood testing, the prevalent use of CEP necessitates evaluating its regimen recommendations. This has led our research to delve deeper into understanding CEP regimens, considering the experiences of travestis amidst distinct sociocultural lifestyles and limited access to public endocrinological care services [15,16]. Hence, our objective is to elucidate our observations in monitoring trans individuals utilizing CEP regimens by evaluating hormone levels […] within a cohort of transwomen employing the most common injectable CEP, namely estradiol enanthate with dihydroxyprogesterone acetophenide (EEn/DHPA) and comparing these observations with other GAHT regimens.
Subjects and methods
Estradiol enanthate pharmacokinetics curve
Utilizing a previously published meta-analysis method of estradiol concentration-time data from publicly available information on cisgender women who had used EEn or EEn/DHPA [17], we reanalyzed and integrated data from various studies. A unified single-dose curve for 30 days was created. We employed least squares regression for studies with four or more concentration-time data points (solid lines). We manually adjusted other studies with three data points to fit into a single-dose curve.
Each study’s data were adjusted for baseline estradiol levels or endogenous estradiol production and then normalized by 10 mg. The V3C Fitter and Desmos tools, accessible online at https://alyw234237.github.io/injectable-e2-simulator/v3c-fitter/ and https://www.desmos.com/calculator/ndgvp2avhj?lang=pt-BR respectively, were utilized for fitting the three-compartment pharmacokinetic model. Estradiol levels from transgender women on EEn/DHPA in this study were presented using a box plot graph featuring percentiles at 10, 25, 50, 75, and 90.
Results
Hormonal levels during the follow-up of feminizing regimens
Scatter plot graphs depicted the measurement of sex hormones (Figure 2). Serum estradiol levels in the EEn/ DHPA group (mean: 186.4pg/mL ± 32.8pg/mL) were significantly higher than those in the group using other therapies (62.2±6.9pg/mL) (Figure 2(A)). Within the EEn/DHPA group, serum FSH levels were significantly lower compared to the other group (Others) (Figure 2(B)). However, no significant difference was found between the groups concerning testosterone (Figure 2(C)) and LH (Figure 2(D)) levels.
Pharmacokinetics of injectable estradiol enanthate
Serum estradiol levels in trans women using EEn/DHPA reached the target levels for this population during hormone therapy, a trend not observed in participants using other feminizing hormone therapies (Table 1). The boxplot graph (Figure 5) illustrates that the median estradiol levels in trans women using EEn/DHPA fell within this population’s expected average range values (100–200pg/mL).
Figure 5. Meta-analysis of estradiol concentration-time data from cisgender women under EEn alone or EEn/DHPA. Fitted data curves from various studies individually and combined into a single-dose curve over 30 days were generated based on an informal meta-analysis of published estradiol concentration-time data from cisgender women under EEn or EEn/DHPA [17]. For studies with four or more concentration-time data points (solid lines) and the fit of combined data (thick black line), least squares regression to a three-compartment pharmacokinetic model was employed. A single-dose curve was manually adjusted for studies with three data points (dashed lines). Data from each study were adjusted for endogenous estradiol production via baseline or trough estradiol levels subtraction and normalized by 10mg. The graph illustrates estradiol levels from the transwoman cohort in a boxplot. The shaded area represents the optimal target range for estradiol levels in transwomen under hormone therapy. The boxplot graph displays the percentiles 10, 25, 50, 75, and 90 for estradiol levels of transwomen under EEn/DHPA in this study (N=53).
Discussion
Our study represents a pioneering contribution to the literature by demonstrating that Brazilian trans women undergoing EEn/DHPA therapy achieved estradiol levels comparable to those observed during the follicular phase in cisgender women. […]
Our study further noted that DHPA demonstrates comparable efficacy to cyproterone or other anti-androgens in achieving optimal LH pituitary suppression and reducing testosterone levels. EEn/ DHPA, an affordable injectable contraceptive widely accessible in South American countries, presents a promising avenue for attaining target hormone levels among transfeminine individuals.
Additionally, our investigation, which reviewed pharmacokinetic data, supports the potential implementation of EEn/DHPA in a 21-day regimen to sustain optimal estradiol levels. While alternative medications exist to inhibit testosterone production and action, their availability varies based on regional healthcare provider systems. […]
EEn/DHPA, commonly used as a long-lasting injectable contraceptive [21–23], has found application in feminizing hormone therapy for transfeminine people, notably in travestis in Brazil [7,8,24,25]. […]
In conclusion, our long-term cohort study suggests that administering parenteral estradiol enanthate with dihydroxyprogesterone acetophenide every three weeks could serve as a practical option for feminizing hormone regimens in transgender women. Nonetheless, adopting an individualized approach that takes into account each individual’s goals, response to prior hormone therapies, and medical history is crucial. This personalized approach is central to improving healthcare provision and ensuring optimal outcomes in bodily changes. By continuing to explore and refine hormone therapy regimens, we can better support the health and well-being of transgender individuals on their gender-affirming journey.
References
[17] Aly. 2021. An informal meta-analysis of estradiol curves with injectable estradiol preparations. Transfeminine Sci. https:// transfemscience.org/articles/injectable-e2-meta-analysis/
Update 8: Misakian et al. (2025)
Misakian, A. L., Kelley, C. E., Sullivan, E. A., Chang, J. J., Singh, G., Kokosa, S., Avila, J., Cooper, H., Liang, J. W., Botzheim, B., Quint, M., Jeevananthan, A., Chi, E., Harmer, M., Hiatt, L., Kowalewski, M., Steinberg, B., Tausinga, T., Tanner, H., Ho, T. F., … Ariel, D. (2025). Injectable Estradiol Use in Transgender and Gender-Diverse Individuals throughout the United States. The Journal of Clinical Endocrinology & Metabolism, dgaf015. [DOI:10.1210/clinem/dgaf015]:
Context: Guidelines for use of injectable estradiol esters (valerate [EV] and cypionate [EC]) among transgender and gender-diverse (TGD) individuals designated male at birth vary considerably, with many providers noting supraphysiologic serum estradiol concentrations based on current dosing recommendations.
Objectives: This work aimed to 1. determine the dose of injectable estradiol (subcutaneous [SC] and intramuscular [IM]) needed to reach guideline-recommended estradiol concentrations for TGD adults using EC/EV; 2. describe the relationship between estradiol concentration relative to timing/dose of last estradiol injection and other covariates; and 3. determine dosing differences between IM/SC EV/EC.
Methods: A cross-sectional retrospective study was conducted across 6 US medical centers including TGD adults on same-dose injectable estradiol for more than 75 days, with confirmed timing of estradiol concentration relative to last injection, from January 1, 2019 to December 31, 2023. Descriptive statistics were used to describe patient characteristics and weighted linear mixed models to evaluate relationship between various covariates and estradiol concentration.
Results: Data from 562 patients were included. Among those injecting every 7 days who reached the guideline-recommended estradiol concentration (n = 131, 27.5%), the median estradiol dose was 4.0 mg (interquartile range, 3.0-5.0 mg). Among all patients, the majority reached supraphysiologic estradiol concentrations (>200 pg/mL [>734 pmol/L]) while dose and timing in the injection cycle were significant covariates for the estradiol concentration. There were no significant dosing differences between IM/SC EV/EC.
Conclusion: Injectable estradiol esters effectively reach guideline-recommended estradiol concentrations but at lower doses than previously recommended. Estradiol concentrations are best interpreted relative to timing of last injection. Route of administration and type of ester do not significantly affect estradiol concentrations.
[…]
And a letter to the editor commenting on the paper:
Milano, C., & Harper, J. (2025). Comments on Injectable Estradiol Use in Transgender and Gender-Diverse Individuals in the US. The Journal of Clinical Endocrinology & Metabolism, dgaf134. [DOI:10.1210/clinem/dgaf134]
Update 9: Slack et al. (2025)
Slack, D. J., Di Via Ioschpe, A., Saturno, M., Kihuwa-Mani, S., Amakiri, U. O., Guerra, D., Karim, S., & Safer, J. D. (2025). Examining the Influence of the Route of Administration and Dose of Estradiol on Serum Estradiol and Testosterone Levels in Feminizing Gender-Affirming Hormone Therapy. Endocrine Practice, 31(1), 19–27. [DOI:10.1016/j.eprac.2024.10.002]:
Introduction: […] This study investigates the effect of route of administration (ROA) and dose of estradiol on estradiol (E2) and testosterone (T) levels in transfeminine individuals.
Methods: We conducted a chart review of 573 patients with an active prescription for estradiol for feminizing GAHT and serum hormone levels available.
Results: […] Intramuscular estradiol was associated with lower T and higher E2 compared to oral and transdermal ROAs (P < .001), with many achieving target hormone levels even at low doses.
Conclusions: […] The intramuscular ROA appears to be the most potent delivery of estradiol with impact on serum hormone levels with doses on the low end of guideline-suggested ranges.
[…]
Update 10: Carlson et al. (2025)
Carlson, S. M., Dominguez, C., Jeevananthan, A., & Crowley, M. J. (2025). Follow-Up Estradiol Levels Based on Regimen Formulation With Guideline-Concordant Gender-Affirming Hormone Therapy. Journal of the Endocrine Society, 9(3), bvae205. [DOI:10.1210/jendso/bvae205]:
Context: Endocrine Society guidelines for dosing of feminizing gender-affirming hormone therapy (GAHT) have remained essentially unchanged since 2009. The Endocrine Society recommends periodic monitoring of serum estradiol levels, with the goal of maintaining levels in the premenopausal cisgender female range (100-200 pg/mL). However, it is not clear whether guideline-concordant dosing consistently produces guideline-recommended levels across common estradiol formulation types (oral pills, parenteral injections, transdermal patches).
Objective: All transgender and nonbinary patients receiving estradiol-based GAHT between October 2015 and March 2023 were reviewed at a single center, with the goal of determining the frequency with which guideline-concordant dosing with different estradiol formulations led to guideline-recommended estradiol levels.
Methods: Demographics, GAHT regimen, and estradiol levels were obtained via chart review, and data were analyzed descriptively.
Results: The analytic population included n = 35 individuals, including n = 9 prescribed oral estradiol pills, n = 11 prescribed parenteral injections, and n = 15 prescribed transdermal patches. With guideline-concordant doses of oral estradiol (mean 2.8 mg daily), the mean follow-up level was 168 pg/mL; 32% of follow-up levels were subtherapeutic and 14% were supratherapeutic. With guideline-concordant doses of parenteral estradiol (mean 5.8 mg weekly), the mean midpoint follow-up level was 342 pg/mL; 91% of midpoint follow-up levels were supratherapeutic. With guideline-concordant doses of transdermal estradiol (mean 0.09 mg/day), the mean follow-up level was 81.5 pg/mL; 70% of follow-up levels were subtherapeutic.
Conclusion: Supratherapeutic follow-up estradiol levels were common with guideline-concordant parenteral estradiol doses, as were subtherapeutic follow-up levels with guideline-concordant transdermal doses. These findings may suggest the need for revision of guideline-recommended estradiol doses for these formulations
[…]
Update 11: Kanin et al. (2025)
Kanin, M., Slack, M., Patel, R., Chen, K. T., Jackson, N., Williams, K. C., & Grock, S. (2025). Injectable Estradiol Dosing Regimens in Transgender and Nonbinary Adults Listed as Male at Birth. Journal of the Endocrine Society, bvaf004. [DOI:10.1210/jendso/bvaf004]:
Context: Many transgender and nonbinary (TGNB) individuals assigned male at birth (AMAB) seek hormone therapy to achieve physical and emotional changes. Standard therapy includes estradiol, with or without an antiandrogen. Our clinical observations suggest that currently recommended injectable estradiol dosing may lead to supratherapeutic estradiol levels.
Objective: We sought to evaluate whether lower-than-recommended doses of injectable estradiol were effective in achieving serum estradiol and testosterone goals.
Methods: We conducted a retrospective cohort study to evaluate injectable estradiol dosing in treatment-naive AMAB individuals initiating hormone therapy. Data from a single provider at an academic center from January 2017 to March 2023 were analyzed. A total of 29 patients were eligible for inclusion. The primary variables of estradiol dosage, serum estradiol, and testosterone levels were analyzed over 15 months.
Results: The average estradiol dose decreased from 4.3 to 3.7 mg weekly (P < .001) during the study period with a final on-treatment estradiol level of 248 pg/mL. All individuals achieved a testosterone level of less than 50 ng/dL during the study period. The average initial on-treatment testosterone level was not significantly different from average final on-treatment measurement of 24.0 mg/dL (P = .95). […]
Conclusion: Lower doses of injectable estradiol can achieve therapeutic estradiol levels with excellent testosterone suppression. […]
[…]
This study had been previously published as a conference abstract:
Kanin, M., Slack, M., Patel, R., Chen, K. T., Jackson, N., Williams, K., & Grock, S. (2024). 8309 Injectable Estradiol Dosing Regimens; A Retrospective Review of Hormone Therapy for Gender-Diverse Adults Assigned Male at Birth. Journal of the Endocrine Society, 8(Suppl 1), bvae163-1706. [DOI:10.1210/jendso/bvae163.1706]
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-An Introduction to Hormone Therapy for Transfeminine People - Transfeminine Science
An Introduction to Hormone Therapy for Transfeminine People
By Aly | First published August 4, 2018 | Last modified December 17, 2025
Abstract / TL;DR
Sex hormones such as estrogen, testosterone, and progesterone are produced by the gonads. The sex hormones mediate the development of the secondary sexual characteristics. Testosterone causes masculinization, while estradiol causes feminization and breast development. Males have high amounts of testosterone, while females have low testosterone but high amounts of estradiol. These hormonal differences are responsible for the physical differences between males and females. Sex hormones and other hormonal medications are used in transfeminine people to shift the hormonal profile from a male-typical one to a female-typical profile. This causes feminization and demasculinization and allows for alleviation of gender dysphoria. The changes caused by transfeminine hormone therapy occur over a period of months to years. There are many different types and forms of hormonal medications, and these medications can be administered by a variety of different routes. Examples include as pills taken by mouth, as patches or gel applied to the skin, and as injections, among others. Different hormonal medications, routes, and doses have differences in efficacy, side effects, risks, costs, convenience, and availability. Hormone therapy should ideally be regularly monitored in transfeminine people with blood tests to ensure effectiveness and safety and to allow for adjustment as necessary.
The Sex Hormones
Types and Effects
The sex hormones include the estrogens (E), progestogens (P), and androgens. A person’s hormonal profile is a product of the type of gonads that they are born with. Natal males have testes while natal females have ovaries. Testes produce large amounts of androgens and small amounts of estrogens whereas ovaries produce high amounts of estrogens and progesterone and low amounts of androgens.
Progestogens have essentially no known role in feminization or pubertal breast development. Rather than acting as mediators of feminization, progestogens have important effects in the female reproductive system and are essential hormones during pregnancy (Wiki). They also oppose the actions of estrogens in certain parts of the body, such as the uterus, vagina, and breasts (Wiki).
In addition to their effects on the body, sex hormones have actions in the brain. These actions influence cognition, emotions, and behavior. For instance, androgens produce pronounced sexual desire and arousal (including spontaneous erections) in men, while estrogens appear to be the major hormones responsible for sexual desire in women (Cappelletti & Wallen, 2016). As another example, testosterone levels have been negatively associated with agreeableness, whereas estrogen levels have been positively associated with this characteristic (Treleaven et al., 2013). Sex hormones also have important effects on health, which can be both positive and negative. For instance, estrogens maintain bone strength and likely protect against heart disease in cisgender women (NAMS, 2022), but also increase the risk of breast cancer (Aly, 2020) and can increase the risk of blood clots (Aly, 2020).
Estrogens, progestogens, and androgens also have antigonadotropic effects. That is, they inhibit the gonadotropin-releasing hormone (GnRH)-induced secretion of the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), from the pituitary gland in the brain. The gonadotropins signal the gonads to make sex hormones and to supply the sperm and egg cells necessary for fertility. Hence, lower levels of the gonadotropins will result in reduced gonadal sex hormone production and diminished fertility. If gonadotropin levels are sufficiently suppressed, the gonads will no longer make sex hormones at all and fertility will cease. The vast majorities of the quantities of estradiol, testosterone, and progesterone in the body are produced by the gonads. Most of the small remaining amounts of these hormones are produced via the adrenal glands of the kidneys.
Normal Hormone Levels
In cisgender females, the sex hormones are largely absent during childhood, gradually ramp up in production in late childhood and adolescence, are present in a cyclical manner during adulthood, and then largely stop being produced following the menopause. Hormone levels vary substantially but in a predictable manner during the normal menstrual cycle in adult premenopausal women. The menstrual cycle lasts about 28 days on average and consists of the following parts:
Luteal phase—latter half of the cycle or days 14–28
Hormone levels during the menstrual cycle are shown in the following graph:
Figure 1: Median estradiol and progesterone levels throughout the menstrual cycle in premenopausal cisgender women (Stricker et al., 2006; Abbott, 2009). The horizontal dashed lines are the average levels over the spanned periods. Other figures available elsewhere show variation between individuals (Graph; Graph; Graph).
As can be seen in the graph, estradiol levels are relatively low and progesterone levels are very low during the follicular phase; estradiol but not progesterone levels briefly surge to very high levels and trigger ovulation during mid-cycle; and estradiol and progesterone levels both undergo a bump and are relatively high during the luteal phase (though estradiol is not as high as during the mid-cycle peak).
The table below shows the circulating levels and production rates of estradiol, progesterone, and testosterone in women and men and allows for comparison between them.
Table 1: Ranges for circulating levelsa and estimated production ratesb of the major sex hormones:
Mean integrated estradiol levels are around 100 pg/mL (367 pmol/L) in premenopausal women and around 25 pg/mL (92 pmol/L) in men. The 95% range for mean estradiol levels in women is around 50 to 250 pg/mL (180–918 pmol/L) (e.g., Abbott, 2009 (Graph); Verdonk et al., 2019 (Graph)). The average production of estradiol by the ovaries in premenopausal women is about 6 mg over the course of one menstrual cycle (i.e., one month) (Rosenfield et al., 2008). This corresponds to a mean rate of about 200 μg/day. Estradiol levels increase slowly during normal female puberty, when breast development and feminization take place. Mean estradiol levels during the different stages of female puberty are quite low—less than about 50 to 60 pg/mL (180–220 pmol/L) until late puberty (Aly, 2020). In postmenopausal women, whose ovaries no longer produce considerable quantities of estrogens, estradiol levels are generally less than 10 to 20 pg/mL (37–73 pmol/L) (Nakamoto, 2016). Estradiol levels below 50 pg/mL (184 pmol/L) in adults are concentration-dependently associated with menopausal symptoms, including hot flashes, depressive mood changes, defeminization (e.g., breast atrophy, loss of feminine fat distribution), accelerated skin aging, and bone density loss with increased risk of bone fracture.
Mean testosterone levels are around 30 ng/dL (1.0 nmol/L) in women and 600 ng/dL (21 nmol/L) in men. Based on these values, testosterone levels are on average about 20-fold higher in men than in women. In men who have undergone gonadectomy (castration or surgical gonadal removal), testosterone levels are similar to those in women (<50 ng/dL [1.7 nmol/L]) (Nishiyama, 2014; Getzenberg & Itty, 2020). The mean or median levels of testosterone in women with polycystic ovary syndrome (PCOS), who often have clinically significant symptoms of androgen excess (e.g., excessive facial/body hair growth), range from 41 to 75 ng/dL (1.4–2.6 nmol/L) per different studies (Balen et al., 1995; Steinberger et al., 1998; Legro et al., 2010; Loh et al., 2020). Hence, it appears that even testosterone levels that are marginally elevated relative to normal female levels may produce undesirable androgenic effects.
The goal of hormone therapy for transfeminine people, otherwise known as feminizing hormone therapy (FHT) or (more in the past) as male-to-female (MtF) hormone replacement therapy (HRT), is to produce feminization and demasculinization of the body as well as alleviation of gender dysphoria. Medication therapy with sex hormones and other sex-hormonal medications is used to mediate these changes. Transfeminine people are given estrogens, progestogens, and antiandrogens (AAs) to supersede gonadal sex hormone production and shift the hormonal profile from male-typical to female-typical.
Transfeminine hormone therapy aims to achieve estradiol and testosterone levels within the normal female range. Commonly recommended ranges for transfeminine people in the literature are 100 to 200 pg/mL (367–734 pmol/L) for estradiol levels and less than 50 ng/dL (1.7 nmol/L) for testosterone levels (Table). However, higher estradiol levels of more than 200 pg/mL (734 pmol/L) can be useful in transfeminine hormone therapy to help suppress testosterone levels. Lower estradiol levels (≤50–60 pg/mL [≤180–220 pmol/L]) are recommended and more appropriate for pubertal and adolescent transfeminine individuals. Sex hormone levels in the blood can be measured with blood tests, in which blood is drawn from a vein using a needle and then analyzed in a laboratory. This is useful in transfeminine people to ensure that the hormonal profile has been satisfactorily altered in line with therapeutic goals—specifically that hormone levels are within female ranges.
Gonadal Suppression
At sufficiently high exposure, estrogens and androgens are able to completely suppress gonadal sex hormone production, while progestogens by themselves are able to partially but substantially suppress gonadal sex hormone production. More specifically, studies in cisgender men and transfeminine people have found that estradiol levels of around 200 pg/mL (734 pmol/L) suppress testosterone levels by about 90% on average (to ~50 ng/dL [1.7 nmol/L]), while estradiol levels of around 500 pg/mL (1,840 pmol/L) suppress testosterone levels by about 95% on average (to ~20–30 ng/dL [0.7–1.0 nmol/L]) (Gooren et al., 1984 [Graph]; Herndon et al., 2023 [Discussion]; Wiki; Graphs). Estradiol levels of below 200 pg/mL (734 pmol/L) also suppress testosterone levels, although to a reduced extent compared to higher levels (Aly, 2019; Krishnamurthy et al., 2023; Slack et al., 2023). In one large study in transfeminine people, the rates of adequate testosterone suppression (to testosterone levels of <50 ng/dL or <1.7 nmol/L) were 24% of individuals at estradiol levels of <100 pg/mL (367 pmol/L), 58% at 100 to 200 pg/mL (367–734 pmol/L), and 77% at >200 pg/mL (>734 pmol/L) (Krishnamurthy et al., 2023).
Figure 2: Estradiol and testosterone levels after a single injection of 320 mg polyestradiol phosphate (PEP) (a long-acting prodrug of estradiol) in men with prostate cancer (Stege et al., 1996). The maximal decrease in testosterone levels occurred with estradiol levels of greater than 200 pg/mL (734 pmol/L) and was about 90% (to roughly 50 ng/dL [1.7 nmol/L]). This figure demonstrates the ability of estradiol to concentration-dependently suppress gonadal testosterone production and circulating testosterone levels in people with testes.
Progestogens on their own are able to maximally suppress testosterone levels by about 50 to 70% (to ~150–300 ng/dL [5.2–10.4 nmol/L] on average) (Aly, 2019; Wiki). In combination with relatively small amounts of estrogen however, there is synergism in the antigonadotropic effect—the suppression of gonadal testosterone production with maximally effective doses of progestogens becomes complete, and testosterone levels are reduced by about 95% (to ~20–30 ng/dL [0.7–1.0 nmol/L]) (Aly, 2019). Hence, the combination of an estrogen and a progestogen can be used to achieve maximal testosterone suppression at lower doses than would be necessary if an estrogen or progestogen were used alone.
The antigonadotropic effects of estrogens and progestogens are taken advantage of in transfeminine hormone therapy to suppress gonadal testosterone production and attain testosterone levels that are more consistent with those in cisgender women. It should be noted that the preceding numbers on testosterone suppression with estrogens and progestogens are averages and there is significant variation between individuals in terms of testosterone suppression. In other words, some may need more or less in terms of hormonal dosages to achieve the same decrease in testosterone levels.
Effects and Timeline
During normal puberty in both males and females, sex hormone exposure increases slowly over a period of several years (Aly, 2020). In relation to this, sexual maturation occurs gradually during normal puberty. In non-adolescent transgender people, adult or higher amounts of hormones are generally administered right away, and this can result in changes in secondary sex characteristics happening more quickly. Most of the effects of feminizing hormone therapy in transfeminine people onset within 1 to 6 months of commencing treatment and complete within 1 to 3 years. The table below is reproduced from literature sources with slight modification and is commonly cited as a timeline of the effects (Table). It is based on a mixture of anecdotal clinical experience, expert opinion, and available clinical studies of hormone therapy in transfeminine people. Due to limited research characterizing the effects of transfeminine hormone therapy at present, the table may or may not be completely accurate.
Table 2: Effects of hormone therapy at typical doses in adult transfeminine people (Wiki):
Effect
Onseta
Completiona
Permanency
Breast development
2–6 months
2–3 years
Permanent
Reduced and slowed growth of facial and body hair
3–12 months
>3 yearsb
Reversible
Cessation and reversal of scalp hair loss
1–3 months
1–2 years
Reversible
Softening of skin and decreased skin oiliness and acne
3–6 months
Unknown
Reversible
Redistribution of body fat in a feminine pattern
3–6 months
2–5 years
Reversible
Decreased muscle mass and strength
3–6 months
1–2 yearsc
Reversible
Widening and rounding of the pelvisd
Unknown
Unknown
Permanent
Changes in mood, emotionality, and behavior
Immediate
Unknown
Reversible
Decreased sex drive and spontaneous erections
1–3 months
3–6 months
Reversible
Erectile dysfunction and decreased ejaculate volume
1–3 months
Variable
Reversible
Decreased sperm production and infertility
Unknown
>3 years
Mixede
Decreased testicular volume
3–6 months
2–3 years
Unknown
Voice changes (e.g., more feminine pitch/resonance)
Nonef
N/A
N/A
Height changes (e.g., decrease)
Noneg
N/A
N/A
a Effects in general may vary significantly between individuals due to factors like genetics, diet/nutrition, hormone levels, etc. b Hormone therapy usually has little influence on facial hair density in transfeminine people. Complete removal of facial and body hair can be achieved with laser hair removal and electrolysis. Temporary hair removal can be achieved with shaving, epilating, waxing, and other methods. c Reduced muscle mass and strength may vary significantly depending on amount of physical exercise. d Pelvic changes occur only in young individuals who have not yet completed growth plate closure (may not occur at all in post-adolescent people). e Only estrogens, particularly at high doses, seem to have the potential for long-lasting or irreversible infertility; impaired fertility caused by antiandrogens is usually readily reversible with discontinuation. fVoice training can be an effective means of feminizing the voice. g Height attainment may be reduced in adolescents, but height is not meaningfully changed or reduced in adults per clinical data (Gooren & Bunck, 2004; Ingram & Thomas, 2019; Hilton & Lundberg, 2021; Talathi et al., 2025).
Breast Development
Breast development is among the most anticipated effects of hormone therapy in transfeminine people (Masumori et al., 2021; Grock et al., 2024). This relates to the key significance of breasts as a feminine characteristic, component of sexual attractiveness, and signal of sex and gender. Breast growth in transfeminine people usually starts within 1 to 6 months and completes over a period of 1 to 3 years (e.g., de Blok et al., 2021). The developed breasts of transfeminine people are highly variable in terms of size and shape, as with natal women (de Blok et al., 2021). Based on available high-quality clinical studies, transfeminine people tend to have much smaller mature breasts than those of natal women on average, and this appears to be the case regardless of hormonal regimen or age at which hormone therapy is commenced (e.g., de Blok et al., 2021; Boogers et al., 2025). The reasons for this are unknown, but one key possibility, observed in animals, is that prenatal androgen exposure limits subsequent breast growth potential. Despite usually modest breast development, many transfeminine people still express overall satisfaction with their breasts (de Blok et al., 2021; Boogers et al., 2025).
Beyond ensuring adequate testosterone suppression and maintaining sufficient estradiol levels above a specific low threshold, there are currently no known or substantiated methods to permanently enhance or optimize breast development. However, research suggests that avoiding high or excessive doses of estradiol and progestogens may be beneficial. In addition, high levels of estradiol, progesterone, and/or prolactin, as with the normal menstrual cycle and pregnancy, are known to induce temporary and reversible breast tenderness and enlargement, for instance due to local fluid retention and lobuloalveolar maturation (Aly, 2020). However, the breast size increases are modest, and high hormone levels come with health risks (Aly, 2020). Surgical breast augmentation is an option to increase breast size if it is unsatisfactory. Some transfeminine people, for instance many non-binary individuals, may wish to avoid or minimize breast growth, and there are possible therapeutic approaches in this area (Aly, 2019).
Additional review content on breast development in transfeminine people exists on this site (e.g., Aly, 2020; Aly, 2020). Breast growth can be measured and tracked with a variety of methods for individuals who are interested in monitoring their progress (Wiki). Photographs and timelines of breast development and feminization with hormone therapy in transfeminine people are available in communities like r/TransTimelines and r/TransBreastTimelines on the social media website Reddit.
Specific Hormonal Medications
The medications that are used in transfeminine hormone therapy include estrogens, progestogens, and antiandrogens. Estrogens produce feminization and testosterone suppression. Progestogens and antiandrogens do not mediate feminization themselves but further suppress and/or block testosterone. Testosterone suppression causes demasculinization and disinhibition of estrogen-mediated feminization. Androgens are sometimes used at low doses in transfeminine people who have low testosterone levels, although they are not required and benefits are uncertain. There are many different types of these hormonal medications available for transfeminine hormone therapy, with different benefits and risks.
Estrogens, progestogens, and antiandrogens are available in a variety of different formulations and for use by many different routes of administration in transfeminine people. The route of administration influences the absorption, distribution, metabolism, and elimination of the hormone in the body, resulting in significant differences between routes in terms of bioavailability, hormone levels in blood and specific tissues, and patterns of metabolites. These differences can have important therapeutic consequences.
Table 3: Major routes of administration of hormonal medications for transfeminine people:
Insertion via surgical incision into fat under skin
Pellet
Vaginal administration is a major additional route of administration of hormonal medications in cisgender women. While vaginal administration via a natal vagina is of course not possible in transfeminine people, neovaginal administration is a possiblility in those who have undergone vaginoplasty. However, the lining of the neovagina is not the vaginal epithelium of natal females but instead is usually skin or colon—depending on the type of vaginoplasty performed (penile inversion or sigmoid colon vaginoplasty, respectively). For this reason, neovaginal administration in transfeminine people is likely more similar in its properties to transdermal and rectal administration—depending on the type of neovagina—than to vaginal administration in cisgender women. It is noteworthy that the vaginal and rectal routes are said to be similar in their properties for hormonal medications however (Goletiani, Keith, & Gorsky, 2007; Wiki). Moreover, absorption of estradiol via neovaginas constructed from peritoneum (internal abdominal lining)—a less commonly employed vaginoplasty approach in transfeminine people—was reported in one study to be similar to that with vaginal administration of estradiol in cisgender women (Willemsen et al., 1985). As such, neovaginal administration may be an additional possible route for certain transfeminine people depending on the circumstances. However, this route still remains to be more adequately characterized.
An often-encountered question from people who take hormonal medications is whether there is an optimal time of the day to take them (Colonnello et al., 2025). As of present, there is little research in this area, and the answer to the question is essentially unknown (Colonnello et al., 2025). In any case, there is currently no evidence or persuasive theoretical basis to favor specific times of day to take these medications (Colonnello et al., 2025). In all likelihood, it makes little or no difference.
Estrogens
Estradiol, the primary bioidentical form normally found in the human body, is the main estrogen that is used in transfeminine hormone therapy. Estradiol hemihydrate (EH) is another form that is essentially identical to and interchangeable with estradiol. Estradiol esters are also sometimes used in place of estradiol. They are prodrugs of estradiol (i.e., are converted into estradiol in the body) and have essentially identical biological activity to estradiol. However, they have longer durations when used by injection due to slower absorption from the injection site, and this allows them to be administered less often. Some examples of major estradiol esters include estradiol valerate (EV; Progynova, Progynon Depot, Delestrogen) and estradiol cypionate (EC; Depo-Estradiol). Polyestradiol phosphate (PEP; Estradurin) is an injectable estradiol prodrug in the form of a polymer (i.e., linked chain of estradiol molecules) which is metabolized slowly and has a very long duration.
Non-bioidentical estrogens such as ethinylestradiol (EE; found in birth control pills), conjugated estrogens (CEEs; Premarin; used in menopausal hormone therapy), and diethylstilbestrol (DES; widely used previously but now abandoned) are resistant to metabolism in the liver and have disproportionate effects on estrogen-modulated liver synthesis when compared to bioidentical estrogens like estradiol (Aly, 2020). As a result, they have stronger influence on coagulation and greater risk of certain health problems like blood clots and associated cardiovascular issues (Aly, 2020). For this reason, as well as the fact that relatively high doses of estrogens may be needed for testosterone suppression, non-bioidentical estrogens should ideally never be used in transfeminine hormone therapy.
Estradiol dose-dependently suppresses testosterone levels in people with testes. Physiological and guideline-based levels of estradiol (<200 pg/mL or <734 pmol/L) are often not sufficient to suppress testosterone levels into the female range in transfeminine people who have not had their gonads removed (e.g., Liang et al., 2018; Krishnamurthy et al., 2023; Slack et al., 2023). As a result, estradiol is generally used in combination with an antiandrogen or progestogen in transfeminine hormone therapy (Hembree et al., 2017; Coleman et al., 2022; Rose et al., 2023). This results in partial suppression of testosterone levels by estradiol and further suppression or blockade of the remaining testosterone by the antiandrogen or progestogen. While combination therapy can be effective in fully suppressing or blocking testosterone (e.g., Aly, 2019; Aly, 2020), testosterone suppression can also still remain incomplete with antiandrogens and progestogens in certain forms (e.g., Aly, 2018; Jain, Kwan, & Forcier, 2019). In contrast to physiological estradiol levels, supraphysiological levels of estradiol are able to consistently and fully suppress testosterone levels into the normal female range with estradiol alone in transfeminine people (e.g., Gooren et al., 1984 [Graph]; Igo & Visram, 2021; Herndon et al., 2023 [Discussion]). This alternative approach, often referred to as high-dose estradiol monotherapy, has the advantage of avoiding the side effects, risks, and costs of antiandrogens and progestogens. However, it has the disadvantage of exposure to supraphysiological estradiol levels that are above those recommended by guidelines and that may have greater health risks. Physiological estradiol doses and combination therapy are more often used in transfeminine people treated by clinicians, whereas high-dose estradiol monotherapy is more frequently encountered in transfeminine people on DIY hormone therapy.
The feminizing effects of estradiol appear to be maximal at relatively low levels in the absence of androgens. Higher doses of estradiol and supraphysiological estradiol levels, aside from allowing for greater testosterone suppression, are not known to result in better feminization in transfeminine people (Deutsch, 2016; Nolan & Cheung, 2021). In fact, there is some indication that higher estrogen doses early into hormone therapy could actually result in worse breast development. Hence, the therapeutic emphasis in transfeminine hormone therapy is more on testosterone suppression than on achieving a specific estradiol level, at least above a certain low threshold level. Higher doses of estrogens, including of estradiol, also have a greater risk of adverse health effects such as blood clots and cardiovascular problems (Aly, 2020). As such, the use of physiological doses of estradiol is optimal in transfeminine people. At the same time however, high estrogen doses can be useful for improving testosterone suppression when it is inadequate, and the absolute risks, in the case of non-oral bioidentical estradiol, are low and are more important in people with specific risk factors (e.g., older age, physical inactivity, obesity, concomitant progestogen use, smoking, surgery, and rare thrombophilic abnormalities). If more adequate testosterone suppression is necessary, limitedly supraphysiological doses of non-oral estradiol may have a reasonable ratio of benefit to risk in this context, at least in those without relevant risk factors for estrogen-related complications (e.g., many healthy young people) (Aly, 2020).
Estradiol and estradiol esters are usually used orally, sublingually, transdermally, by injection (intramuscularly or subcutaneously), or by implant in transfeminine hormone therapy (Wiki).
Oral Estradiol
Estradiol is used orally in the form of tablets of estradiol (Wiki; Graphs). Alternatively, oral estradiol valerate tablets are used in some countries, for instance in many European countries. The only real difference between these oral estradiol forms is that estradiol valerate contains slightly less estradiol by weight (~76% of that of estradiol) due to its ester component and hence requires somewhat higher doses (~1.3-fold) in comparison for equivalent estradiol levels (Wiki; Table). Oral estradiol has a duration suitable for once-daily administration. Oral administration of estradiol is a very convenient and inexpensive route, which makes it the most popular and widely used form of estradiol in transfeminine people. Oral estradiol has relatively low bioavailability (~5%), and there is substantial variability between people in terms of estradiol levels achieved with the same dose. Hence, in some transfeminine people estradiol levels may be low with oral estradiol, and testosterone suppression may be inadequate depending on the antiandrogen.
A major drawback of oral estradiol is that it results in excessive levels of estradiol in the liver due to the first pass that occurs with oral administration and has a disproportionate impact on estrogen-modulated liver synthesis (Aly, 2020). This in turn increases coagulation and the risk of associated health complications like blood clots and cardiovascular problems (Aly, 2020). These particular health concerns are largely allayed if estradiol is taken non-orally at reasonable and non-excessive doses. Hence non-oral forms of estradiol, like transdermal estradiol, although less convenient and often more expensive than oral estradiol, are preferable in transfeminine hormone therapy. It is recommended that all transfeminine people who are over 40 to 45 years of age use non-oral routes due to the greater risk of blood clots and cardiovascular problems that occurs with age (Aly, 2020; Coleman et al., 2022). Oral estradiol is not a good choice for high-dose estradiol monotherapy in transfeminine people due to the high estradiol levels required and the greater risks than with non-oral routes. In addition to its disproportionate liver impact, oral estradiol results in unphysiological levels of estradiol metabolites like estrone and estrone sulfate when compared to non-oral forms. The clinical implications of this, if any, are unknown. Oral and non-oral estradiol have in any case been found to have similar effectiveness in terms of feminization and breast development in transfeminine people in available studies (Sam, 2020).
Sublingual Estradiol
Oral estradiol tablets can be taken sublingually instead of orally. Sublingual use of estradiol tablets has several-fold higher bioavailability relative to oral administration and hence achieves much higher overall estradiol levels in comparison (Sam, 2021; Wiki; Graphs). Sublingual use of oral estradiol tablets can be employed instead of oral administration to reduce doses and hence medication costs or to produce higher estradiol levels for the purpose of achieving better testosterone suppression when needed. However, sublingual estradiol is very spiky in terms of estradiol levels when compared to oral estradiol and has a short duration of highly elevated estradiol levels. As such, it may be advisable for sublingual estradiol to be used in divided doses multiple times throughout the day in order to maintain at least somewhat steadier estradiol levels. The therapeutic implications for transfeminine people of the spikiness of sublingual estradiol, for instance in terms of testosterone suppression and health risks, have been little-studied and are mostly unknown. In any case, when used as a form of high-dose estradiol monotherapy and taken multiple times per day, strong though still incomplete testosterone suppression has been observed (Yaish et al., 2023). Oral estradiol valerate tablets can be taken sublingually instead of orally similarly to estradiol and are likewise highly effective when used in this way (Aly, 2019; Wiki). Due to partial swallowing of tablets, sublingual estradiol may in practice be a mixture of sublingual and oral administration and may have some of the same health risks of oral estradiol (Wiki). Buccal administration of estradiol appears to have similar properties as sublingual administration but is much less researched in comparison and is not used as often in transfeminine people (Wiki).
Transdermal Estradiol
Transdermal estradiol is available in the form of patches, gel, emulsions, and sprays (Wiki). These forms are usually applied to skin areas such as the arms, abdomen, or buttocks. Gel, emulsions, and sprays are applied and left to dry for a short period, whereas patches are applied and remain adhesed to the skin for a specified amount of time. Due to rate-limited absorption through the skin, there is a depot effect with transdermal estradiol and this route has a long duration with very steady estradiol levels. As a result, estradiol gel, emulsions, and sprays are all suitable for once-daily use. Patches stay applied and continuously deliver estradiol for either 3–4 days or 7 days depending on the patch brand (Table). Transdermal estradiol is more expensive than oral estradiol. Gel, emulsions, and sprays may be less convenient than oral administration, but patches can be more convenient due to their infrequent application. However, patches can sometimes cause application site problems like redness and irritation and can occasionally come off prematurely due to adhesive failure. As with oral estradiol, there is substantial variability in estradiol levels with transdermal estradiol, and some transfeminine people may have poor absorption, low estradiol levels, and inadequate testosterone suppression with this route. Estradiol sprays, such as Lenzetto, have been found to achieve very low estradiol levels that are probably not therapeutically adequate for use in transfeminine hormone therapy (Aly, 2020; Graph).
Transdermal estradiol is the form of estradiol most commonly used in transfeminine people who are over 40 years of age due to its lower health risks relative to oral estradiol. Transdermal estradiol gel is not a favorable option for high-dose estradiol monotherapy as it has difficulty achieving the high estradiol levels needed for adequate testosterone suppression (Aly, 2019). On the other hand, transdermal estradiol patches can be an effective option for high-dose estradiol monotherapy if multiple 100 μg/day patches are used, although this can require the use of many patches and can be expensive (Wiki). Different skin sites absorb transdermal estradiol to different extents (Wiki). Genital application of transdermal estradiol, specifically to the scrotum or neolabia, is particularly better-absorbed than conventional skin sites and can result in much higher estradiol levels than usual (Aly, 2019). This can be useful for reducing doses and hence medication costs or for achieving higher estradiol levels for better testosterone suppression when needed, for instance in the context of high-dose estradiol monotherapy. Transdermal estradiol should not be applied to the breasts as this is not known to result in improved breast development and the potential health consequences of doing so are unknown (e.g., influence on breast cancer risk).
Injectable Estradiol
Injectable estradiol preparations can be administered via either intramuscular or subcutaneous injection (Wiki; Wiki; Graphs). There is a depot effect with injection of estradiol esters such that they are slowly absorbed from the injection site and have a prolonged duration. This ranges from days to months depending on the ester. Commonly used injectable estradiol esters, which all have short to moderate durations, include estradiol valerate (EV), estradiol cypionate (EC), estradiol enanthate (EEn), and estradiol benzoate (EB). Longer-acting injectable estradiol esters, such as estradiol undecylate (EU) and polyestradiol phosphate (PEP), have been discontinued and are no longer pharmaceutically available. In the case of intramuscular injection, common injection sites include the deltoid muscle (upper arm), vastus lateralis and rectus femoris muscles (thigh), and ventrogluteal muscle (buttocks). Subcutaneous injection of estradiol injectables, while less commonly used, has comparable pharmacokinetics to intramuscular injection, and is easier, less painful, and more convenient in comparison (Wiki). However, the maximum volume that can be safely and comfortably injected subcutaneously (1.5–3 mL) is less than that which can be injected intramuscularly (2–5 mL) (Hopkins, & Arias, 2013; Usach et al., 2019). Injectable estradiol tends to be fairly inexpensive, but may be less convenient than other routes due to the need for regular injections. There may also be a risk of internal scar tissue build-up long-term. Estradiol injectables have been discontinued in many parts of the world (e.g., most of Europe), and their availability is limited. In recent years, many transfeminine people have turned to black market homebrewed injectable estradiol preparations to use this route.
Injectable estradiol preparations are typically used at higher doses than other forms of estradiol, and can easily achieve very high levels of estradiol. This can be useful for testosterone suppression, making this form of estradiol likely the best choice for high-dose estradiol monotherapy in transfeminine people. However, the high doses that are possible with injectable estradiol preparations can also easily lead to overdosage and unnecessarily increased risks (e.g., Aly, 2020). Resources are available on this site for guiding selection of appropriate doses and intervals of injectable estradiol esters in transfeminine people. This includes a simulator and informal meta-analysis of estradiol levels with these preparations (Aly, 2021; Aly, 2021) and a table providing approximate equivalent doses between injectable estradiol esters and other estradiol routes and forms (Aly, 2020). It is notable and unfortunate that currently recommended doses and intervals for injectable estradiol esters by transgender care guidelines (e.g., 10–40 mg/2 weeks estradiol valerate) appear to be highly excessive and too widely spaced, and are likely to be therapeutically inadvisable (Aly, 2021). Doses and intervals of injectable estradiol esters recommended by the present author for use as a means of high-dose estradiol monotherapy, targeting mean estradiol levels of around 300 pg/mL (1,100 pmol/L), are provided below (Table 4).
Table 4: Recommended doses and intervals of injectable estradiol esters for high-dose estradiol monotherapy (targeting estradiol levels of around 300 pg/mL [1,100 pmol/L]):
a Injection interval. b Doses and intervals for estradiol undecylate are extrapolated and hypothetical (Aly, 2021).
These doses and intervals should be considered a starting point, and should be fine-tuned as necessary based on blood tests. In terms of injection intervals, the shorter interval, the more stable the estradiol levels, but the more often that injections need to be done. Doses may be increased if estradiol levels are too low and testosterone suppression is inadequate, and doses may be decreased if estradiol levels are too high so long as adequate testosterone suppression is maintained. Doses should be lower (targeting mean estradiol levels of 100–200 pg/mL [367–734 pmol/L]) if combined with an antiandrogen or progestogen as these agents will help with testosterone suppression. Similarly, doses should be lower following surgical gonadal removal as testosterone suppression will no longer be necessary.
Estradiol Pellets
Estradiol implants are pellets of pure crystalline hormone and are surgically placed into subcutaneous fat by a physician (Wiki). They are slowly absorbed by the body following implantation, and new implants are given once every 4 to 6 months. Due to the need for minor surgery, their high cost, and limited availability, estradiol implants are not as commonly used as other estradiol routes. Notably, almost all pharmaceutical estradiol implants throughout the world have been discontinued, and the implants that remain available are almost exclusively compounded products provided by compounding pharmacies. Dosage adjustment with estradiol implants is also more difficult than with other estradiol routes. Despite their various practical limitations however, estradiol implants allow for very steady estradiol levels, and their very long duration can allow for unusual convenience among available estradiol forms.
Additional Notes
Table 5: Available forms and recommended doses of estradiol for adulta transfeminine people:
a Estradiol doses in pubertal adolescent transfeminine people should be lower to mimic estradiol exposure during normal female puberty (Aly, 2020). b May be advisable to use divided doses 2 to 4 times per day (i.e., once every 6 to 12 hours) instead of once per day (Sam, 2021). c This estradiol form achieves very low estradiol levels at typical doses that don’t appear to be well-suited for transfeminine hormone therapy (Aly, 2020; Graph). d Estradiol valerate contains about 75% of the same amount of estradiol as estradiol so doses are about 1.3-fold higher for the same estradiol levels (Aly, 2019; Sam, 2021). e Doses and intervals for estradiol undecylate are extrapolated and hypothetical (Aly, 2021). f A higher initial loading dose of e.g., 240 or 320 mg polyestradiol phosphate can be used for the first one or two injections to reach steady-state estradiol levels more quickly. However, this preparation has recently been discontinued and appears to no longer be available.
Additional informational resources are available in terms of estradiol levels (Wiki; Table) and approximate equivalent doses (Aly, 2020) with different forms, routes, and doses of estradiol.
There is high variability between individuals in the levels of estradiol achieved during estradiol therapy. That is, estradiol levels during treatment with the same dosage of estradiol can differ substantially between individuals. This variability is greatest with oral and transdermal estradiol but is also considerable even with injectable estradiol preparations and other estradiol forms. As such, estradiol doses are not absolute and should be individualized on a case-by-case basis in conjunction with blood work as a guide. It should also be noted that due to fluctuations in estradiol concentrations with certain routes, levels of estradiol can vary considerably from one blood test to another. This is most notable with sublingual estradiol and injectable estradiol. The fluctuations in estradiol levels with these routes are predictable and must be understood when interpreting blood work results. Differences in blood test results can be minimized with informed and consistent timing of blood draws.
If or when the gonads are surgically removed, testosterone suppression is no longer needed in transfeminine people. As a result, estradiol doses, if they are high or supraphysiological, can be lowered to more closely approximate normal physiological levels in cisgender women.
Progestogens
Progestogens include progesterone and progestins. Progestins are synthetic progestogens derived from structural modification of progesterone or testosterone. There are dozens of different progestins and these progestins can be divided into a variety of different structural classes with varying properties (Table). Examples of some major progestins of different classes include the 17α-hydroxyprogesterone derivative medroxyprogesterone acetate (MPA; Provera, Depo-Provera), the 19-nortestosterone derivative norethisterone (NET; many brand names), the retroprogesterone derivative dydrogesterone (Duphaston), and the 17α-spirolactone derivative drospirenone (Slynd, Yasmin). Progestins were developed because they have a more favorable disposition in the body than progesterone for use as medications. Only a few clinically used progestins have been employed in transfeminine hormone therapy. However, progestogens largely produce the same progestogenic effects, with a few exceptions, and theoretically almost any progestogen could be used.
Besides helping with testosterone suppression, progestogens are of no clear or known benefit for feminization or breast development in transfeminine people. While some transfeminine people anecdotally claim to experience improved breast development with progestogens, an involvement of progestogens in improving breast size or shape is controversial and is not supported by theory nor evidence at present (Wiki; Aly, 2020). It is possible that premature introduction of progestogens, particularly at high doses, could actually have an unfavorable influence on breast development (Aly, 2020). Many transfeminine people have also anecdotally claimed that progestogens have a beneficial effect on their sexual desire. However, a review of the literature by the present author found that neither progesterone nor progestins positively influence sexual desire in humans (Aly, 2020). Instead, the available evidence suggests either a neutral influence or an inhibitory effect of progestogens on sexual desire, although the latter may be specific only to high doses of progestogens (Aly, 2020). Claims have been made that progesterone may have beneficial effects on mood in transfeminine people as well, but clinical support for such notions is likewise lacking at this time (Coleman et al., 2022; Nolan et al., 2022). It is notable that progesterone at luteal-phase levels, due to its neurosteroid metabolites like allopregnanolone, actually appears to worsen mood in around 30% of cisgender women, and produces more overt negative reactions, which constitute the diagnoses of premenstrual syndrome (PMS) and premenstrual dysphoric disorder (PMDD), in around 2 to 10% of women (Bäckström et al., 2011; Edler Schiller, Schmidt, & Rubinow, 2014; Sundström-Poromaa et al., 2020). More research is needed to evaluate the possible beneficial effects of progestogens in transfeminine people.
Most clinically used progestogens have off-target activities in addition to their progestogenic activity, and these activities may be desirable or undesirable depending on the action in question (Kuhl, 2005; Stanczyk et al., 2013; Wiki; Table). Progesterone has a variety of neurosteroid as well as other activities that can result in central nervous system effects among others which are not shared by progestins. MPA as well as NET and its derivatives have weak androgenic activity, which is unfavorable in the context of transfeminine hormone therapy. NET and certain related progestins produce ethinylestradiol as a metabolite at high doses and hence can produce ethinylestradiol-like estrogenic effects, including increased risk of blood clots and associated cardiovascular problems. Other off-target actions of progestogens include antiandrogenic, glucocorticoid, and antimineralocorticoid activities. These actions can result in differences in therapeutic effectiveness (e.g., androgen suppression or blockade) as well as side effects and health risks. Some notable progestins without undesirable off-target activities (i.e., androgenic or glucocorticoid activity) include low-dose CPA, drospirenone (DRSP), dienogest, nomegestrol acetate (NOMAC), dydrogesterone, and hydroxyprogesterone caproate (OHPC). However, of these progestins, only CPA has been considerably used and studied in transfeminine people.
The addition of progestogens to estrogen therapy has been associated with a number of unfavorable health effects. These include increased risk of blood clots (Wiki; Aly, 2020), coronary heart disease (Wiki), and breast cancer (Wiki; Aly, 2020). High doses of progestogens are also associated with increased risk of certain non-cancerousbrain tumors including meningiomas and prolactinomas (Wiki; Aly, 2020). The coronary heart disease risk may be due to changes in blood lipids caused by the weak androgenic activity of certain progestogens, but the rest of the aforementioned risks are probably due to their progestogenic activity (Stanczyk et al., 2013; Jiang & Tian, 2017). Aside from health risks, progestogens have also been associated with adverse mood changes (Wiki; Wiki). However, besides the case of progesterone and its neurosteroid metabolites, these effects of progestogens are controversial and are not well-supported by evidence (Wiki; Wiki). Progestogens are otherwise generally well-tolerated and are regarded as producing little in the way of side effects.
In contrast to certain progestins, progesterone has no unfavorable off-target hormonal activities. Due to its lack of androgenic activity, progesterone has no adverse influence on blood lipids and is not expected to raise the risk of coronary heart disease. The addition of oral progesterone to estrogen therapy notably has not been associated with increased risk of blood clots (Wiki). In addition, oral progesterone seems to have less risk of breast cancer than progestins with shorter-term therapy, although this is notably not the case with longer-term exposure (Wiki; Aly, 2020). Consequently, it has been suggested that progesterone, for reasons that have yet to be fully elucidated, may be a safer progestogen than progestins and that it should be the preferred progestogen for hormone therapy in cisgender women and transfeminine people. However, there are also theoretical arguments against such notions. Oral progesterone is known to produce very low progesterone levels and to have only weak progestogenic effects at typical doses (Aly, 2018; Wiki). The seemingly better safety of oral progesterone may simply be an artifact of the low progesterone levels that occur with it, and hence of progestogenic dosage. Non-oral progesterone, at doses resulting in physiological and full progestogenic strength, has never been properly evaluated in terms of health outcomes, and may have similar risks as progestins (Aly, 2018; Wiki).
Due to their lack of known influence on feminization and breast development and their known and possible adverse effects and risks, progestogens are not routinely used in transfeminine hormone therapy at present. Major transgender health guidelines note the limitations of the available evidence on progestogens for transfeminine people and have mixed attitudes on their use, either explictly recommending against their use (Coleman et al., 2022—WPATH SOC8), taking a more neutral stance (Hembree et al., 2017—Endocrine Society guidelines), or being permissive of their use (Deutsch, 2016—UCSF guidelines). There is however a very major exception to the preceding in the form of CPA, an antiandrogen which is widely used in transfeminine hormone therapy to suppress testosterone production and which happens to be a powerful progestogen at the typical doses used in transfeminine people. CPA will be described below in the section on antiandrogens. Although progestogens have various health risks, cisgender women of course have progesterone, and the absolute risks of progestogens are very low in healthy young people. Risks like breast cancer also are exposure-dependent and take many years to develop. The testosterone suppression provided by progestogens can furthermore be very useful in transfeminine people, as is widely taken advantage of with CPA. Given these considerations, a limited duration of progestogen therapy in transfeminine people, for instance a few years to help suppress testosterone levels before surgical gonadal removal, may be considered quite acceptable.
Progesterone can be used in transfeminine people by oral administration, sublingual administration, rectal administration, or by intramuscular or subcutaneous injection (Wiki). Progestins are usually used via oral administration, but certain progestins are also available in injectable formulations (Wiki).
Oral Progesterone
Progesterone is most commonly taken orally. It is used by this route in the form of oil-filled capsules containing 100 or 200 mg micronized progesterone under brand names such as Prometrium, Utrogestan, and Microgest (Wiki). Despite its widespread use, levels of progesterone via oral administration have been found using state-of-the-art assays (LC–MS) to be very low (<2 ng/mL [<6.4 nmol/L] at 100 mg/day) and inadequate for satisfactory progestogenic effects in various areas (Aly, 2018; Wiki). In relation to this, even high doses of oral progesterone (400 mg/day) showed no antigonadotropic effect or testosterone suppression in cisgender men (Aly, 2018; Wiki). This is in major contrast to non-oral forms of progesterone and to progestins, which produce dose-dependent and robust testosterone suppression (Aly, 2019; Wiki). In addition to its low progestogenic potency, oral progesterone is excessively converted into neurosteroid metabolites like allopregnanolone and pregnanolone. These metabolites act as potent GABAA receptor positive allosteric modulators, and can produce undesirable alcohol-like side effects such as sedation, cognitive, memory, and motor impairment, and mood changes (Wiki; Wiki). As such, while inconvenient, non-oral routes are greatly preferable for progesterone.
Sublingual Progesterone
Sublingual progesterone tablets exist and are marketed under the brand name Luteina but today are only available in Poland and Ukraine (Wiki). Oral progesterone could theoretically be taken sublingually, analogously to sublingual use of oral estradiol. However, because oral progesterone is formulated as oil-filled capsules, this makes it difficult and unpleasant to use by sublingual administration. Buccal progesterone, which would be expected to have similar characteristics to those of sublingual progesterone, has been used in medicine in the past, but is no longer marketed today (Wiki).
Rectal Progesterone
Progesterone is approved for use by rectal administration in the form of suppositories under the brand name Cyclogest (Wiki). This product is marketed in only a limited number of countries however, although it is available in the United Kingdom (Wiki). While not approved for use by rectal administration, oral progesterone capsules can be taken rectally instead of orally, and using them in this way may allow for much higher progesterone levels than would be achieved by oral administration due to avoidance of most first-pass metabolism. Rectal administration of oral progesterone capsules has not been formally studied, but oral progesterone capsules have been administered vaginally in cisgender women with success (Miles et al., 1994; Wang et al., 2019), and the vaginal and rectal routes are said to have similar pharmacokinetics in general (Goletiani, Keith, & Gorsky, 2007; Wiki). Hence, there is good theoretical basis for rectal administration of oral progesterone capsules being an effective route of progesterone. Whereas oral progesterone achieves very low levels of progesterone, rectal progesterone can readily achieve normal luteal-phase levels of progesterone (Wiki). Although inconvenient, rectal administration may be the overall best route of administration of progesterone for transfeminine people. A significant subset of transfeminine people on progestogens take progesterone rectally (Chang et al., 2024).
Injectable Progesterone
Progesterone by injection is available as an oil solution for intramuscular injection under brand names such as Proluton, Progestaject, and Gestone (Wiki) and as an aqueous solution for subcutaneous injection under the brand name Prolutex (Wiki). Oil solutions of progesterone for intramuscular injection are widely available, whereas the aqueous solution of progesterone for subcutaneous injection is available only in some European countries (Wiki). Injectable progesterone, regardless of route, has a relatively short duration and must be injected once every one to three days (Wiki; Wiki). This makes it too inconvenient to use for most people. Unlike with estradiol, progesterone esters with longer durations than progesterone itself by injection are not chemically possible as progesterone has no hydroxyl groups available for esterification (Wiki). Injectable aqueous suspensions of microcrystalline progesterone were previously marketed and had a duration of 1 to 2 weeks, but these preparations were associated with pain at the injection site and were eventually discontinued (Aly, 2019; Wiki).
Other Progesterone Routes
Other progesterone routes, such as transdermal progesterone and subcutaneous progesterone pellets, are also known, but are not available as pharmaceutical drugs and are little-used medically (Wiki). This is related to the low potency of progesterone and difficulty achieving progesterone levels high enough for adequate therapeutic effects with these routes (Wiki; Wiki). In addition, progesterone pellets tend to be extruded at high rates (Wiki). In any case, certain compounding pharmacies may make forms of progesterone that could be used by these routes.
Oral and Injectable Progestins
Most progestins are taken orally in the form of solid tablets (Wiki). In contrast to progesterone, progestins, owing to their synthetic nature, are resistant to metabolism in the intestines and liver and have high oral bioavailability. In addition, unlike the case of the estrogen receptors, the progesterone receptors are expressed minimally or not at all in the liver, and there is no known first pass influence of progestogenic activity on liver synthesis (Lax, 1987; Stanczyk, Mathews, & Cortessis, 2017). As a result, there are no apparent problems with oral administration in the case of purely progestogenic progestins. However, some progestins have liver-impacting off-target hormonal actions, such as androgenic, estrogenic, and/or glucocorticoid activity, and this can result in adverse effects like unfavorable lipid changes or procoagulation—which may be augmented by the first pass with oral administration.
A selection of progestins are available in injectable formulations, including for intramuscular or subcutaneous injection (Wiki). Some of the more notable ones include medroxyprogesterone acetate (MPA), norethisterone enanthate (NETE), hydroxyprogesterone caproate (OHPC), and algestone acetophenide (dihydroxyprogesterone acetophenide; DHPA) (Wiki). In addition to being used alone, injectable progestins are used together with estradiol esters in combined injectable contraceptives (Wiki). These preparations are often used as a means of hormone therapy by transfeminine people in Latin America. Whereas injectable progesterone has a duration measured in days, injectable progestins have durations ranging from weeks to months, and can be injected much less often in comparison (Table).
Additional Notes
Table 6: Available forms and recommended doses of progestogens for transfeminine people:
For progesterone levels with different forms, routes, and doses of progesterone, see the table here (only LC–MS and IA + CS assays for oral progesterone) and the graphs here.
As with estradiol, there is high variability between individuals in progesterone levels. Conversely, there is less variability between individuals in the case of progestins.
After removal of the gonads, progestogen doses can be lowered or adjusted to approximate normal female physiological exposure or they can be discontinued entirely.
Antiandrogens
Aside from estrogens and progestogens, there is another class of hormonal medications used in transfeminine hormone therapy known as antiandrogens (AAs). These medications reduce the effects of androgens in the body by either decreasing androgen production and thereby lowering androgen levels or by directly blocking the actions of androgens. They work via a variety of different mechanisms of action, and include androgen receptor antagonists, antigonadotropins, and androgen synthesis inhibitors.
Androgen receptor antagonists act by directly blocking the effects of androgens, including testosterone, DHT, and other androgens, at the level of their biological target. They bind to the androgen receptor without activating it, thereby displacing androgens from the receptor. Due to the nature of their mechanism of action as competitive blockers of androgens, the antiandrogenic efficacy of androgen receptor antagonists is both highly dose-dependent and fundamentally dependent on testosterone levels. They do not act by lowering testosterone levels, although some androgen receptor antagonists may have additional antiandrogenic actions that result in decreased testosterone levels. Because androgen receptor antagonists do not work by lowering testosterone levels, blood work can be less informative for them compared to antiandrogens that suppress testosterone levels. Androgen receptor antagonists include steroidal antiandrogens (SAAs) like spironolactone (Aldactone) and cyproterone acetate (CPA; Androcur) and nonsteroidal antiandrogens (NSAAs) like bicalutamide (Casodex).
Antigonadotropins suppress the gonadal production of androgens by inhibiting the GnRH-mediated secretion of gonadotropins from the pituitary gland. They include estrogens and progestogens. In addition, GnRH agonists such as leuprorelin (Lupron) and GnRH antagonists such as elagolix (Orilissa) act similarly and could likewise be described as antigonadotropins.
Androgen synthesis inhibitors inhibit the enzyme-mediated synthesis of androgens. They include 5α-reductase inhibitors (5α-RIs) like finasteride (Propecia) and dutasteride (Avodart). There are also other types of androgen synthesis inhibitors, for instance potent 17α-hydroxylase/17,20-lyase inhibitors like ketoconazole (Nizoral) and abiraterone acetate (Zytiga). However, these agents have limitations (e.g., toxicity, high cost, and lack of experience) and have not been used in transfeminine hormone therapy.
Although antigonadotropins and androgen synthesis inhibitors have antiandrogenic effects secondary to decreased androgen levels, they are not usually referred to as “antiandrogens”. Instead, this term is most commonly reserved to refer specifically to androgen receptor antagonists. However, antigonadotropins and androgen synthesis inhibitors may nonetheless be described as antiandrogens as well.
After removal of the gonads, antiandrogens can be discontinued. If unwanted androgen-dependent symptoms, such as acne, seborrhea, or scalp hair loss, persist despite full suppression or ablation of gonadal testosterone, then a lower dose of an androgen receptor antagonist, such as 100 to 200 mg/day spironolactone or 12.5 to 25 mg/day bicalutamide, can be continued to treat these symptoms.
Table 7: Available forms and recommended doses of antiandrogens for transfeminine people:
a For CPA, this dose range is specifically one-quarter of a 10-mg tablet to one full 10-mg tablet per day (2.5–10 mg/day) or a quarter of a 50-mg tablet every other day or every 2 to 3 days (4.2–12.5 mg/day). A dosage of 5–10 mg/day or 6.25–12.5 mg/day is likely to ensure maximal testosterone suppression, while lower doses may be less effective (Aly, 2019). b For spironolactone and bicalutamide, it is assumed that testosterone levels are substantially suppressed (≤200 ng/dL [<6.9 nmol/L]). If testosterone levels are not suppressed to this range, then higher doses may be warranted. c Spironolactone and its metabolites have relatively short half-lives, and twice-daily administration in divided doses (e.g., 100–200 mg twice per day) is recommended.
Figure 3: Suppression of gonadal testosterone production and circulating testosterone levels (ng/dL) with estradiol in combination with different antiandrogens over one year of hormone therapy in transfeminine people (Sofer et al., 2020). The estradiol forms included oral tablets 2–8 mg/day, transdermal gel 2.5–5 mg/day, and transdermal patches 50–200 μg/day. The antiandrogens included spironolactone 50–200 mg/day (n=16), cyproterone acetate (n=41), and GnRH agonists (specifically triptorelin 3.75 mg/month or goserelin 3.6 mg/month by injection) (n=10) (Sofer et al., 2020). It should be noted that lower doses of cyproterone acetate (10–12.5 mg/day) show equal testosterone suppression to higher doses (25–100 mg/day) and higher doses should no longer be used (Aly, 2019). The dashed horizontal line corresponds to the upper limit of the normal female range for testosterone levels.
As an antiandrogen, CPA has a dual mechanism of action of suppressing testosterone levels via its progestogenic and hence antigonadotropic activity and of acting as an androgen receptor antagonist (Aly, 2019). The progestogenic activity of CPA is of far greater potency than its androgen receptor antagonism however (Aly, 2019). The dose of CPA used as a progestogen in cisgender women is about 2 mg per day, which produces similar progestogenic effects to those of physiological luteal-phase levels of progesterone (e.g., suppression of gonadotropin secretion, ovulation inhibition, and endometrial transformation and protection) (Aly, 2019). Conversely, much higher doses of CPA of 50 to 300 mg/day have typically been used for androgen-dependent indications (Aly, 2019). These high doses of CPA result in profound progestogenic overdosage and associated side effects and risks (Aly, 2019). In transfeminine people, CPA has historically been used at doses of 50 to 150 mg/day (Aly, 2019). However, CPA doses have dramatically fallen in recent years, and today doses of no more than 10 to 12.5 mg/day are recommended (Aly, 2019; Coleman et al., 2022—WPATH SOC8). These lower doses of CPA still produce strong progestogenic effects, and in combination with estradiol, are equally effective as higher doses in suppressing testosterone levels (Aly, 2019; Meyer et al., 2020; Even Zohar et al., 2021; Kuijpers et al., 2021; Coleman et al., 2022). Even lower doses of CPA, for instance 5 to 6.25 mg/day, are currently being studied, and may still be fully effective (Aly, 2019).
Given by itself without estrogen, CPA typically suppresses testosterone levels in people with testes by about 50 to 70%, down to about 150 to 300 ng/dL (5.2–10.4 nmol/L) (Meriggiola et al., 2002; Toorians et al., 2003; Giltay et al., 2004; T’Sjoen et al., 2005; Tack et al., 2017; Zitzmann et al., 2017; Aly, 2019). Lower doses of CPA alone (e.g., 10 mg/day) show the same degree of testosterone suppression as higher doses of CPA alone (e.g., 50–100 mg/day), indicating that the antigonadotropic effects of CPA are maximal at relatively low therapeutic doses of this medication (Aly, 2019). This is on the order of about 5 to 10 times the ovulation-inhibiting dosage of CPA in cisgender women, a dose–response relationship that has also been observed with a number of other progestogens (Aly, 2019). Per the preceding, CPA alone, regardless of dosage, is unable to reduce testosterone levels into the normal female range (<50 ng/dL [<1.7 nmol/L]). But when CPA is combined with estradiol, even at relatively small doses of estradiol, it consistently suppresses testosterone levels into the normal female range (Aly, 2019; Angus et al., 2019; Gava et al., 2020; Sofer et al., 2020; Collet et al., 2022). However, it appears that a certain minimum level of estradiol, perhaps around 60 pg/mL (220 pmol/L) on average, is required for this to occur (Aly, 2019). Estradiol levels lower than this threshold in those taking CPA, which can occasionally be encountered in transfeminine people due to estradiol being dosed too low, have the potential to compromise full testosterone suppression (Aly, 2019).
In addition to testosterone suppression, CPA can dose-dependently block the androgen receptor (Aly, 2019). However, relatively high doses of CPA are needed to considerably antagonize the androgen receptor (e.g., 50–300 mg/day), and lower doses (e.g., ≤12.5 mg/day) may not be able to do this to a meaningful degree (Aly, 2019). As such, lower doses of CPA may essentially be purely progestogenic, with minimal or no androgen receptor antagonism. In this regard, referring to CPA at such doses as an “antiandrogen”—rather than as a “progestogen”—may be considered somewhat of a misnomer. Higher doses of CPA (>12.5 mg/day) can no longer be considered safe due to the massive progestogenic overdosage that occurs with them, and should no longer be used in transfeminine people. Moreover, as testosterone levels are usually suppressed into the normal female range in transfeminine people taking estradiol plus CPA, there is no actual need for any additional androgen receptor blockade (Aly, 2019).
CPA has been reported to produce various side effects. Some of these side effects include fatigue and a degree of weight gain (Belisle & Love, 1986; Hammerstein, 1990; Martinez-Martin et al., 2022). CPA might be able to produce a magnitude of sexual dysfunction (e.g., reduced sexual desire) beyond that expected with testosterone suppression alone (Wiki; Aly, 2019). It may also have a small risk of depressive mood changes (Wiki). In transfeminine people, CPA has been documented to produce pregnancy-like breast changes (i.e., lobuloalveolar development of the mammary glands) (Kanhai et al., 2000). In relation to this, CPA sometimes causes lactation as a side effect (Dewhurst & Underhill, 1979; Gooren, Harmsen-Louman, & van Kessel, 1985; Schlatterer et al., 1998; Bazarra-Castro, 2009). Concerns have been raised about premature introduction of progestogens—particularly at high doses like with CPA—and possible adverse influence on breast development (Aly, 2020). However, little data exists in humans to substantiate such concerns at present. The side effects of CPA are assumed to be dose-dependent, and using the lowest effective doses is expected to minimize its side effects.
CPA is usually taken orally in the form of tablets (e.g., 10, 50, and 100 mg) (Wiki). Under the brand name Androcur Depot, it is also available as a long-lasting 300 mg depot injectable in some countries (Wiki). However, this formulation is not commonly used in transfeminine people, and happens to correspond to very high doses in terms of CPA exposure. A pill cutter (Amazon) can be used to split CPA tablets and achieve lower doses (e.g., 12.5 mg doses with 50-mg tablets). CPA has a relatively long elimination half-life of about 1.6 to 4.3 days (Wiki; Aly, 2019). As such, it can be taken once daily, or even as infrequently as once every 2 or 3 days, if needed (Aly, 2019). In addition to splitting of CPA tablets, dosing CPA once every 2 or 3 days can also be useful for achieving lower doses (Aly, 2019).
As already described, CPA is a powerful progestogen even at the relatively low doses now used in transfeminine people (e.g., 5–12.5 mg/day). As such, there is no need, nor point, in adding another progestogen, for instance progesterone, in those who are taking CPA—at least if the goal of doing so is to produce progestogenic effects. This is something that is often overlooked in people taking CPA, and can result in increased costs, side effects, and inconvenience without any expected benefit.
Spironolactone
Spironolactone (Aldactone) is an antiandrogen and antimineralocorticoid. It is widely used as an antiandrogen in cisgender women for treatment of androgen-dependent hair and skin conditions like acne, hirsutism (excessive facial/body hair growth), and scalp hair loss, in cisgender women for treatment of hyperandrogenism (high androgen levels) due to polycystic ovary syndrome (PCOS), and in transfeminine people as a component of hormone therapy. Spironolactone is particularly widely used in transfeminine people in the United States, where it is the most commonly used antiandrogen in this population. As an antimineralocorticoid, the original and primary use of spironolactone in medicine, it is used to treat heart failure, high blood pressure, high mineralocorticoid levels, low potassium levels, and conditions of excess fluid retention like nephrotic syndrome and ascites, among others (Wiki). In terms of its antiandrogenic actions, spironolactone is a relatively weak androgen receptor antagonist as well as a weak androgen synthesis inhibitor (Wiki). The androgen synthesis inhibition of spironolactone is mediated specifically via inhibition of 17α-hydroxylase and 17,20-lyase (Wiki). Spironolactone does not appear to have meaningful progestogenic activity, 5α-reductase inhibition, or direct estrogenic activity (Wiki). However, indirect estrogenic effects secondary to its antiandrogenic activity (e.g., breast development and feminization) can occur with it at sufficiently high doses (Wiki).
Spironolactone shows limited and highly inconsistent effects on testosterone levels in clinical studies in cisgender men, cisgender women, and transfeminine people, with most studies finding no change in levels, some studies finding a decrease in levels, and a small number even finding an increase in levels (Aly, 2018). In spite of this, studies commonly find that spironolactone still produces antiandrogenic effects even when androgen levels remain unchanged. Hence, the primary mechanism of action of spironolactone as an antiandrogen appears to be androgen receptor blockade. In relation to this, in transfeminine people taking spironolactone as an antiandrogen, the estrogen component of the regimen is likely to be the main or possibly sole agent suppressing testosterone production. This is in part based on studies in transfeminine people comparing estradiol plus spironolactone to estradiol alone (e.g., Leinung, 2014; Leinung, Feustel, & Joseph, 2018; Angus et al., 2019) and on studies comparing testosterone levels with different doses of spironolactone (e.g., Liang et al., 2018; SoRelle et al., 2019; Allen et al., 2021). Due to the minimal influence of spironolactone on testosterone production, testosterone levels are not usually suppressed into the female range in transfeminine people taking estradiol plus spironolactone, with testosterone levels often remaining well above this range (e.g., 50–450 ng/dL [1.7–15.6 nmol/L] on average) (Leinung, 2014; Leinung, Feustel, & Joseph, 2018; Liang et al., 2018; Angus et al., 2019; Jain, Kwan, & Forcier, 2019; SoRelle et al., 2019; Sofer et al., 2020; Burinkul et al., 2021). However, testosterone levels do tend to decline gradually over time in transfeminine people on this regimen (e.g., Liang et al., 2018; Sofer et al., 2020 (Graph); Allen et al., 2021).
Due to its relatively weak androgen receptor antagonism, spironolactone is likely best-suited for blocking female-range or somewhat-higher testosterone levels (e.g., <100 ng/dL [<3.5 nmol/L]) (Aly, 2018). This is based on clinical dose-ranging studies of spironolactone (typically using 50–200 mg/day) in healthy cisgender women and cisgender women with PCOS (Goodfellow et al., 1984; Lobo et al., 1985; Hammerstein, 1990; James, Jamerson, & Aguh, 2022) as well as comparative studies of spironolactone against the more-potent antiandrogen flutamide (Cusan et al., 1994; Erenus et al., 1994; Shaw, 1996). The clinical antiandrogenic efficacy of spironolactone has been very limitedly assessed in transfeminine people to date, and is largely unknown (Angus et al., 2021). In any case, the antiandrogenic efficacy of spironolactone in cisgender women with androgen-dependent hair and skin conditions is well-established, and the medication thus does appear to be effective so long as testosterone levels are not too high (Brown et al., 2009; van Zuuren & Fedorowicz, 2016; Layton et al., 2017; Barrionuevo et al., 2018; James, Jamerson, & Aguh, 2022). In addition, higher doses of spironolactone (e.g., 300–400 mg/day) may be more useful for blocking higher testosterone levels in transfeminine people, and are allowed for by transgender care guidelines (Aly, 2020).
Consequent to spironolactone’s limited and inconsistent influence on testosterone levels and its relatively weak androgen receptor antagonism, estradiol plus spironolactone regimens will likely not be fully effective in terms of testosterone suppression for many transfeminine people. This is liable to result in suboptimal demasculinization, feminization, and breast development in these individuals. Other antiandrogenic approaches, such as bicalutamide, CPA, GnRH modulators, and high-dose estradiol monotherapy, will likely be more effective in these cases owing either to their ability to more potently block androgens or their capacity to reliably reduce testosterone levels into the female range. If testosterone levels are still too high with estradiol plus spironolactone, a switch to a different antiandrogen, increasing to a higher dosage of estradiol, or addition of a clinically antigonadotropic progestogen (e.g., non-oral progesterone or a progestin) should be considered.
Spironolactone is a strong antimineralocorticoid, or antagonist of the mineralocorticoid receptor, the biological target of the mineralocorticoid steroid hormones aldosterone and 11-deoxycorticosterone. This is an action that spironolactone shares with progesterone, although spironolactone is a much more potent antimineralocorticoid than progesterone. The mineralocorticoid receptor is involved in regulating electrolyte and fluid balances, among other roles. Spironolactone is associated with modestly lowered blood pressure, which may be considered a beneficial effect of its antimineralocorticoid activity (Martinez-Martin et al., 2022). Although spironolactone is usually well-tolerated, it can sometimes produce antimineralocorticoid side effects such as excessively lowered blood pressure, dizziness, fatigue, urinary frequency, and increased cortisol levels, among others (Kellner & Wiedemann, 2008; Kim & Del Rosso, 2012; Zaenglein et al., 2016; Layton et al., 2017; James, Jamerson, & Aguh, 2022). It has been argued by some in the online transgender community that spironolactone, via its antimineralocorticoid activity and increased cortisol levels, may increase visceral fat in transfeminine people (Aly, 2020). However, evidence does not support this hypothetical side effect at present (Aly, 2020). Available data also do not support spironolactone stunting breast development in transfeminine people or producing serious neuropsychiatric side effects, such as prominent depressive mood changes.
In people who are at-risk for hyperkalemia, dietary restriction to limit intake of potassium-rich foods is often recommended (Roscioni et al., 2012; Cupisti et al., 2018). This is often encountered in transgender health as transfeminine people being told “not to eat bananas”, which are said to be high in potassium. However, limiting dietary potassium with spironolactone to avoid hyperkalemia is theoretical and not actually evidence-based, with data so far contradicting its efficacy (St-Jules, Goldfarb, & Sevick, 2016; St-Jules & Fouque, 2020; Babich, Kalantar-Zadeh, & Joshi, 2022; St-Jules & Fouque, 2022). As such, routine restriction of dietary potassium with spironolactone may not be warranted.
Aside from its antimineralocorticoid activity, spironolactone has been reported to increase levels of LDL (“bad”) cholesterol levels and to decrease levels of HDL (“good”) cholesterol in women with PCOS (Nakhjavani et al., 2009). However, findings appear to be conflicting, with other studies not finding unfavorable influences on cholesterol levels with spironolactone (Polyzos et al., 2011). Long-term, adverse effects on cholesterol levels could result in an increase in the risk of coronary heart disease.
Spironolactone is taken orally in the form of tablets (e.g., 25, 50, and 100 mg) (Wiki). It is a prodrug of several active metabolites, including 7α-thiomethylspironolactone, 6β-hydroxy-7α-thiomethylspironolactone, and canrenone (7α-desthioacetyl-δ6-spironolactone) (Wiki). Spironolactone and these active metabolites have elimination half-lives of 1.4 hours, 13.8 hours, 15.0 hours, and 16.5 hours, respectively (Wiki). Due to the relatively short duration of elevated drug levels with spironolactone and its active metabolites (Graph), twice-daily administration of spironolactone in divided doses may be more optimal than once-daily intake and is advised (Reiter et al., 2010).
Bicalutamide
Bicalutamide (Casodex) is a nonsteroidal antiandrogen (NSAA) which acts as a potent and highly selective androgen receptor antagonist (Wiki). It is primarily used in the treatment of prostate cancer in cisgender men. Prostate cancer is an androgen-dependent cancer which antiandrogens can help to slow the progression of, and this use constitutes the vast majority of prescriptions for bicalutamide (Wiki). In addition to prostate cancer, although to a much lesser extent, bicalutamide has been used in the treatment of hirsutism (excessive facial/body hair growth), scalp hair loss, and polycystic ovary syndrome (PCOS) in cisgender women, peripheral or gonadotropin-independent precocious puberty (a rare form of precocious puberty in which antigonadotropins such as GnRH agonists are not effective) in cisgender boys, and priapism in cisgender men (Wiki). Bicalutamide is also becoming increasingly adopted for use as an antiandrogen in transfeminine people (Aly, 2020; Wiki). However, its use in transgender health is still very limited, and well-regarded transgender care guidelines either recommend against its use (Deutsch, 2016—UCSF guidelines; Coleman et al., 2022—WPATH SOC8) or are only cautiously permissive of its use (Thompson et al., 2021—Fenway Health guidelines). This is due to a lack of studies of bicalutamide in transfeminine people and its potential risks. Nonetheless, a small but growing number of clinicians are using bicalutamide in transfeminine people or are willing to prescribe it, with these clinicians located particularly in the United States. A single small clinical study has assessed bicalutamide in transfeminine people so far, specifically as a puberty blocker in 13 transfeminine adolescents who were denied insurance coverage for GnRH agonists (Neyman, Fuqua, & Eugster, 2019). (Update: More studies of bicalutamide in transfeminine people have since been published, see Aly (2020).)
Bicalutamide is a much more potent androgen receptor antagonist than either spironolactone or CPA (Wiki; Neyman, Fuqua, & Eugster, 2019). It is typically used in transfeminine people at a dosage of 25 to 50 mg/day, although this dosage has been arbitrarily selected and is not based on clinical data. Nonetheless, due to its relatively high potency as an androgen receptor antagonist and concomitant suppression of testosterone levels by estradiol, these doses may be adequate for testosterone blockade for many transfeminine people. At higher doses (>50 mg/day), bicalutamide is able to substantially block male-range testosterone levels (>300 ng/dL [>10.4 nmol/L]) based on studies of bicalutamide monotherapy in cisgender men with prostate cancer (Wiki). This is something that spironolactone and CPA are not capable of in the same way. Owing to its selectivity for the androgen receptor, bicalutamide has no off-target hormonal activity and produces almost no side effects in women (Wiki; Erem, 2013; Moretti et al., 2018). The only apparent side effect of bicalutamide in a rigorous clinical trial of the drug for hirsutism in cisgender women was significantly increased total and LDL (“bad”) cholesterol levels (Moretti et al., 2018). Hence, bicalutamide tends to be very well-tolerated. The relative lack of side effects with bicalutamide is in contrast to other antiandrogens like spironolactone and CPA, which are not pure androgen receptor antagonists and have off-target hormonal actions like antimineralocorticoid activity or strong progestogenic activity with consequent side effects and risks.
As a selective androgen receptor antagonist, bicalutamide taken by itself does not decrease testosterone production or levels but rather increases them (Wiki). This is due to a loss of androgen receptor-mediated negative feedback on gonadotropin secretion and a consequent compensatory upregulation of gonadal testosterone production (Wiki). Bicalutamide more than blocks the effects of any increase in testosterone it causes, and in fact fundamentally cannot increase testosterone levels more than it can block them (Wiki). In addition, increases in testosterone levels with bicalutamide will be blunted or abolished if it is combined with an adequate dose of an antigonadotropin such as estradiol (Wiki; Wiki). Since estradiol is made from testosterone in the body, bicalutamide taken alone also preserves and increases estradiol production and levels (Wiki). Because of this, although bicalutamide has no other important intrinsic hormonal activity besides its antiandrogenic activity, it produces robust indirect estrogenic effects including feminization and breast development even when it is not combined with estrogen (Wiki; Wiki; Neyman, Fuqua, & Eugster, 2019). This has important implications for the use of bicalutamide as a puberty blocker in transfeminine adolescents, as bicalutamide does not actually block puberty like conventional puberty blockers (GnRH agonists) but instead has the effect of dose-dependently converting male puberty into female puberty (Wiki; Neyman, Fuqua, & Eugster, 2019).
Bicalutamide has certain health risks, which has been a major reason that it has not been more readily adopted in transfeminine hormone therapy (Aly, 2020). It has a small risk of liver toxicity (Wiki; Aly, 2020) and of lung toxicity (Wiki). Abnormal liver function tests (LFTs), such as elevated liver enzymes and elevated bilirubin, occurred in about 3.4% of men with bicalutamide monotherapy plus standard care versus 1.9% of men with placebo plus standard care in the Early Prostate Trial (EPC) clinical programme after 3.0 years of follow-up (Wiki). In clinical trials, treatment with bicalutamide had to be discontinued in about 0.3 to 1.5% of men due to LFTs that became too highly elevated and could have progressed to serious liver toxicity (Wiki). To date, there are around 10 published case reports of serious liver toxicity, including cases of death, with bicalutamide, all of which have been in men with prostate cancer (Wiki; Table; Aly, 2020). There have also been a few unpublished reports of serious liver toxicity including deaths with bicalutamide in transfeminine people (Aly, 2020). However, these reports have not been confirmed, and they may or may not be reliable. In addition to the preceding reports, hundreds of additional instances of liver complications in people taking bicalutamide exist in the United States FDA Adverse Event Reporting System (FAERS) database (Wiki; FDA). Abnormal LFTs with bicalutamide usually occur within the first 3 to 6 months of treatment (Kolvenbag & Blackledge, 1996; Casodex FDA Label), and all case reports of liver toxicity with bicalutamide have had an onset of less than 6 months (Table). The liver toxicity of bicalutamide is not known to be dose-dependent across its clinically used dose range (Wiki). Abnormal LFTs have occurred with bicalutamide (at rates of 2.9% to 11.4%) even at relatively low doses in cisgender women (e.g., 10–50 mg/day) (de Melo Carvalho, 2022). Due to its risk of liver toxicity, periodic liver monitoring is strongly advised with bicalutamide, especially within the first 6 months of treatment. Possible signs of liver toxicity include nausea, vomiting, abdominal pain, fatigue, appetite loss, flu-like symptoms, dark urine, and jaundice (yellowing of the skin/eyes) (Wiki).
In terms of its lung toxicity risk, bicalutamide has been associated rarely with interstitial pneumonitis, which can lead to pulmonary fibrosis and can be fatal, and also less often with eosinophilic lung disease (Wiki; Table). As of writing, 15 published case reports of interstitial pneumonitis and 2 case reports of eosinophilic lung disease in association with bicalutamide therapy exist, likewise all in men with prostate cancer (Table). As with liver toxicity, hundreds of additional cases of interstitial pneumonitis in people taking bicalutamide exist in the United States FAERS database (Wiki; FDA). It has been estimated that interstitial pneumonitis with bicalutamide occurs at a rate of around 1 in 10,000 people, although this may be an underestimate due to under-reporting (Wiki; Ahmad & Graham, 2003). Asian people may be especially likely to experience lung toxicity with bicalutamide and other NSAAs, as much higher incidences have been observed in this population (Mahler et al., 1996; Wu et al., 2022). There is no laboratory test for routine monitoring of lung changes with bicalutamide. Possible signs of relevant lung toxicity include dyspnea (difficulty breathing or shortness of breath), coughing, and pharyngitis (inflammation of the throat, typically manifesting as sore throat) (Wiki).
Aside from liver and lung toxicity, bicalutamide monotherapy has been found in cisgender men with prostate cancer to increase the risk of death due to causes other than prostate cancer (Iversen et al., 2004; Iversen et al., 2006; Wellington & Keam, 2006; Jia & Spratt, 2022; Wiki). This led to marketing authorization of bicalutamide for treatment of the earliest stage of prostate cancer being revoked and to the drug being abandoned for this use (Wiki). Bicalutamide remains approved and used for treatment of later stages of prostate cancer, as the antiandrogenic benefits of bicalutamide against prostate cancer outweigh any adverse influence it has on non-prostate-cancer mortality in these more severe stages. The mechanisms underlying the increase in risk of death with bicalutamide in men are unknown (Wiki). It is also unclear whether bicalutamide could likewise increase the risk of death in transfeminine people. Limitations of generalizing these studies to transfeminine people include the men in the trials being relatively old and ill, a relatively high dosage of bicalutamide (150 mg/day) being used in the trials for an extended duration (e.g., 5 years), the question of whether the risks were due to androgen deprivation or to specific drug-related toxicity of bicalutamide, and estradiol levels with bicalutamide monotherapy in men with prostate cancer being only about 30 to 50 pg/mL (110–184 pmol/L) (Wiki). The preceding estradiol levels are well above castrate levels and are sufficient for a substantial degree of estrogenic effect, but are nonetheless below those recommended for transfeminine people and potentially needed for full sex-hormone replacement (which are ≥50 pg/mL [≥184 pmol/L]). In any case, as the specific mechanisms underlying the increased mortality risk with bicalutamide seen in men with prostate cancer are uncertain, and as clinical safety data showing that the risk does not generalize do not exist, it remains a possibility that bicalutamide could also increase the risk of death in transfeminine people.
Bicalutamide is taken orally in the form of tablets (e.g., 50 and 150 mg) (Wiki). Due to saturation of absorption in the gastrointestinal tract, the oral bioavailability of bicalutamide progressively starts to decrease above a dosage of about 150 mg/day, and there is no further increase in bicalutamide levels above 300 mg/day (Wiki; Graph). Bicalutamide has a very long elimination half-life of about 6 to 10 days (Wiki; Graphs). As a result, it does not necessarily have to be taken daily, and can be dosed less often (in proportionally higher doses)—for instance twice weekly or even once weekly—if this is more convenient or otherwise desired. Due to its long half-life, bicalutamide requires about 4 to 12 weeks to fully reach steady-state levels (Wiki; Graph; Wiki). However, about 50% of steady state is reached within 1 week of administration of bicalutamide, and about 80 to 90% of steady state is reached after 3 to 4 weeks (Wiki; Graph; Wiki). Loading doses of bicalutamide can be taken to reach steady state more quickly if desired. Animal studies originally suggested that bicalutamide did not cross the blood–brain barrier and hence was peripherally selective (i.e., did not block androgen receptors in the brain) (Wiki). However, subsequent clinical studies found that this was not similarly the case in humans, in whom bicalutamide shows clear and robust centrally mediated antiandrogenic effects (Wiki).
Older NSAAs related to bicalutamide like flutamide (Eulexin) and nilutamide (Anandron, Nilandron) have much greater risks in comparison to bicalutamide and should not be used in transfeminine people. Nilutamide was previously characterized as an antiandrogen in transfeminine people in several studies, but was not further pursued probably due to its very high incidence of lung toxicity and other side effects (Aly, 2020; Wiki; Wiki). Flutamide has been used limitedly as an antiandrogen in transfeminine people in the past, but should no longer be used due to a much higher risk of liver toxicity than bicalutamide as well as other side effects and drawbacks (Aly, 2020; Wiki). Other newer and more-potent NSAAs like enzalutamide (Xtandi), apalutamide (Erleada), and darolutamide (Nubeqa) also have risks and have been studied and used little outside of prostate cancer to date.
5α-Reductase Inhibitors
Testosterone is converted into DHT within certain tissues in the body (Swerdloff et al., 2017). DHT is an androgen metabolite of testosterone with several-fold higher activity than testosterone. The transformation of testosterone into DHT is mediated by the enzyme 5α-reductase. The tissues in which 5α-reductase is present and testosterone is converted into DHT are limited but most importantly include the skin, hair follicles, and prostate gland. Although DHT is more potent than testosterone, it is thought to have minimal biological role as a circulating hormone (Horton, 1992; Swerdloff et al., 2017). Instead, testosterone serves as the main circulating androgen, and the role of DHT is thought to be mainly via local metabolism and potentiation of testosterone into DHT within certain tissues.
5α-Reductase inhibitors (5α-RIs), such as finasteride (Proscar, Propecia) and dutasteride (Avodart), inhibit 5α-reductase and thereby block the conversion of testosterone into DHT. This results in marked decreases in circulating and within-tissue levels of DHT. Due to the primary role of DHT as a mediator in tissues rather than as circulating hormone, the antiandrogenic efficacy of 5α-RIs is limited. This is evidenced by the fact that they are well-tolerated in cisgender men and do not cause notable demasculinization in these individuals (Hirshburg, 2016). The medical use of 5α-RIs is mainly restricted to the treatment of scalp hair loss in men and women, hirsutism (excessive facial/body hair) in women, and prostate enlargement in men. They might also be useful for acne in women, but evidence of this is very limited (Wiki). Due to their specificity, 5α-RIs are inappropriate as general antiandrogens in transfeminine people. Moreover, DHT levels decrease in tandem with testosterone levels with suppression of testosterone production in transfeminine hormone therapy, and routine use of 5α-RIs in transfeminine people with testosterone levels within the female range is of limited usefulness and can be considered unnecessary (Gooren et al., 2016; Irwig, 2020; Prince & Safer, 2020; Glintborg et al., 2021). In any case, 5α-RIs may be useful in transfeminine people on hormone therapy who have persistent body hair growth or scalp hair loss—as they have been shown to be in cisgender women (Barrionuevo et al., 2018; Prince & Safer, 2020). However, it is notable that evidence of effectiveness in cisgender women is better for androgen receptor antagonists for such indications (van Zuuren et al., 2015). This is intuitive as androgen receptor antagonists block both testosterone and DHT whereas 5α-RIs only prevent conversion of testosterone into DHT. Hence, although 5α-RIs strongly reduce or eliminate DHT and their net effect is antiandrogenic, they do not decrease testosterone levels and in fact increase them.
There are three subtypes of 5α-reductase. Dutasteride inhibits all three subtypes of 5α-reductase whereas finasteride only inhibits two of the subtypes. As a result of this, dutasteride is a more complete 5α-RI than finasteride. Dutasteride decreases DHT levels in the blood by up to 98% while finasteride can only decrease them by around 65 to 70%. As nearly all circulating DHT originates from synthesis in peripheral tissues, these decreases indicate parallel reductions in tissue DHT production (Horton, 1992). In accordance with these findings, dutasteride has been found to be more effective than finasteride in the treatment of scalp hair loss in men (Zhou et al., 2018; Dhurat et al., 2020; Wiki). For these reasons, although both finasteride and dutasteride are effective 5α-RIs, dutasteride may be the preferable choice if a 5α-RI is used (Zhou et al., 2018; Dhurat et al., 2020).
A potentially undesirable effect of 5α-RIs in transfeminine people is that they may increase circulating testosterone levels to a degree in those in whom testosterone production isn’t fully suppressed (Leinung, Feustel, & Joseph, 2018; Aly, 2019; Traish et al., 2019; Irwig, 2020; Glintborg et al., 2021). It appears that DHT adds significantly to negative feedback on gonadotropin secretion in the pituitary gland in people with testes who have low testosterone levels relative to the normal male range (Traish et al., 2019). The therapeutic implications of this for transfeminine people, if any, are uncertain.
Clinical dose-ranging studies have found that lower doses of finasteride and dutasteride than are typically used still provide substantial or near-maximal 5α-reductase inhibition (Gormley et al., 1990; Vermeulen et al., 1991; Sudduth & Koronkowski, 1993; Drake et al., 1999; Roberts et al., 1999; Clark et al., 2004; Frye, 2006; Olsen et al., 2006; Harcha et al., 2014; Kuhl & Wiegratz, 2017). In one study with finasteride for instance, DHT levels decreased by 49.5% at 0.05 mg/day, 68.6% at 0.2 mg/day, 71.4% at 1 mg/day, and 72.2% at 5 mg/day (Drake et al., 1999). Parallel reductions in DHT levels were seen locally in the scalp (Drake et al., 1999). In a study with dutasteride, DHT levels were decreased by 52.9% at 0.05 mg/day, 94.7% at 0.5 mg/day, 97.7% at 2.5 mg/day, and 98.4% at 5 mg/day (Clark et al., 2004). Based on these findings, 5α-RIs can potentially be taken at lower doses to help reduce medication costs if needed. Finasteride tablets can be split to achieve smaller doses. Conversely, dutasteride cannot be split as it is formulated as an oil capsule. However, dutasteride has a long half-life, and instead of dividing pills, it can be taken less frequently (e.g., once every few days) as a means of reducing dosage.
5α-Reductase inhibitors are taken orally in the form of tablets and capsules. Compoundedtopical formulations of finasteride also exist (Marks et al., 2020). However, caution is advised with these preparations as they have been found to be excessively dosed and to produce equivalent systemic DHT suppression as oral finasteride formulations (Marks et al., 2020). Lower-concentration formulations of topical finsteride on the other hand may be more locally selective (Marks et al., 2020).
Table 8: Available forms and recommended doses of 5α-reductase inhibitors for transfeminine people:
GnRH agonists and antagonists (GnRHa), also known as GnRH receptor agonists and antagonists or GnRH modulators, are antiandrogens which work by preventing the effects of GnRH in the pituitary gland and thereby suppressing LH and FSH secretion. Receptor agonists normally activate receptors while receptor antagonists block and thereby inhibit the activation of receptors. Due to a physiological quirk however, GnRH agonists and antagonists have the same effects in the pituitary gland. This is because GnRH is secreted in pulses under normal physiological circumstances, and when the GnRH receptor is unnaturally activated in a continuous manner, as with exogenous GnRH agonists, the GnRH receptor in the pituitary gland is strongly desensitized to the point of becoming inactive. Consequently, both GnRH agonists and GnRH antagonists have the effect of abolishing gonadal sex hormone production. This, in turn, reduces testosterone levels into the castrate or normal female range (both <50 ng/dL or <1.7 nmol/L) in people with testes. GnRHa are like a reversible gonadectomy, and for this reason, are also sometimes referred to as “medical castration”. Provided that an estrogen is taken in combination with a GnRHa to prevent sex hormone deficiency, these medications have essentially no known side effects or risks. For these reasons, GnRHa are the ideal antiandrogens for use in transfeminine people.
GnRHa are widely used to suppess puberty in adolescent transgender individuals. Unfortunately however, they are very expensive (e.g., ~US$10,000 per year) and medical insurance does not usually cover them for adult transgender people. Consequently, GnRHa are not commonly used in adult transfeminine people at this time. An exception is in the United Kingdom, where GnRH agonists are covered for all adult transgender people by the National Health Service (NHS). Another exception is buserelin (Suprefact), which has become available very inexpensively in its nasal spray form from certain Eastern European online pharmacies in recent years (Aly, 2018).
GnRH agonists cause a brief flare in testosterone levels at the start of therapy prior to the GnRH receptors in the pituitary gland becoming desensitized (Wiki). Testosterone levels increase by up to about 1.5- to 2-fold for about 1 week and then decrease thereafter (Wiki). Castrate or female-range levels of testosterone are generally reached within 2 to 4 weeks (Wiki). In contrast to GnRH agonists, there is no testosterone flare with GnRH antagonists and testosterone levels start decreasing immediately, reaching castrate levels within a few days (Wiki; Graph). This is because GnRH antagonists work by blocking the GnRH receptor without initially activating it, and hence desensitization of the receptor is not necessary for their action. If desired, the testosterone flare at the initiation of GnRH agonist therapy can be prevented or blunted with the use of antigonadotropins, for instance estrogens and progestogens, as well as with potent androgen receptor antagonists such as bicalutamide (Wiki).
GnRH agonists must be injected subcutaneously or intramuscularly once per day or once every one to six months depending on the formulation employed (buserelin, goserelin, leuprorelin, triptorelin). Alternatively, they can be surgically implanted once a year (histrelin, leuprorelin) or used as a nasal spray two to three times per day (buserelin, nafarelin). The first GnRH antagonists were developed for use by once-monthly intramuscular or subcutaneous injection (abarelix, degarelix). More recently, orally administered GnRH antagonists such as elagolix and relugolix have been introduced for medical use. They are taken in the form of tablets once or twice daily.
Table 9: Available forms and recommended doses of GnRH agonists for transfeminine people:
a 500 μg 3x/day for the first week then 200 μg/day. b 800 μg 3x/day for the first week then 400 μg 3x/day. c 500 μg 2x/day can be used instead of 400 μg 3x/day but is less effective (70% decrease in testosterone levels (to ~180 ng/dL [6.2 nmol/L]) instead of 90% decrease (to ~50 ng/dL [1.7 nmol/L]) per available studies of buserelin in men with prostate cancer) (Aly, 2018; Wiki).
Table 10: Available forms and recommended doses of GnRH antagonists for transfeminine people:
a First month is 240 mg then 80 mg per month thereafter. b 150 mg 1x/day is less effective than 200 mg 2x/day (which provides full gonadal sex-hormone suppression in cisgender women) (Wiki). c 80–120 mg/day for full gonadal sex-hormone suppression and 20–40 mg/day for substantial but partial gonadal sex-hormone suppression (MacLean et al., 2015; DailyMed).
Other Hormonal Medications
Androgens and Anabolic Steroids
In addition to estrogens, progestogens, and antiandrogens, androgens/anabolic steroids (AAS) are sometimes added to transfeminine hormone therapy. This is when testosterone levels are low (e.g., below the female average of 30 ng/dL [1.0 nmol/L]) and androgen replacement is desired. It has been proposed that adequate levels of testosterone may provide benefits such as increased sexual desire, improved mood and energy, positive effects on skin health and cellulite (Avram, 2004), and increased muscle size and strength (Huang & Basaria, 2017). However, there is insufficient clinical evidence to support such benefits at present, and androgens can produce adverse effects in cisgender women and transfeminine people, for instance acne, hirsutism, scalp hair loss, and masculinization (Wiki). For transfeminine people who nonetheless desire androgen replacement therapy, possible options for androgen medications include testosterone and its esters, dehydroepiandrosterone (DHEA; prasterone), and nandroloneesters such as nandrolone decanoate (ND) (Aly, 2020; Table), among others.
Monitoring of Therapy
Transfeminine people on hormone therapy should undergo regular laboratory monitoring in the form of blood work to assess efficacy and monitor for safety. Total estradiol levels and total testosterone levels should be measured to assess the effectiveness of therapy—that is, whether hormone levels are in appropriate ranges for cisgender females—and determine whether medication adjustments may be necessary. Levels of free testosterone, free estradiol, estrone (E1), dihydrotestosterone (DHT), luteinizing hormone (LH), follicle-stimulating hormone (FSH), and sex hormone-binding globulin (SHBG) can also be measured to provide further information although they’re not absolutely necessary. If progesterone is used as a part of hormone therapy, progesterone levels can be measured to provide insight on the degree of progesterone exposure. In addition to hormone blood tests, transfeminine people can monitor their physical changes with hormone therapy, such as breast development and other aspects of feminization, using various physical and digital measurement methods (e.g., Wiki).
Certain therapeutic situations can result in inaccurate lab blood work results. Monitoring of progesterone levels with oral progesterone using immunoassay-based blood tests can result in falsely high readings for progesterone levels due to cross-reactivity with high levels of progesterone metabolites such as allopregnanolone (Aly, 2018; Wiki). Instead of immunoassay-based tests, mass spectrometry-based tests should be used to determine progesterone levels with oral progesterone (Aly, 2018; Wiki). Conversely, either type of test may be used to measure progesterone levels with non-oral progesterone therapy. High-dose biotin (vitamin B7) supplements can interfere with the accuracy of immunoassay-based hormone blood tests, causing falsely low or falsely high readings (Samarasinghe et al., 2017; Avery, 2019; Bowen et al., 2019; FDA, 2019; Luong, Male, & Glennon, 2019). Transdermal estradiol formulations applied to the arm can result in contamination of blood draws taken from the same arm and can result in falsely high readings for estradiol levels (Vihtamäkia, Luukkaala, & Tuimala, 2004).
Certain cancers are known to be hormone-sensitive and their incidence can be influenced by hormone therapy. Screening for breast and prostate cancer is recommended in transfeminine people (Sterling & Garcia, 2020; Iwamoto et al., 2021). The risk of breast cancer appears to be dramatically increased with transfeminine hormone therapy, perhaps especially with progestogens (Aly, 2020). However, the risk still remains lower than in cisgender women (Aly, 2020). The incidence of prostate cancer is greatly decreased with hormone therapy in transfeminine people as a consequence of androgen deprivation, but the risk is not abolished and prostate cancer can still occur (de Nie et al., 2020). The prostate gland is not removed with vaginoplasty, so transfeminine people who have undergone vaginoplasty will also require monitoring for prostate cancer still. Testicular cancer is not known to be a hormone-dependent cancer and its incidence does not appear to be increased with hormone therapy in transfeminine people (Bensley et al., 2021; de Nie et al., 2021; Jacoby et al., 2021).
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+An Introduction to Hormone Therapy for Transfeminine People - Transfeminine Science
An Introduction to Hormone Therapy for Transfeminine People
By Aly | First published August 4, 2018 | Last modified December 17, 2025
Abstract / TL;DR
Sex hormones such as estrogen, testosterone, and progesterone are produced by the gonads. The sex hormones mediate the development of the secondary sexual characteristics. Testosterone causes masculinization, while estradiol causes feminization and breast development. Males have high amounts of testosterone, while females have low testosterone but high amounts of estradiol. These hormonal differences are responsible for the physical differences between males and females. Sex hormones and other hormonal medications are used in transfeminine people to shift the hormonal profile from a male-typical one to a female-typical profile. This causes feminization and demasculinization and allows for alleviation of gender dysphoria. The changes caused by transfeminine hormone therapy occur over a period of months to years. There are many different types and forms of hormonal medications, and these medications can be administered by a variety of different routes. Examples include as pills taken by mouth, as patches or gel applied to the skin, and as injections, among others. Different hormonal medications, routes, and doses have differences in efficacy, side effects, risks, costs, convenience, and availability. Hormone therapy should ideally be regularly monitored in transfeminine people with blood tests to ensure effectiveness and safety and to allow for adjustment as necessary.
The Sex Hormones
Types and Effects
The sex hormones include the estrogens (E), progestogens (P), and androgens. A person’s hormonal profile is a product of the type of gonads that they are born with. Natal males have testes while natal females have ovaries. Testes produce large amounts of androgens and small amounts of estrogens whereas ovaries produce high amounts of estrogens and progesterone and low amounts of androgens.
Progestogens have essentially no known role in feminization or pubertal breast development. Rather than acting as mediators of feminization, progestogens have important effects in the female reproductive system and are essential hormones during pregnancy (Wiki). They also oppose the actions of estrogens in certain parts of the body, such as the uterus, vagina, and breasts (Wiki).
In addition to their effects on the body, sex hormones have actions in the brain. These actions influence cognition, emotions, and behavior. For instance, androgens produce pronounced sexual desire and arousal (including spontaneous erections) in men, while estrogens appear to be the major hormones responsible for sexual desire in women (Cappelletti & Wallen, 2016). As another example, testosterone levels have been negatively associated with agreeableness, whereas estrogen levels have been positively associated with this characteristic (Treleaven et al., 2013). Sex hormones also have important effects on health, which can be both positive and negative. For instance, estrogens maintain bone strength and likely protect against heart disease in cisgender women (NAMS, 2022), but also increase the risk of breast cancer (Aly, 2020) and can increase the risk of blood clots (Aly, 2020).
Estrogens, progestogens, and androgens also have antigonadotropic effects. That is, they inhibit the gonadotropin-releasing hormone (GnRH)-induced secretion of the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), from the pituitary gland in the brain. The gonadotropins signal the gonads to make sex hormones and to supply the sperm and egg cells necessary for fertility. Hence, lower levels of the gonadotropins will result in reduced gonadal sex hormone production and diminished fertility. If gonadotropin levels are sufficiently suppressed, the gonads will no longer make sex hormones at all and fertility will cease. The vast majorities of the quantities of estradiol, testosterone, and progesterone in the body are produced by the gonads. Most of the small remaining amounts of these hormones are produced via the adrenal glands of the kidneys.
Normal Hormone Levels
In cisgender females, the sex hormones are largely absent during childhood, gradually ramp up in production in late childhood and adolescence, are present in a cyclical manner during adulthood, and then largely stop being produced following the menopause. Hormone levels vary substantially but in a predictable manner during the normal menstrual cycle in adult premenopausal women. The menstrual cycle lasts about 28 days on average and consists of the following parts:
Luteal phase—latter half of the cycle or days 14–28
Hormone levels during the menstrual cycle are shown in the following graph:
Figure 1: Median estradiol and progesterone levels throughout the menstrual cycle in premenopausal cisgender women (Stricker et al., 2006; Abbott, 2009). The horizontal dashed lines are the average levels over the spanned periods. Other figures available elsewhere show variation between individuals (Graph; Graph; Graph).
As can be seen in the graph, estradiol levels are relatively low and progesterone levels are very low during the follicular phase; estradiol but not progesterone levels briefly surge to very high levels and trigger ovulation during mid-cycle; and estradiol and progesterone levels both undergo a bump and are relatively high during the luteal phase (though estradiol is not as high as during the mid-cycle peak).
The table below shows the circulating levels and production rates of estradiol, progesterone, and testosterone in women and men and allows for comparison between them.
Table 1: Ranges for circulating levelsa and estimated production ratesb of the major sex hormones:
Mean integrated estradiol levels are around 100 pg/mL (367 pmol/L) in premenopausal women and around 25 pg/mL (92 pmol/L) in men. The 95% range for mean estradiol levels in women is around 50 to 250 pg/mL (180–918 pmol/L) (e.g., Abbott, 2009 (Graph); Verdonk et al., 2019 (Graph)). The average production of estradiol by the ovaries in premenopausal women is about 6 mg over the course of one menstrual cycle (i.e., one month) (Rosenfield et al., 2008). This corresponds to a mean rate of about 200 μg/day. Estradiol levels increase slowly during normal female puberty, when breast development and feminization take place. Mean estradiol levels during the different stages of female puberty are quite low—less than about 50 to 60 pg/mL (180–220 pmol/L) until late puberty (Aly, 2020). In postmenopausal women, whose ovaries no longer produce considerable quantities of estrogens, estradiol levels are generally less than 10 to 20 pg/mL (37–73 pmol/L) (Nakamoto, 2016). Estradiol levels below 50 pg/mL (184 pmol/L) in adults are concentration-dependently associated with menopausal symptoms, including hot flashes, depressive mood changes, defeminization (e.g., breast atrophy, loss of feminine fat distribution), accelerated skin aging, and bone density loss with increased risk of bone fracture.
Mean testosterone levels are around 30 ng/dL (1.0 nmol/L) in women and 600 ng/dL (21 nmol/L) in men. Based on these values, testosterone levels are on average about 20-fold higher in men than in women. In men who have undergone gonadectomy (castration or surgical gonadal removal), testosterone levels are similar to those in women (<50 ng/dL [1.7 nmol/L]) (Nishiyama, 2014; Getzenberg & Itty, 2020). The mean or median levels of testosterone in women with polycystic ovary syndrome (PCOS), who often have clinically significant symptoms of androgen excess (e.g., excessive facial/body hair growth), range from 41 to 75 ng/dL (1.4–2.6 nmol/L) per different studies (Balen et al., 1995; Steinberger et al., 1998; Legro et al., 2010; Loh et al., 2020). Hence, it appears that even testosterone levels that are marginally elevated relative to normal female levels may produce undesirable androgenic effects.
The goal of hormone therapy for transfeminine people, otherwise known as feminizing hormone therapy (FHT) or (more in the past) as male-to-female (MtF) hormone replacement therapy (HRT), is to produce feminization and demasculinization of the body as well as alleviation of gender dysphoria. Medication therapy with sex hormones and other sex-hormonal medications is used to mediate these changes. Transfeminine people are given estrogens, progestogens, and antiandrogens (AAs) to supersede gonadal sex hormone production and shift the hormonal profile from male-typical to female-typical.
Transfeminine hormone therapy aims to achieve estradiol and testosterone levels within the normal female range. Commonly recommended ranges for transfeminine people in the literature are 100 to 200 pg/mL (367–734 pmol/L) for estradiol levels and less than 50 ng/dL (1.7 nmol/L) for testosterone levels (Table). However, higher estradiol levels of more than 200 pg/mL (734 pmol/L) can be useful in transfeminine hormone therapy to help suppress testosterone levels. Lower estradiol levels (≤50–60 pg/mL [≤180–220 pmol/L]) are recommended and more appropriate for pubertal and adolescent transfeminine individuals. Sex hormone levels in the blood can be measured with blood tests, in which blood is drawn from a vein using a needle and then analyzed in a laboratory. This is useful in transfeminine people to ensure that the hormonal profile has been satisfactorily altered in line with therapeutic goals—specifically that hormone levels are within female ranges.
Gonadal Suppression
At sufficiently high exposure, estrogens and androgens are able to completely suppress gonadal sex hormone production, while progestogens by themselves are able to partially but substantially suppress gonadal sex hormone production. More specifically, studies in cisgender men and transfeminine people have found that estradiol levels of around 200 pg/mL (734 pmol/L) suppress testosterone levels by about 90% on average (to ~50 ng/dL [1.7 nmol/L]), while estradiol levels of around 500 pg/mL (1,840 pmol/L) suppress testosterone levels by about 95% on average (to ~20–30 ng/dL [0.7–1.0 nmol/L]) (Gooren et al., 1984 [Graph]; Herndon et al., 2023 [Discussion]; Wiki; Graphs). Estradiol levels of below 200 pg/mL (734 pmol/L) also suppress testosterone levels, although to a reduced extent compared to higher levels (Aly, 2019; Krishnamurthy et al., 2023; Slack et al., 2023). In one large study in transfeminine people, the rates of adequate testosterone suppression (to testosterone levels of <50 ng/dL or <1.7 nmol/L) were 24% of individuals at estradiol levels of <100 pg/mL (367 pmol/L), 58% at 100 to 200 pg/mL (367–734 pmol/L), and 77% at >200 pg/mL (>734 pmol/L) (Krishnamurthy et al., 2023).
Figure 2: Estradiol and testosterone levels after a single injection of 320 mg polyestradiol phosphate (PEP) (a long-acting prodrug of estradiol) in men with prostate cancer (Stege et al., 1996). The maximal decrease in testosterone levels occurred with estradiol levels of greater than 200 pg/mL (734 pmol/L) and was about 90% (to roughly 50 ng/dL [1.7 nmol/L]). This figure demonstrates the ability of estradiol to concentration-dependently suppress gonadal testosterone production and circulating testosterone levels in people with testes.
Progestogens on their own are able to maximally suppress testosterone levels by about 50 to 70% (to ~150–300 ng/dL [5.2–10.4 nmol/L] on average) (Aly, 2019; Wiki). In combination with relatively small amounts of estrogen however, there is synergism in the antigonadotropic effect—the suppression of gonadal testosterone production with maximally effective doses of progestogens becomes complete, and testosterone levels are reduced by about 95% (to ~20–30 ng/dL [0.7–1.0 nmol/L]) (Aly, 2019). Hence, the combination of an estrogen and a progestogen can be used to achieve maximal testosterone suppression at lower doses than would be necessary if an estrogen or progestogen were used alone.
The antigonadotropic effects of estrogens and progestogens are taken advantage of in transfeminine hormone therapy to suppress gonadal testosterone production and attain testosterone levels that are more consistent with those in cisgender women. It should be noted that the preceding numbers on testosterone suppression with estrogens and progestogens are averages and there is significant variation between individuals in terms of testosterone suppression. In other words, some may need more or less in terms of hormonal dosages to achieve the same decrease in testosterone levels.
Effects and Timeline
During normal puberty in both males and females, sex hormone exposure increases slowly over a period of several years (Aly, 2020). In relation to this, sexual maturation occurs gradually during normal puberty. In non-adolescent transgender people, adult or higher amounts of hormones are generally administered right away, and this can result in changes in secondary sex characteristics happening more quickly. Most of the effects of feminizing hormone therapy in transfeminine people onset within 1 to 6 months of commencing treatment and complete within 1 to 3 years. The table below is reproduced from literature sources with slight modification and is commonly cited as a timeline of the effects (Table). It is based on a mixture of anecdotal clinical experience, expert opinion, and available clinical studies of hormone therapy in transfeminine people. Due to limited research characterizing the effects of transfeminine hormone therapy at present, the table may or may not be completely accurate.
Table 2: Effects of hormone therapy at typical doses in adult transfeminine people (Wiki):
Effect
Onseta
Completiona
Permanency
Breast development
2–6 months
2–3 years
Permanent
Reduced and slowed growth of facial and body hair
3–12 months
>3 yearsb
Reversible
Cessation and reversal of scalp hair loss
1–3 months
1–2 years
Reversible
Softening of skin and decreased skin oiliness and acne
3–6 months
Unknown
Reversible
Redistribution of body fat in a feminine pattern
3–6 months
2–5 years
Reversible
Decreased muscle mass and strength
3–6 months
1–2 yearsc
Reversible
Widening and rounding of the pelvisd
Unknown
Unknown
Permanent
Changes in mood, emotionality, and behavior
Immediate
Unknown
Reversible
Decreased sex drive and spontaneous erections
1–3 months
3–6 months
Reversible
Erectile dysfunction and decreased ejaculate volume
1–3 months
Variable
Reversible
Decreased sperm production and infertility
Unknown
>3 years
Mixede
Decreased testicular volume
3–6 months
2–3 years
Unknown
Voice changes (e.g., more feminine pitch/resonance)
Nonef
N/A
N/A
Height changes (e.g., decrease)
Noneg
N/A
N/A
a Effects in general may vary significantly between individuals due to factors like genetics, diet/nutrition, hormone levels, etc. b Hormone therapy usually has little influence on facial hair density in transfeminine people. Complete removal of facial and body hair can be achieved with laser hair removal and electrolysis. Temporary hair removal can be achieved with shaving, epilating, waxing, and other methods. c Reduced muscle mass and strength may vary significantly depending on amount of physical exercise. d Pelvic changes occur only in young individuals who have not yet completed growth plate closure (may not occur at all in post-adolescent people). e Only estrogens, particularly at high doses, seem to have the potential for long-lasting or irreversible infertility; impaired fertility caused by antiandrogens is usually readily reversible with discontinuation. fVoice training can be an effective means of feminizing the voice. g Height attainment may be reduced in adolescents, but height is not meaningfully changed or reduced in adults per clinical data (Gooren & Bunck, 2004; Ingram & Thomas, 2019; Hilton & Lundberg, 2020; Talathi et al., 2025).
Breast Development
Breast development is among the most anticipated effects of hormone therapy in transfeminine people (Masumori et al., 2021; Grock et al., 2024). This relates to the key significance of breasts as a feminine characteristic, component of sexual attractiveness, and signal of sex and gender. Breast growth in transfeminine people usually starts within 1 to 6 months and completes over a period of 1 to 3 years (e.g., de Blok et al., 2021). The developed breasts of transfeminine people are highly variable in terms of size and shape, as with natal women (de Blok et al., 2021). Based on available high-quality clinical studies, transfeminine people tend to have much smaller mature breasts than those of natal women on average, and this appears to be the case regardless of hormonal regimen or age at which hormone therapy is commenced (e.g., de Blok et al., 2021; Boogers et al., 2025). The reasons for this are unknown, but one key possibility, observed in animals, is that prenatal androgen exposure limits subsequent breast growth potential. Despite usually modest breast development, many transfeminine people still express overall satisfaction with their breasts (de Blok et al., 2021; Boogers et al., 2025).
Beyond ensuring adequate testosterone suppression and maintaining sufficient estradiol levels above a specific low threshold, there are currently no known or substantiated methods to permanently enhance or optimize breast development. However, research suggests that avoiding high or excessive doses of estradiol and progestogens may be beneficial. In addition, high levels of estradiol, progesterone, and/or prolactin, as with the normal menstrual cycle and pregnancy, are known to induce temporary and reversible breast tenderness and enlargement, for instance due to local fluid retention and lobuloalveolar maturation (Aly, 2020). However, the breast size increases are modest, and high hormone levels come with health risks (Aly, 2020). Surgical breast augmentation is an option to increase breast size if it is unsatisfactory. Some transfeminine people, for instance many non-binary individuals, may wish to avoid or minimize breast growth, and there are possible therapeutic approaches in this area (Aly, 2019).
Additional review content on breast development in transfeminine people exists on this site (e.g., Aly, 2020; Aly, 2020). Breast growth can be measured and tracked with a variety of methods for individuals who are interested in monitoring their progress (Wiki). Photographs and timelines of breast development and feminization with hormone therapy in transfeminine people are available in communities like r/TransTimelines and r/TransBreastTimelines on the social media website Reddit.
Specific Hormonal Medications
The medications that are used in transfeminine hormone therapy include estrogens, progestogens, and antiandrogens. Estrogens produce feminization and testosterone suppression. Progestogens and antiandrogens do not mediate feminization themselves but further suppress and/or block testosterone. Testosterone suppression causes demasculinization and disinhibition of estrogen-mediated feminization. Androgens are sometimes used at low doses in transfeminine people who have low testosterone levels, although they are not required and benefits are uncertain. There are many different types of these hormonal medications available for transfeminine hormone therapy, with different benefits and risks.
Estrogens, progestogens, and antiandrogens are available in a variety of different formulations and for use by many different routes of administration in transfeminine people. The route of administration influences the absorption, distribution, metabolism, and elimination of the hormone in the body, resulting in significant differences between routes in terms of bioavailability, hormone levels in blood and specific tissues, and patterns of metabolites. These differences can have important therapeutic consequences.
Table 3: Major routes of administration of hormonal medications for transfeminine people:
Insertion via surgical incision into fat under skin
Pellet
Vaginal administration is a major additional route of administration of hormonal medications in cisgender women. While vaginal administration via a natal vagina is of course not possible in transfeminine people, neovaginal administration is a possiblility in those who have undergone vaginoplasty. However, the lining of the neovagina is not the vaginal epithelium of natal females but instead is usually skin or colon—depending on the type of vaginoplasty performed (penile inversion or sigmoid colon vaginoplasty, respectively). For this reason, neovaginal administration in transfeminine people is likely more similar in its properties to transdermal and rectal administration—depending on the type of neovagina—than to vaginal administration in cisgender women. It is noteworthy that the vaginal and rectal routes are said to be similar in their properties for hormonal medications however (Goletiani, Keith, & Gorsky, 2007; Wiki). Moreover, absorption of estradiol via neovaginas constructed from peritoneum (internal abdominal lining)—a less commonly employed vaginoplasty approach in transfeminine people—was reported in one study to be similar to that with vaginal administration of estradiol in cisgender women (Willemsen et al., 1985). As such, neovaginal administration may be an additional possible route for certain transfeminine people depending on the circumstances. However, this route still remains to be more adequately characterized.
An often-encountered question from people who take hormonal medications is whether there is an optimal time of the day to take them (Colonnello et al., 2025). As of present, there is little research in this area, and the answer to the question is essentially unknown (Colonnello et al., 2025). In any case, there is currently no evidence or persuasive theoretical basis to favor specific times of day to take these medications (Colonnello et al., 2025). In all likelihood, it makes little or no difference.
Estrogens
Estradiol, the primary bioidentical form normally found in the human body, is the main estrogen that is used in transfeminine hormone therapy. Estradiol hemihydrate (EH) is another form that is essentially identical to and interchangeable with estradiol. Estradiol esters are also sometimes used in place of estradiol. They are prodrugs of estradiol (i.e., are converted into estradiol in the body) and have essentially identical biological activity to estradiol. However, they have longer durations when used by injection due to slower absorption from the injection site, and this allows them to be administered less often. Some examples of major estradiol esters include estradiol valerate (EV; Progynova, Progynon Depot, Delestrogen) and estradiol cypionate (EC; Depo-Estradiol). Polyestradiol phosphate (PEP; Estradurin) is an injectable estradiol prodrug in the form of a polymer (i.e., linked chain of estradiol molecules) which is metabolized slowly and has a very long duration.
Non-bioidentical estrogens such as ethinylestradiol (EE; found in birth control pills), conjugated estrogens (CEEs; Premarin; used in menopausal hormone therapy), and diethylstilbestrol (DES; widely used previously but now abandoned) are resistant to metabolism in the liver and have disproportionate effects on estrogen-modulated liver synthesis when compared to bioidentical estrogens like estradiol (Aly, 2020). As a result, they have stronger influence on coagulation and greater risk of certain health problems like blood clots and associated cardiovascular issues (Aly, 2020). For this reason, as well as the fact that relatively high doses of estrogens may be needed for testosterone suppression, non-bioidentical estrogens should ideally never be used in transfeminine hormone therapy.
Estradiol dose-dependently suppresses testosterone levels in people with testes. Physiological and guideline-based levels of estradiol (<200 pg/mL or <734 pmol/L) are often not sufficient to suppress testosterone levels into the female range in transfeminine people who have not had their gonads removed (e.g., Liang et al., 2018; Krishnamurthy et al., 2023; Slack et al., 2023). As a result, estradiol is generally used in combination with an antiandrogen or progestogen in transfeminine hormone therapy (Hembree et al., 2017; Coleman et al., 2022; Rose et al., 2023). This results in partial suppression of testosterone levels by estradiol and further suppression or blockade of the remaining testosterone by the antiandrogen or progestogen. While combination therapy can be effective in fully suppressing or blocking testosterone (e.g., Aly, 2019; Aly, 2020), testosterone suppression can also still remain incomplete with antiandrogens and progestogens in certain forms (e.g., Aly, 2018; Jain, Kwan, & Forcier, 2019). In contrast to physiological estradiol levels, supraphysiological levels of estradiol are able to consistently and fully suppress testosterone levels into the normal female range with estradiol alone in transfeminine people (e.g., Gooren et al., 1984 [Graph]; Igo & Visram, 2021; Herndon et al., 2023 [Discussion]). This alternative approach, often referred to as high-dose estradiol monotherapy, has the advantage of avoiding the side effects, risks, and costs of antiandrogens and progestogens. However, it has the disadvantage of exposure to supraphysiological estradiol levels that are above those recommended by guidelines and that may have greater health risks. Physiological estradiol doses and combination therapy are more often used in transfeminine people treated by clinicians, whereas high-dose estradiol monotherapy is more frequently encountered in transfeminine people on DIY hormone therapy.
The feminizing effects of estradiol appear to be maximal at relatively low levels in the absence of androgens. Higher doses of estradiol and supraphysiological estradiol levels, aside from allowing for greater testosterone suppression, are not known to result in better feminization in transfeminine people (Deutsch, 2016; Nolan & Cheung, 2021). In fact, there is some indication that higher estrogen doses early into hormone therapy could actually result in worse breast development. Hence, the therapeutic emphasis in transfeminine hormone therapy is more on testosterone suppression than on achieving a specific estradiol level, at least above a certain low threshold level. Higher doses of estrogens, including of estradiol, also have a greater risk of adverse health effects such as blood clots and cardiovascular problems (Aly, 2020). As such, the use of physiological doses of estradiol is optimal in transfeminine people. At the same time however, high estrogen doses can be useful for improving testosterone suppression when it is inadequate, and the absolute risks, in the case of non-oral bioidentical estradiol, are low and are more important in people with specific risk factors (e.g., older age, physical inactivity, obesity, concomitant progestogen use, smoking, surgery, and rare thrombophilic abnormalities). If more adequate testosterone suppression is necessary, limitedly supraphysiological doses of non-oral estradiol may have a reasonable ratio of benefit to risk in this context, at least in those without relevant risk factors for estrogen-related complications (e.g., many healthy young people) (Aly, 2020).
Estradiol and estradiol esters are usually used orally, sublingually, transdermally, by injection (intramuscularly or subcutaneously), or by implant in transfeminine hormone therapy (Wiki).
Oral Estradiol
Estradiol is used orally in the form of tablets of estradiol (Wiki; Graphs). Alternatively, oral estradiol valerate tablets are used in some countries, for instance in many European countries. The only real difference between these oral estradiol forms is that estradiol valerate contains slightly less estradiol by weight (~76% of that of estradiol) due to its ester component and hence requires somewhat higher doses (~1.3-fold) in comparison for equivalent estradiol levels (Wiki; Table). Oral estradiol has a duration suitable for once-daily administration. Oral administration of estradiol is a very convenient and inexpensive route, which makes it the most popular and widely used form of estradiol in transfeminine people. Oral estradiol has relatively low bioavailability (~5%), and there is substantial variability between people in terms of estradiol levels achieved with the same dose. Hence, in some transfeminine people estradiol levels may be low with oral estradiol, and testosterone suppression may be inadequate depending on the antiandrogen.
A major drawback of oral estradiol is that it results in excessive levels of estradiol in the liver due to the first pass that occurs with oral administration and has a disproportionate impact on estrogen-modulated liver synthesis (Aly, 2020). This in turn increases coagulation and the risk of associated health complications like blood clots and cardiovascular problems (Aly, 2020). These particular health concerns are largely allayed if estradiol is taken non-orally at reasonable and non-excessive doses. Hence non-oral forms of estradiol, like transdermal estradiol, although less convenient and often more expensive than oral estradiol, are preferable in transfeminine hormone therapy. It is recommended that all transfeminine people who are over 40 to 45 years of age use non-oral routes due to the greater risk of blood clots and cardiovascular problems that occurs with age (Aly, 2020; Coleman et al., 2022). Oral estradiol is not a good choice for high-dose estradiol monotherapy in transfeminine people due to the high estradiol levels required and the greater risks than with non-oral routes. In addition to its disproportionate liver impact, oral estradiol results in unphysiological levels of estradiol metabolites like estrone and estrone sulfate when compared to non-oral forms. The clinical implications of this, if any, are unknown. Oral and non-oral estradiol have in any case been found to have similar effectiveness in terms of feminization and breast development in transfeminine people in available studies (Sam, 2020).
Sublingual Estradiol
Oral estradiol tablets can be taken sublingually instead of orally. Sublingual use of estradiol tablets has several-fold higher bioavailability relative to oral administration and hence achieves much higher overall estradiol levels in comparison (Sam, 2021; Wiki; Graphs). Sublingual use of oral estradiol tablets can be employed instead of oral administration to reduce doses and hence medication costs or to produce higher estradiol levels for the purpose of achieving better testosterone suppression when needed. However, sublingual estradiol is very spiky in terms of estradiol levels when compared to oral estradiol and has a short duration of highly elevated estradiol levels. As such, it may be advisable for sublingual estradiol to be used in divided doses multiple times throughout the day in order to maintain at least somewhat steadier estradiol levels. The therapeutic implications for transfeminine people of the spikiness of sublingual estradiol, for instance in terms of testosterone suppression and health risks, have been little-studied and are mostly unknown. In any case, when used as a form of high-dose estradiol monotherapy and taken multiple times per day, strong though still incomplete testosterone suppression has been observed (Yaish et al., 2023). Oral estradiol valerate tablets can be taken sublingually instead of orally similarly to estradiol and are likewise highly effective when used in this way (Aly, 2019; Wiki). Due to partial swallowing of tablets, sublingual estradiol may in practice be a mixture of sublingual and oral administration and may have some of the same health risks of oral estradiol (Wiki). Buccal administration of estradiol appears to have similar properties as sublingual administration but is much less researched in comparison and is not used as often in transfeminine people (Wiki).
Transdermal Estradiol
Transdermal estradiol is available in the form of patches, gel, emulsions, and sprays (Wiki). These forms are usually applied to skin areas such as the arms, abdomen, or buttocks. Gel, emulsions, and sprays are applied and left to dry for a short period, whereas patches are applied and remain adhesed to the skin for a specified amount of time. Due to rate-limited absorption through the skin, there is a depot effect with transdermal estradiol and this route has a long duration with very steady estradiol levels. As a result, estradiol gel, emulsions, and sprays are all suitable for once-daily use. Patches stay applied and continuously deliver estradiol for either 3–4 days or 7 days depending on the patch brand (Table). Transdermal estradiol is more expensive than oral estradiol. Gel, emulsions, and sprays may be less convenient than oral administration, but patches can be more convenient due to their infrequent application. However, patches can sometimes cause application site problems like redness and irritation and can occasionally come off prematurely due to adhesive failure. As with oral estradiol, there is substantial variability in estradiol levels with transdermal estradiol, and some transfeminine people may have poor absorption, low estradiol levels, and inadequate testosterone suppression with this route. Estradiol sprays, such as Lenzetto, have been found to achieve very low estradiol levels that are probably not therapeutically adequate for use in transfeminine hormone therapy (Aly, 2020; Graph).
Transdermal estradiol is the form of estradiol most commonly used in transfeminine people who are over 40 years of age due to its lower health risks relative to oral estradiol. Transdermal estradiol gel is not a favorable option for high-dose estradiol monotherapy as it has difficulty achieving the high estradiol levels needed for adequate testosterone suppression (Aly, 2019). On the other hand, transdermal estradiol patches can be an effective option for high-dose estradiol monotherapy if multiple 100 μg/day patches are used, although this can require the use of many patches and can be expensive (Wiki). Different skin sites absorb transdermal estradiol to different extents (Wiki). Genital application of transdermal estradiol, specifically to the scrotum or neolabia, is particularly better-absorbed than conventional skin sites and can result in much higher estradiol levels than usual (Aly, 2019). This can be useful for reducing doses and hence medication costs or for achieving higher estradiol levels for better testosterone suppression when needed, for instance in the context of high-dose estradiol monotherapy. Transdermal estradiol should not be applied to the breasts as this is not known to result in improved breast development and the potential health consequences of doing so are unknown (e.g., influence on breast cancer risk).
Injectable Estradiol
Injectable estradiol preparations can be administered via either intramuscular or subcutaneous injection (Wiki; Wiki; Graphs). There is a depot effect with injection of estradiol esters such that they are slowly absorbed from the injection site and have a prolonged duration. This ranges from days to months depending on the ester. Commonly used injectable estradiol esters, which all have short to moderate durations, include estradiol valerate (EV), estradiol cypionate (EC), estradiol enanthate (EEn), and estradiol benzoate (EB). Longer-acting injectable estradiol esters, such as estradiol undecylate (EU) and polyestradiol phosphate (PEP), have been discontinued and are no longer pharmaceutically available. In the case of intramuscular injection, common injection sites include the deltoid muscle (upper arm), vastus lateralis and rectus femoris muscles (thigh), and ventrogluteal muscle (buttocks). Subcutaneous injection of estradiol injectables, while less commonly used, has comparable pharmacokinetics to intramuscular injection, and is easier, less painful, and more convenient in comparison (Wiki). However, the maximum volume that can be safely and comfortably injected subcutaneously (1.5–3 mL) is less than that which can be injected intramuscularly (2–5 mL) (Hopkins, & Arias, 2013; Usach et al., 2019). Injectable estradiol tends to be fairly inexpensive, but may be less convenient than other routes due to the need for regular injections. There may also be a risk of internal scar tissue build-up long-term. Estradiol injectables have been discontinued in many parts of the world (e.g., most of Europe), and their availability is limited. In recent years, many transfeminine people have turned to black market homebrewed injectable estradiol preparations to use this route.
Injectable estradiol preparations are typically used at higher doses than other forms of estradiol, and can easily achieve very high levels of estradiol. This can be useful for testosterone suppression, making this form of estradiol likely the best choice for high-dose estradiol monotherapy in transfeminine people. However, the high doses that are possible with injectable estradiol preparations can also easily lead to overdosage and unnecessarily increased risks (e.g., Aly, 2020). Resources are available on this site for guiding selection of appropriate doses and intervals of injectable estradiol esters in transfeminine people. This includes a simulator and informal meta-analysis of estradiol levels with these preparations (Aly, 2021; Aly, 2021) and a table providing approximate equivalent doses between injectable estradiol esters and other estradiol routes and forms (Aly, 2020). It is notable and unfortunate that currently recommended doses and intervals for injectable estradiol esters by transgender care guidelines (e.g., 10–40 mg/2 weeks estradiol valerate) appear to be highly excessive and too widely spaced, and are likely to be therapeutically inadvisable (Aly, 2021). Doses and intervals of injectable estradiol esters recommended by the present author for use as a means of high-dose estradiol monotherapy, targeting mean estradiol levels of around 300 pg/mL (1,100 pmol/L), are provided below (Table 4).
Table 4: Recommended doses and intervals of injectable estradiol esters for high-dose estradiol monotherapy (targeting estradiol levels of around 300 pg/mL [1,100 pmol/L]):
a Injection interval. b Doses and intervals for estradiol undecylate are extrapolated and hypothetical (Aly, 2021).
These doses and intervals should be considered a starting point, and should be fine-tuned as necessary based on blood tests. In terms of injection intervals, the shorter interval, the more stable the estradiol levels, but the more often that injections need to be done. Doses may be increased if estradiol levels are too low and testosterone suppression is inadequate, and doses may be decreased if estradiol levels are too high so long as adequate testosterone suppression is maintained. Doses should be lower (targeting mean estradiol levels of 100–200 pg/mL [367–734 pmol/L]) if combined with an antiandrogen or progestogen as these agents will help with testosterone suppression. Similarly, doses should be lower following surgical gonadal removal as testosterone suppression will no longer be necessary.
Estradiol Pellets
Estradiol implants are pellets of pure crystalline hormone and are surgically placed into subcutaneous fat by a physician (Wiki). They are slowly absorbed by the body following implantation, and new implants are given once every 4 to 6 months. Due to the need for minor surgery, their high cost, and limited availability, estradiol implants are not as commonly used as other estradiol routes. Notably, almost all pharmaceutical estradiol implants throughout the world have been discontinued, and the implants that remain available are almost exclusively compounded products provided by compounding pharmacies. Dosage adjustment with estradiol implants is also more difficult than with other estradiol routes. Despite their various practical limitations however, estradiol implants allow for very steady estradiol levels, and their very long duration can allow for unusual convenience among available estradiol forms.
Additional Notes
Table 5: Available forms and recommended doses of estradiol for adulta transfeminine people:
a Estradiol doses in pubertal adolescent transfeminine people should be lower to mimic estradiol exposure during normal female puberty (Aly, 2020). b May be advisable to use divided doses 2 to 4 times per day (i.e., once every 6 to 12 hours) instead of once per day (Sam, 2021). c This estradiol form achieves very low estradiol levels at typical doses that don’t appear to be well-suited for transfeminine hormone therapy (Aly, 2020; Graph). d Estradiol valerate contains about 75% of the same amount of estradiol as estradiol so doses are about 1.3-fold higher for the same estradiol levels (Aly, 2019; Sam, 2021). e Doses and intervals for estradiol undecylate are extrapolated and hypothetical (Aly, 2021). f A higher initial loading dose of e.g., 240 or 320 mg polyestradiol phosphate can be used for the first one or two injections to reach steady-state estradiol levels more quickly. However, this preparation has recently been discontinued and appears to no longer be available.
Additional informational resources are available in terms of estradiol levels (Wiki; Table) and approximate equivalent doses (Aly, 2020) with different forms, routes, and doses of estradiol.
There is high variability between individuals in the levels of estradiol achieved during estradiol therapy. That is, estradiol levels during treatment with the same dosage of estradiol can differ substantially between individuals. This variability is greatest with oral and transdermal estradiol but is also considerable even with injectable estradiol preparations and other estradiol forms. As such, estradiol doses are not absolute and should be individualized on a case-by-case basis in conjunction with blood work as a guide. It should also be noted that due to fluctuations in estradiol concentrations with certain routes, levels of estradiol can vary considerably from one blood test to another. This is most notable with sublingual estradiol and injectable estradiol. The fluctuations in estradiol levels with these routes are predictable and must be understood when interpreting blood work results. Differences in blood test results can be minimized with informed and consistent timing of blood draws.
If or when the gonads are surgically removed, testosterone suppression is no longer needed in transfeminine people. As a result, estradiol doses, if they are high or supraphysiological, can be lowered to more closely approximate normal physiological levels in cisgender women.
Progestogens
Progestogens include progesterone and progestins. Progestins are synthetic progestogens derived from structural modification of progesterone or testosterone. There are dozens of different progestins and these progestins can be divided into a variety of different structural classes with varying properties (Table). Examples of some major progestins of different classes include the 17α-hydroxyprogesterone derivative medroxyprogesterone acetate (MPA; Provera, Depo-Provera), the 19-nortestosterone derivative norethisterone (NET; many brand names), the retroprogesterone derivative dydrogesterone (Duphaston), and the 17α-spirolactone derivative drospirenone (Slynd, Yasmin). Progestins were developed because they have a more favorable disposition in the body than progesterone for use as medications. Only a few clinically used progestins have been employed in transfeminine hormone therapy. However, progestogens largely produce the same progestogenic effects, with a few exceptions, and theoretically almost any progestogen could be used.
Besides helping with testosterone suppression, progestogens are of no clear or known benefit for feminization or breast development in transfeminine people. While some transfeminine people anecdotally claim to experience improved breast development with progestogens, an involvement of progestogens in improving breast size or shape is controversial and is not supported by theory nor evidence at present (Wiki; Aly, 2020). It is possible that premature introduction of progestogens, particularly at high doses, could actually have an unfavorable influence on breast development (Aly, 2020). Many transfeminine people have also anecdotally claimed that progestogens have a beneficial effect on their sexual desire. However, a review of the literature by the present author found that neither progesterone nor progestins positively influence sexual desire in humans (Aly, 2020). Instead, the available evidence suggests either a neutral influence or an inhibitory effect of progestogens on sexual desire, although the latter may be specific only to high doses of progestogens (Aly, 2020). Claims have been made that progesterone may have beneficial effects on mood in transfeminine people as well, but clinical support for such notions is likewise lacking at this time (Coleman et al., 2022; Nolan et al., 2022). It is notable that progesterone at luteal-phase levels, due to its neurosteroid metabolites like allopregnanolone, actually appears to worsen mood in around 30% of cisgender women, and produces more overt negative reactions, which constitute the diagnoses of premenstrual syndrome (PMS) and premenstrual dysphoric disorder (PMDD), in around 2 to 10% of women (Bäckström et al., 2011; Edler Schiller, Schmidt, & Rubinow, 2014; Sundström-Poromaa et al., 2020). More research is needed to evaluate the possible beneficial effects of progestogens in transfeminine people.
Most clinically used progestogens have off-target activities in addition to their progestogenic activity, and these activities may be desirable or undesirable depending on the action in question (Kuhl, 2005; Stanczyk et al., 2013; Wiki; Table). Progesterone has a variety of neurosteroid as well as other activities that can result in central nervous system effects among others which are not shared by progestins. MPA as well as NET and its derivatives have weak androgenic activity, which is unfavorable in the context of transfeminine hormone therapy. NET and certain related progestins produce ethinylestradiol as a metabolite at high doses and hence can produce ethinylestradiol-like estrogenic effects, including increased risk of blood clots and associated cardiovascular problems. Other off-target actions of progestogens include antiandrogenic, glucocorticoid, and antimineralocorticoid activities. These actions can result in differences in therapeutic effectiveness (e.g., androgen suppression or blockade) as well as side effects and health risks. Some notable progestins without undesirable off-target activities (i.e., androgenic or glucocorticoid activity) include low-dose CPA, drospirenone (DRSP), dienogest, nomegestrol acetate (NOMAC), dydrogesterone, and hydroxyprogesterone caproate (OHPC). However, of these progestins, only CPA has been considerably used and studied in transfeminine people.
The addition of progestogens to estrogen therapy has been associated with a number of unfavorable health effects. These include increased risk of blood clots (Wiki; Aly, 2020), coronary heart disease (Wiki), and breast cancer (Wiki; Aly, 2020). High doses of progestogens are also associated with increased risk of certain non-cancerousbrain tumors including meningiomas and prolactinomas (Wiki; Aly, 2020). The coronary heart disease risk may be due to changes in blood lipids caused by the weak androgenic activity of certain progestogens, but the rest of the aforementioned risks are probably due to their progestogenic activity (Stanczyk et al., 2013; Jiang & Tian, 2017). Aside from health risks, progestogens have also been associated with adverse mood changes (Wiki; Wiki). However, besides the case of progesterone and its neurosteroid metabolites, these effects of progestogens are controversial and are not well-supported by evidence (Wiki; Wiki). Progestogens are otherwise generally well-tolerated and are regarded as producing little in the way of side effects.
In contrast to certain progestins, progesterone has no unfavorable off-target hormonal activities. Due to its lack of androgenic activity, progesterone has no adverse influence on blood lipids and is not expected to raise the risk of coronary heart disease. The addition of oral progesterone to estrogen therapy notably has not been associated with increased risk of blood clots (Wiki). In addition, oral progesterone seems to have less risk of breast cancer than progestins with shorter-term therapy, although this is notably not the case with longer-term exposure (Wiki; Aly, 2020). Consequently, it has been suggested that progesterone, for reasons that have yet to be fully elucidated, may be a safer progestogen than progestins and that it should be the preferred progestogen for hormone therapy in cisgender women and transfeminine people. However, there are also theoretical arguments against such notions. Oral progesterone is known to produce very low progesterone levels and to have only weak progestogenic effects at typical doses (Aly, 2018; Wiki). The seemingly better safety of oral progesterone may simply be an artifact of the low progesterone levels that occur with it, and hence of progestogenic dosage. Non-oral progesterone, at doses resulting in physiological and full progestogenic strength, has never been properly evaluated in terms of health outcomes, and may have similar risks as progestins (Aly, 2018; Wiki).
Due to their lack of known influence on feminization and breast development and their known and possible adverse effects and risks, progestogens are not routinely used in transfeminine hormone therapy at present. Major transgender health guidelines note the limitations of the available evidence on progestogens for transfeminine people and have mixed attitudes on their use, either explictly recommending against their use (Coleman et al., 2022—WPATH SOC8), taking a more neutral stance (Hembree et al., 2017—Endocrine Society guidelines), or being permissive of their use (Deutsch, 2016—UCSF guidelines). There is however a very major exception to the preceding in the form of CPA, an antiandrogen which is widely used in transfeminine hormone therapy to suppress testosterone production and which happens to be a powerful progestogen at the typical doses used in transfeminine people. CPA will be described below in the section on antiandrogens. Although progestogens have various health risks, cisgender women of course have progesterone, and the absolute risks of progestogens are very low in healthy young people. Risks like breast cancer also are exposure-dependent and take many years to develop. The testosterone suppression provided by progestogens can furthermore be very useful in transfeminine people, as is widely taken advantage of with CPA. Given these considerations, a limited duration of progestogen therapy in transfeminine people, for instance a few years to help suppress testosterone levels before surgical gonadal removal, may be considered quite acceptable.
Progesterone can be used in transfeminine people by oral administration, sublingual administration, rectal administration, or by intramuscular or subcutaneous injection (Wiki). Progestins are usually used via oral administration, but certain progestins are also available in injectable formulations (Wiki).
Oral Progesterone
Progesterone is most commonly taken orally. It is used by this route in the form of oil-filled capsules containing 100 or 200 mg micronized progesterone under brand names such as Prometrium, Utrogestan, and Microgest (Wiki). Despite its widespread use, levels of progesterone via oral administration have been found using state-of-the-art assays (LC–MS) to be very low (<2 ng/mL [<6.4 nmol/L] at 100 mg/day) and inadequate for satisfactory progestogenic effects in various areas (Aly, 2018; Wiki). In relation to this, even high doses of oral progesterone (400 mg/day) showed no antigonadotropic effect or testosterone suppression in cisgender men (Aly, 2018; Wiki). This is in major contrast to non-oral forms of progesterone and to progestins, which produce dose-dependent and robust testosterone suppression (Aly, 2019; Wiki). In addition to its low progestogenic potency, oral progesterone is excessively converted into neurosteroid metabolites like allopregnanolone and pregnanolone. These metabolites act as potent GABAA receptor positive allosteric modulators, and can produce undesirable alcohol-like side effects such as sedation, cognitive, memory, and motor impairment, and mood changes (Wiki; Wiki). As such, while inconvenient, non-oral routes are greatly preferable for progesterone.
Sublingual Progesterone
Sublingual progesterone tablets exist and are marketed under the brand name Luteina but today are only available in Poland and Ukraine (Wiki). Oral progesterone could theoretically be taken sublingually, analogously to sublingual use of oral estradiol. However, because oral progesterone is formulated as oil-filled capsules, this makes it difficult and unpleasant to use by sublingual administration. Buccal progesterone, which would be expected to have similar characteristics to those of sublingual progesterone, has been used in medicine in the past, but is no longer marketed today (Wiki).
Rectal Progesterone
Progesterone is approved for use by rectal administration in the form of suppositories under the brand name Cyclogest (Wiki). This product is marketed in only a limited number of countries however, although it is available in the United Kingdom (Wiki). While not approved for use by rectal administration, oral progesterone capsules can be taken rectally instead of orally, and using them in this way may allow for much higher progesterone levels than would be achieved by oral administration due to avoidance of most first-pass metabolism. Rectal administration of oral progesterone capsules has not been formally studied, but oral progesterone capsules have been administered vaginally in cisgender women with success (Miles et al., 1994; Wang et al., 2019), and the vaginal and rectal routes are said to have similar pharmacokinetics in general (Goletiani, Keith, & Gorsky, 2007; Wiki). Hence, there is good theoretical basis for rectal administration of oral progesterone capsules being an effective route of progesterone. Whereas oral progesterone achieves very low levels of progesterone, rectal progesterone can readily achieve normal luteal-phase levels of progesterone (Wiki). Although inconvenient, rectal administration may be the overall best route of administration of progesterone for transfeminine people. A significant subset of transfeminine people on progestogens take progesterone rectally (Chang et al., 2024).
Injectable Progesterone
Progesterone by injection is available as an oil solution for intramuscular injection under brand names such as Proluton, Progestaject, and Gestone (Wiki) and as an aqueous solution for subcutaneous injection under the brand name Prolutex (Wiki). Oil solutions of progesterone for intramuscular injection are widely available, whereas the aqueous solution of progesterone for subcutaneous injection is available only in some European countries (Wiki). Injectable progesterone, regardless of route, has a relatively short duration and must be injected once every one to three days (Wiki; Wiki). This makes it too inconvenient to use for most people. Unlike with estradiol, progesterone esters with longer durations than progesterone itself by injection are not chemically possible as progesterone has no hydroxyl groups available for esterification (Wiki). Injectable aqueous suspensions of microcrystalline progesterone were previously marketed and had a duration of 1 to 2 weeks, but these preparations were associated with pain at the injection site and were eventually discontinued (Aly, 2019; Wiki).
Other Progesterone Routes
Other progesterone routes, such as transdermal progesterone and subcutaneous progesterone pellets, are also known, but are not available as pharmaceutical drugs and are little-used medically (Wiki). This is related to the low potency of progesterone and difficulty achieving progesterone levels high enough for adequate therapeutic effects with these routes (Wiki; Wiki). In addition, progesterone pellets tend to be extruded at high rates (Wiki). In any case, certain compounding pharmacies may make forms of progesterone that could be used by these routes.
Oral and Injectable Progestins
Most progestins are taken orally in the form of solid tablets (Wiki). In contrast to progesterone, progestins, owing to their synthetic nature, are resistant to metabolism in the intestines and liver and have high oral bioavailability. In addition, unlike the case of the estrogen receptors, the progesterone receptors are expressed minimally or not at all in the liver, and there is no known first pass influence of progestogenic activity on liver synthesis (Lax, 1987; Stanczyk, Mathews, & Cortessis, 2017). As a result, there are no apparent problems with oral administration in the case of purely progestogenic progestins. However, some progestins have liver-impacting off-target hormonal actions, such as androgenic, estrogenic, and/or glucocorticoid activity, and this can result in adverse effects like unfavorable lipid changes or procoagulation—which may be augmented by the first pass with oral administration.
A selection of progestins are available in injectable formulations, including for intramuscular or subcutaneous injection (Wiki). Some of the more notable ones include medroxyprogesterone acetate (MPA), norethisterone enanthate (NETE), hydroxyprogesterone caproate (OHPC), and algestone acetophenide (dihydroxyprogesterone acetophenide; DHPA) (Wiki). In addition to being used alone, injectable progestins are used together with estradiol esters in combined injectable contraceptives (Wiki). These preparations are often used as a means of hormone therapy by transfeminine people in Latin America. Whereas injectable progesterone has a duration measured in days, injectable progestins have durations ranging from weeks to months, and can be injected much less often in comparison (Table).
Additional Notes
Table 6: Available forms and recommended doses of progestogens for transfeminine people:
For progesterone levels with different forms, routes, and doses of progesterone, see the table here (only LC–MS and IA + CS assays for oral progesterone) and the graphs here.
As with estradiol, there is high variability between individuals in progesterone levels. Conversely, there is less variability between individuals in the case of progestins.
After removal of the gonads, progestogen doses can be lowered or adjusted to approximate normal female physiological exposure or they can be discontinued entirely.
Antiandrogens
Aside from estrogens and progestogens, there is another class of hormonal medications used in transfeminine hormone therapy known as antiandrogens (AAs). These medications reduce the effects of androgens in the body by either decreasing androgen production and thereby lowering androgen levels or by directly blocking the actions of androgens. They work via a variety of different mechanisms of action, and include androgen receptor antagonists, antigonadotropins, and androgen synthesis inhibitors.
Androgen receptor antagonists act by directly blocking the effects of androgens, including testosterone, DHT, and other androgens, at the level of their biological target. They bind to the androgen receptor without activating it, thereby displacing androgens from the receptor. Due to the nature of their mechanism of action as competitive blockers of androgens, the antiandrogenic efficacy of androgen receptor antagonists is both highly dose-dependent and fundamentally dependent on testosterone levels. They do not act by lowering testosterone levels, although some androgen receptor antagonists may have additional antiandrogenic actions that result in decreased testosterone levels. Because androgen receptor antagonists do not work by lowering testosterone levels, blood work can be less informative for them compared to antiandrogens that suppress testosterone levels. Androgen receptor antagonists include steroidal antiandrogens (SAAs) like spironolactone (Aldactone) and cyproterone acetate (CPA; Androcur) and nonsteroidal antiandrogens (NSAAs) like bicalutamide (Casodex).
Antigonadotropins suppress the gonadal production of androgens by inhibiting the GnRH-mediated secretion of gonadotropins from the pituitary gland. They include estrogens and progestogens. In addition, GnRH agonists such as leuprorelin (Lupron) and GnRH antagonists such as elagolix (Orilissa) act similarly and could likewise be described as antigonadotropins.
Androgen synthesis inhibitors inhibit the enzyme-mediated synthesis of androgens. They include 5α-reductase inhibitors (5α-RIs) like finasteride (Propecia) and dutasteride (Avodart). There are also other types of androgen synthesis inhibitors, for instance potent 17α-hydroxylase/17,20-lyase inhibitors like ketoconazole (Nizoral) and abiraterone acetate (Zytiga). However, these agents have limitations (e.g., toxicity, high cost, and lack of experience) and have not been used in transfeminine hormone therapy.
Although antigonadotropins and androgen synthesis inhibitors have antiandrogenic effects secondary to decreased androgen levels, they are not usually referred to as “antiandrogens”. Instead, this term is most commonly reserved to refer specifically to androgen receptor antagonists. However, antigonadotropins and androgen synthesis inhibitors may nonetheless be described as antiandrogens as well.
After removal of the gonads, antiandrogens can be discontinued. If unwanted androgen-dependent symptoms, such as acne, seborrhea, or scalp hair loss, persist despite full suppression or ablation of gonadal testosterone, then a lower dose of an androgen receptor antagonist, such as 100 to 200 mg/day spironolactone or 12.5 to 25 mg/day bicalutamide, can be continued to treat these symptoms.
Table 7: Available forms and recommended doses of antiandrogens for transfeminine people:
a For CPA, this dose range is specifically one-quarter of a 10-mg tablet to one full 10-mg tablet per day (2.5–10 mg/day) or a quarter of a 50-mg tablet every other day or every 2 to 3 days (4.2–12.5 mg/day). A dosage of 5–10 mg/day or 6.25–12.5 mg/day is likely to ensure maximal testosterone suppression, while lower doses may be less effective (Aly, 2019). b For spironolactone and bicalutamide, it is assumed that testosterone levels are substantially suppressed (≤200 ng/dL [<6.9 nmol/L]). If testosterone levels are not suppressed to this range, then higher doses may be warranted. c Spironolactone and its metabolites have relatively short half-lives, and twice-daily administration in divided doses (e.g., 100–200 mg twice per day) is recommended.
Figure 3: Suppression of gonadal testosterone production and circulating testosterone levels (ng/dL) with estradiol in combination with different antiandrogens over one year of hormone therapy in transfeminine people (Sofer et al., 2020). The estradiol forms included oral tablets 2–8 mg/day, transdermal gel 2.5–5 mg/day, and transdermal patches 50–200 μg/day. The antiandrogens included spironolactone 50–200 mg/day (n=16), cyproterone acetate (n=41), and GnRH agonists (specifically triptorelin 3.75 mg/month or goserelin 3.6 mg/month by injection) (n=10) (Sofer et al., 2020). It should be noted that lower doses of cyproterone acetate (10–12.5 mg/day) show equal testosterone suppression to higher doses (25–100 mg/day) and higher doses should no longer be used (Aly, 2019). The dashed horizontal line corresponds to the upper limit of the normal female range for testosterone levels.
As an antiandrogen, CPA has a dual mechanism of action of suppressing testosterone levels via its progestogenic and hence antigonadotropic activity and of acting as an androgen receptor antagonist (Aly, 2019). The progestogenic activity of CPA is of far greater potency than its androgen receptor antagonism however (Aly, 2019). The dose of CPA used as a progestogen in cisgender women is about 2 mg per day, which produces similar progestogenic effects to those of physiological luteal-phase levels of progesterone (e.g., suppression of gonadotropin secretion, ovulation inhibition, and endometrial transformation and protection) (Aly, 2019). Conversely, much higher doses of CPA of 50 to 300 mg/day have typically been used for androgen-dependent indications (Aly, 2019). These high doses of CPA result in profound progestogenic overdosage and associated side effects and risks (Aly, 2019). In transfeminine people, CPA has historically been used at doses of 50 to 150 mg/day (Aly, 2019). However, CPA doses have dramatically fallen in recent years, and today doses of no more than 10 to 12.5 mg/day are recommended (Aly, 2019; Coleman et al., 2022—WPATH SOC8). These lower doses of CPA still produce strong progestogenic effects, and in combination with estradiol, are equally effective as higher doses in suppressing testosterone levels (Aly, 2019; Meyer et al., 2020; Even Zohar et al., 2021; Kuijpers et al., 2021; Coleman et al., 2022). Even lower doses of CPA, for instance 5 to 6.25 mg/day, are currently being studied, and may still be fully effective (Aly, 2019).
Given by itself without estrogen, CPA typically suppresses testosterone levels in people with testes by about 50 to 70%, down to about 150 to 300 ng/dL (5.2–10.4 nmol/L) (Meriggiola et al., 2002; Toorians et al., 2003; Giltay et al., 2004; T’Sjoen et al., 2005; Tack et al., 2017; Zitzmann et al., 2017; Aly, 2019). Lower doses of CPA alone (e.g., 10 mg/day) show the same degree of testosterone suppression as higher doses of CPA alone (e.g., 50–100 mg/day), indicating that the antigonadotropic effects of CPA are maximal at relatively low therapeutic doses of this medication (Aly, 2019). This is on the order of about 5 to 10 times the ovulation-inhibiting dosage of CPA in cisgender women, a dose–response relationship that has also been observed with a number of other progestogens (Aly, 2019). Per the preceding, CPA alone, regardless of dosage, is unable to reduce testosterone levels into the normal female range (<50 ng/dL [<1.7 nmol/L]). But when CPA is combined with estradiol, even at relatively small doses of estradiol, it consistently suppresses testosterone levels into the normal female range (Aly, 2019; Angus et al., 2019; Gava et al., 2020; Sofer et al., 2020; Collet et al., 2022). However, it appears that a certain minimum level of estradiol, perhaps around 60 pg/mL (220 pmol/L) on average, is required for this to occur (Aly, 2019). Estradiol levels lower than this threshold in those taking CPA, which can occasionally be encountered in transfeminine people due to estradiol being dosed too low, have the potential to compromise full testosterone suppression (Aly, 2019).
In addition to testosterone suppression, CPA can dose-dependently block the androgen receptor (Aly, 2019). However, relatively high doses of CPA are needed to considerably antagonize the androgen receptor (e.g., 50–300 mg/day), and lower doses (e.g., ≤12.5 mg/day) may not be able to do this to a meaningful degree (Aly, 2019). As such, lower doses of CPA may essentially be purely progestogenic, with minimal or no androgen receptor antagonism. In this regard, referring to CPA at such doses as an “antiandrogen”—rather than as a “progestogen”—may be considered somewhat of a misnomer. Higher doses of CPA (>12.5 mg/day) can no longer be considered safe due to the massive progestogenic overdosage that occurs with them, and should no longer be used in transfeminine people. Moreover, as testosterone levels are usually suppressed into the normal female range in transfeminine people taking estradiol plus CPA, there is no actual need for any additional androgen receptor blockade (Aly, 2019).
CPA has been reported to produce various side effects. Some of these side effects include fatigue and a degree of weight gain (Belisle & Love, 1986; Hammerstein, 1990; Martinez-Martin et al., 2022). CPA might be able to produce a magnitude of sexual dysfunction (e.g., reduced sexual desire) beyond that expected with testosterone suppression alone (Wiki; Aly, 2019). It may also have a small risk of depressive mood changes (Wiki). In transfeminine people, CPA has been documented to produce pregnancy-like breast changes (i.e., lobuloalveolar development of the mammary glands) (Kanhai et al., 2000). In relation to this, CPA sometimes causes lactation as a side effect (Dewhurst & Underhill, 1979; Gooren, Harmsen-Louman, & van Kessel, 1985; Schlatterer et al., 1998; Bazarra-Castro, 2009). Concerns have been raised about premature introduction of progestogens—particularly at high doses like with CPA—and possible adverse influence on breast development (Aly, 2020). However, little data exists in humans to substantiate such concerns at present. The side effects of CPA are assumed to be dose-dependent, and using the lowest effective doses is expected to minimize its side effects.
CPA is usually taken orally in the form of tablets (e.g., 10, 50, and 100 mg) (Wiki). Under the brand name Androcur Depot, it is also available as a long-lasting 300 mg depot injectable in some countries (Wiki). However, this formulation is not commonly used in transfeminine people, and happens to correspond to very high doses in terms of CPA exposure. A pill cutter (Amazon) can be used to split CPA tablets and achieve lower doses (e.g., 12.5 mg doses with 50-mg tablets). CPA has a relatively long elimination half-life of about 1.6 to 4.3 days (Wiki; Aly, 2019). As such, it can be taken once daily, or even as infrequently as once every 2 or 3 days, if needed (Aly, 2019). In addition to splitting of CPA tablets, dosing CPA once every 2 or 3 days can also be useful for achieving lower doses (Aly, 2019).
As already described, CPA is a powerful progestogen even at the relatively low doses now used in transfeminine people (e.g., 5–12.5 mg/day). As such, there is no need, nor point, in adding another progestogen, for instance progesterone, in those who are taking CPA—at least if the goal of doing so is to produce progestogenic effects. This is something that is often overlooked in people taking CPA, and can result in increased costs, side effects, and inconvenience without any expected benefit.
Spironolactone
Spironolactone (Aldactone) is an antiandrogen and antimineralocorticoid. It is widely used as an antiandrogen in cisgender women for treatment of androgen-dependent hair and skin conditions like acne, hirsutism (excessive facial/body hair growth), and scalp hair loss, in cisgender women for treatment of hyperandrogenism (high androgen levels) due to polycystic ovary syndrome (PCOS), and in transfeminine people as a component of hormone therapy. Spironolactone is particularly widely used in transfeminine people in the United States, where it is the most commonly used antiandrogen in this population. As an antimineralocorticoid, the original and primary use of spironolactone in medicine, it is used to treat heart failure, high blood pressure, high mineralocorticoid levels, low potassium levels, and conditions of excess fluid retention like nephrotic syndrome and ascites, among others (Wiki). In terms of its antiandrogenic actions, spironolactone is a relatively weak androgen receptor antagonist as well as a weak androgen synthesis inhibitor (Wiki). The androgen synthesis inhibition of spironolactone is mediated specifically via inhibition of 17α-hydroxylase and 17,20-lyase (Wiki). Spironolactone does not appear to have meaningful progestogenic activity, 5α-reductase inhibition, or direct estrogenic activity (Wiki). However, indirect estrogenic effects secondary to its antiandrogenic activity (e.g., breast development and feminization) can occur with it at sufficiently high doses (Wiki).
Spironolactone shows limited and highly inconsistent effects on testosterone levels in clinical studies in cisgender men, cisgender women, and transfeminine people, with most studies finding no change in levels, some studies finding a decrease in levels, and a small number even finding an increase in levels (Aly, 2018). In spite of this, studies commonly find that spironolactone still produces antiandrogenic effects even when androgen levels remain unchanged. Hence, the primary mechanism of action of spironolactone as an antiandrogen appears to be androgen receptor blockade. In relation to this, in transfeminine people taking spironolactone as an antiandrogen, the estrogen component of the regimen is likely to be the main or possibly sole agent suppressing testosterone production. This is in part based on studies in transfeminine people comparing estradiol plus spironolactone to estradiol alone (e.g., Leinung, 2014; Leinung, Feustel, & Joseph, 2018; Angus et al., 2019) and on studies comparing testosterone levels with different doses of spironolactone (e.g., Liang et al., 2018; SoRelle et al., 2019; Allen et al., 2021). Due to the minimal influence of spironolactone on testosterone production, testosterone levels are not usually suppressed into the female range in transfeminine people taking estradiol plus spironolactone, with testosterone levels often remaining well above this range (e.g., 50–450 ng/dL [1.7–15.6 nmol/L] on average) (Leinung, 2014; Leinung, Feustel, & Joseph, 2018; Liang et al., 2018; Angus et al., 2019; Jain, Kwan, & Forcier, 2019; SoRelle et al., 2019; Sofer et al., 2020; Burinkul et al., 2021). However, testosterone levels do tend to decline gradually over time in transfeminine people on this regimen (e.g., Liang et al., 2018; Sofer et al., 2020 (Graph); Allen et al., 2021).
Due to its relatively weak androgen receptor antagonism, spironolactone is likely best-suited for blocking female-range or somewhat-higher testosterone levels (e.g., <100 ng/dL [<3.5 nmol/L]) (Aly, 2018). This is based on clinical dose-ranging studies of spironolactone (typically using 50–200 mg/day) in healthy cisgender women and cisgender women with PCOS (Goodfellow et al., 1984; Lobo et al., 1985; Hammerstein, 1990; James, Jamerson, & Aguh, 2022) as well as comparative studies of spironolactone against the more-potent antiandrogen flutamide (Cusan et al., 1994; Erenus et al., 1994; Shaw, 1996). The clinical antiandrogenic efficacy of spironolactone has been very limitedly assessed in transfeminine people to date, and is largely unknown (Angus et al., 2021). In any case, the antiandrogenic efficacy of spironolactone in cisgender women with androgen-dependent hair and skin conditions is well-established, and the medication thus does appear to be effective so long as testosterone levels are not too high (Brown et al., 2009; van Zuuren & Fedorowicz, 2016; Layton et al., 2017; Barrionuevo et al., 2018; James, Jamerson, & Aguh, 2022). In addition, higher doses of spironolactone (e.g., 300–400 mg/day) may be more useful for blocking higher testosterone levels in transfeminine people, and are allowed for by transgender care guidelines (Aly, 2020).
Consequent to spironolactone’s limited and inconsistent influence on testosterone levels and its relatively weak androgen receptor antagonism, estradiol plus spironolactone regimens will likely not be fully effective in terms of testosterone suppression for many transfeminine people. This is liable to result in suboptimal demasculinization, feminization, and breast development in these individuals. Other antiandrogenic approaches, such as bicalutamide, CPA, GnRH modulators, and high-dose estradiol monotherapy, will likely be more effective in these cases owing either to their ability to more potently block androgens or their capacity to reliably reduce testosterone levels into the female range. If testosterone levels are still too high with estradiol plus spironolactone, a switch to a different antiandrogen, increasing to a higher dosage of estradiol, or addition of a clinically antigonadotropic progestogen (e.g., non-oral progesterone or a progestin) should be considered.
Spironolactone is a strong antimineralocorticoid, or antagonist of the mineralocorticoid receptor, the biological target of the mineralocorticoid steroid hormones aldosterone and 11-deoxycorticosterone. This is an action that spironolactone shares with progesterone, although spironolactone is a much more potent antimineralocorticoid than progesterone. The mineralocorticoid receptor is involved in regulating electrolyte and fluid balances, among other roles. Spironolactone is associated with modestly lowered blood pressure, which may be considered a beneficial effect of its antimineralocorticoid activity (Martinez-Martin et al., 2022). Although spironolactone is usually well-tolerated, it can sometimes produce antimineralocorticoid side effects such as excessively lowered blood pressure, dizziness, fatigue, urinary frequency, and increased cortisol levels, among others (Kellner & Wiedemann, 2008; Kim & Del Rosso, 2012; Zaenglein et al., 2016; Layton et al., 2017; James, Jamerson, & Aguh, 2022). It has been argued by some in the online transgender community that spironolactone, via its antimineralocorticoid activity and increased cortisol levels, may increase visceral fat in transfeminine people (Aly, 2020). However, evidence does not support this hypothetical side effect at present (Aly, 2020). Available data also do not support spironolactone stunting breast development in transfeminine people or producing serious neuropsychiatric side effects, such as prominent depressive mood changes.
In people who are at-risk for hyperkalemia, dietary restriction to limit intake of potassium-rich foods is often recommended (Roscioni et al., 2012; Cupisti et al., 2018). This is often encountered in transgender health as transfeminine people being told “not to eat bananas”, which are said to be high in potassium. However, limiting dietary potassium with spironolactone to avoid hyperkalemia is theoretical and not actually evidence-based, with data so far contradicting its efficacy (St-Jules, Goldfarb, & Sevick, 2016; St-Jules & Fouque, 2020; Babich, Kalantar-Zadeh, & Joshi, 2022; St-Jules & Fouque, 2022). As such, routine restriction of dietary potassium with spironolactone may not be warranted.
Aside from its antimineralocorticoid activity, spironolactone has been reported to increase levels of LDL (“bad”) cholesterol levels and to decrease levels of HDL (“good”) cholesterol in women with PCOS (Nakhjavani et al., 2009). However, findings appear to be conflicting, with other studies not finding unfavorable influences on cholesterol levels with spironolactone (Polyzos et al., 2011). Long-term, adverse effects on cholesterol levels could result in an increase in the risk of coronary heart disease.
Spironolactone is taken orally in the form of tablets (e.g., 25, 50, and 100 mg) (Wiki). It is a prodrug of several active metabolites, including 7α-thiomethylspironolactone, 6β-hydroxy-7α-thiomethylspironolactone, and canrenone (7α-desthioacetyl-δ6-spironolactone) (Wiki). Spironolactone and these active metabolites have elimination half-lives of 1.4 hours, 13.8 hours, 15.0 hours, and 16.5 hours, respectively (Wiki). Due to the relatively short duration of elevated drug levels with spironolactone and its active metabolites (Graph), twice-daily administration of spironolactone in divided doses may be more optimal than once-daily intake and is advised (Reiter et al., 2010).
Bicalutamide
Bicalutamide (Casodex) is a nonsteroidal antiandrogen (NSAA) which acts as a potent and highly selective androgen receptor antagonist (Wiki). It is primarily used in the treatment of prostate cancer in cisgender men. Prostate cancer is an androgen-dependent cancer which antiandrogens can help to slow the progression of, and this use constitutes the vast majority of prescriptions for bicalutamide (Wiki). In addition to prostate cancer, although to a much lesser extent, bicalutamide has been used in the treatment of hirsutism (excessive facial/body hair growth), scalp hair loss, and polycystic ovary syndrome (PCOS) in cisgender women, peripheral or gonadotropin-independent precocious puberty (a rare form of precocious puberty in which antigonadotropins such as GnRH agonists are not effective) in cisgender boys, and priapism in cisgender men (Wiki). Bicalutamide is also becoming increasingly adopted for use as an antiandrogen in transfeminine people (Aly, 2020; Wiki). However, its use in transgender health is still very limited, and well-regarded transgender care guidelines either recommend against its use (Deutsch, 2016—UCSF guidelines; Coleman et al., 2022—WPATH SOC8) or are only cautiously permissive of its use (Thompson et al., 2021—Fenway Health guidelines). This is due to a lack of studies of bicalutamide in transfeminine people and its potential risks. Nonetheless, a small but growing number of clinicians are using bicalutamide in transfeminine people or are willing to prescribe it, with these clinicians located particularly in the United States. A single small clinical study has assessed bicalutamide in transfeminine people so far, specifically as a puberty blocker in 13 transfeminine adolescents who were denied insurance coverage for GnRH agonists (Neyman, Fuqua, & Eugster, 2019). (Update: More studies of bicalutamide in transfeminine people have since been published, see Aly (2020).)
Bicalutamide is a much more potent androgen receptor antagonist than either spironolactone or CPA (Wiki; Neyman, Fuqua, & Eugster, 2019). It is typically used in transfeminine people at a dosage of 25 to 50 mg/day, although this dosage has been arbitrarily selected and is not based on clinical data. Nonetheless, due to its relatively high potency as an androgen receptor antagonist and concomitant suppression of testosterone levels by estradiol, these doses may be adequate for testosterone blockade for many transfeminine people. At higher doses (>50 mg/day), bicalutamide is able to substantially block male-range testosterone levels (>300 ng/dL [>10.4 nmol/L]) based on studies of bicalutamide monotherapy in cisgender men with prostate cancer (Wiki). This is something that spironolactone and CPA are not capable of in the same way. Owing to its selectivity for the androgen receptor, bicalutamide has no off-target hormonal activity and produces almost no side effects in women (Wiki; Erem, 2013; Moretti et al., 2018). The only apparent side effect of bicalutamide in a rigorous clinical trial of the drug for hirsutism in cisgender women was significantly increased total and LDL (“bad”) cholesterol levels (Moretti et al., 2018). Hence, bicalutamide tends to be very well-tolerated. The relative lack of side effects with bicalutamide is in contrast to other antiandrogens like spironolactone and CPA, which are not pure androgen receptor antagonists and have off-target hormonal actions like antimineralocorticoid activity or strong progestogenic activity with consequent side effects and risks.
As a selective androgen receptor antagonist, bicalutamide taken by itself does not decrease testosterone production or levels but rather increases them (Wiki). This is due to a loss of androgen receptor-mediated negative feedback on gonadotropin secretion and a consequent compensatory upregulation of gonadal testosterone production (Wiki). Bicalutamide more than blocks the effects of any increase in testosterone it causes, and in fact fundamentally cannot increase testosterone levels more than it can block them (Wiki). In addition, increases in testosterone levels with bicalutamide will be blunted or abolished if it is combined with an adequate dose of an antigonadotropin such as estradiol (Wiki; Wiki). Since estradiol is made from testosterone in the body, bicalutamide taken alone also preserves and increases estradiol production and levels (Wiki). Because of this, although bicalutamide has no other important intrinsic hormonal activity besides its antiandrogenic activity, it produces robust indirect estrogenic effects including feminization and breast development even when it is not combined with estrogen (Wiki; Wiki; Neyman, Fuqua, & Eugster, 2019). This has important implications for the use of bicalutamide as a puberty blocker in transfeminine adolescents, as bicalutamide does not actually block puberty like conventional puberty blockers (GnRH agonists) but instead has the effect of dose-dependently converting male puberty into female puberty (Wiki; Neyman, Fuqua, & Eugster, 2019).
Bicalutamide has certain health risks, which has been a major reason that it has not been more readily adopted in transfeminine hormone therapy (Aly, 2020). It has a small risk of liver toxicity (Wiki; Aly, 2020) and of lung toxicity (Wiki). Abnormal liver function tests (LFTs), such as elevated liver enzymes and elevated bilirubin, occurred in about 3.4% of men with bicalutamide monotherapy plus standard care versus 1.9% of men with placebo plus standard care in the Early Prostate Trial (EPC) clinical programme after 3.0 years of follow-up (Wiki). In clinical trials, treatment with bicalutamide had to be discontinued in about 0.3 to 1.5% of men due to LFTs that became too highly elevated and could have progressed to serious liver toxicity (Wiki). To date, there are around 10 published case reports of serious liver toxicity, including cases of death, with bicalutamide, all of which have been in men with prostate cancer (Wiki; Table; Aly, 2020). There have also been a few unpublished reports of serious liver toxicity including deaths with bicalutamide in transfeminine people (Aly, 2020). However, these reports have not been confirmed, and they may or may not be reliable. In addition to the preceding reports, hundreds of additional instances of liver complications in people taking bicalutamide exist in the United States FDA Adverse Event Reporting System (FAERS) database (Wiki; FDA). Abnormal LFTs with bicalutamide usually occur within the first 3 to 6 months of treatment (Kolvenbag & Blackledge, 1996; Casodex FDA Label), and all case reports of liver toxicity with bicalutamide have had an onset of less than 6 months (Table). The liver toxicity of bicalutamide is not known to be dose-dependent across its clinically used dose range (Wiki). Abnormal LFTs have occurred with bicalutamide (at rates of 2.9% to 11.4%) even at relatively low doses in cisgender women (e.g., 10–50 mg/day) (de Melo Carvalho, 2022). Due to its risk of liver toxicity, periodic liver monitoring is strongly advised with bicalutamide, especially within the first 6 months of treatment. Possible signs of liver toxicity include nausea, vomiting, abdominal pain, fatigue, appetite loss, flu-like symptoms, dark urine, and jaundice (yellowing of the skin/eyes) (Wiki).
In terms of its lung toxicity risk, bicalutamide has been associated rarely with interstitial pneumonitis, which can lead to pulmonary fibrosis and can be fatal, and also less often with eosinophilic lung disease (Wiki; Table). As of writing, 15 published case reports of interstitial pneumonitis and 2 case reports of eosinophilic lung disease in association with bicalutamide therapy exist, likewise all in men with prostate cancer (Table). As with liver toxicity, hundreds of additional cases of interstitial pneumonitis in people taking bicalutamide exist in the United States FAERS database (Wiki; FDA). It has been estimated that interstitial pneumonitis with bicalutamide occurs at a rate of around 1 in 10,000 people, although this may be an underestimate due to under-reporting (Wiki; Ahmad & Graham, 2003). Asian people may be especially likely to experience lung toxicity with bicalutamide and other NSAAs, as much higher incidences have been observed in this population (Mahler et al., 1996; Wu et al., 2022). There is no laboratory test for routine monitoring of lung changes with bicalutamide. Possible signs of relevant lung toxicity include dyspnea (difficulty breathing or shortness of breath), coughing, and pharyngitis (inflammation of the throat, typically manifesting as sore throat) (Wiki).
Aside from liver and lung toxicity, bicalutamide monotherapy has been found in cisgender men with prostate cancer to increase the risk of death due to causes other than prostate cancer (Iversen et al., 2004; Iversen et al., 2006; Wellington & Keam, 2006; Jia & Spratt, 2022; Wiki). This led to marketing authorization of bicalutamide for treatment of the earliest stage of prostate cancer being revoked and to the drug being abandoned for this use (Wiki). Bicalutamide remains approved and used for treatment of later stages of prostate cancer, as the antiandrogenic benefits of bicalutamide against prostate cancer outweigh any adverse influence it has on non-prostate-cancer mortality in these more severe stages. The mechanisms underlying the increase in risk of death with bicalutamide in men are unknown (Wiki). It is also unclear whether bicalutamide could likewise increase the risk of death in transfeminine people. Limitations of generalizing these studies to transfeminine people include the men in the trials being relatively old and ill, a relatively high dosage of bicalutamide (150 mg/day) being used in the trials for an extended duration (e.g., 5 years), the question of whether the risks were due to androgen deprivation or to specific drug-related toxicity of bicalutamide, and estradiol levels with bicalutamide monotherapy in men with prostate cancer being only about 30 to 50 pg/mL (110–184 pmol/L) (Wiki). The preceding estradiol levels are well above castrate levels and are sufficient for a substantial degree of estrogenic effect, but are nonetheless below those recommended for transfeminine people and potentially needed for full sex-hormone replacement (which are ≥50 pg/mL [≥184 pmol/L]). In any case, as the specific mechanisms underlying the increased mortality risk with bicalutamide seen in men with prostate cancer are uncertain, and as clinical safety data showing that the risk does not generalize do not exist, it remains a possibility that bicalutamide could also increase the risk of death in transfeminine people.
Bicalutamide is taken orally in the form of tablets (e.g., 50 and 150 mg) (Wiki). Due to saturation of absorption in the gastrointestinal tract, the oral bioavailability of bicalutamide progressively starts to decrease above a dosage of about 150 mg/day, and there is no further increase in bicalutamide levels above 300 mg/day (Wiki; Graph). Bicalutamide has a very long elimination half-life of about 6 to 10 days (Wiki; Graphs). As a result, it does not necessarily have to be taken daily, and can be dosed less often (in proportionally higher doses)—for instance twice weekly or even once weekly—if this is more convenient or otherwise desired. Due to its long half-life, bicalutamide requires about 4 to 12 weeks to fully reach steady-state levels (Wiki; Graph; Wiki). However, about 50% of steady state is reached within 1 week of administration of bicalutamide, and about 80 to 90% of steady state is reached after 3 to 4 weeks (Wiki; Graph; Wiki). Loading doses of bicalutamide can be taken to reach steady state more quickly if desired. Animal studies originally suggested that bicalutamide did not cross the blood–brain barrier and hence was peripherally selective (i.e., did not block androgen receptors in the brain) (Wiki). However, subsequent clinical studies found that this was not similarly the case in humans, in whom bicalutamide shows clear and robust centrally mediated antiandrogenic effects (Wiki).
Older NSAAs related to bicalutamide like flutamide (Eulexin) and nilutamide (Anandron, Nilandron) have much greater risks in comparison to bicalutamide and should not be used in transfeminine people. Nilutamide was previously characterized as an antiandrogen in transfeminine people in several studies, but was not further pursued probably due to its very high incidence of lung toxicity and other side effects (Aly, 2020; Wiki; Wiki). Flutamide has been used limitedly as an antiandrogen in transfeminine people in the past, but should no longer be used due to a much higher risk of liver toxicity than bicalutamide as well as other side effects and drawbacks (Aly, 2020; Wiki). Other newer and more-potent NSAAs like enzalutamide (Xtandi), apalutamide (Erleada), and darolutamide (Nubeqa) also have risks and have been studied and used little outside of prostate cancer to date.
5α-Reductase Inhibitors
Testosterone is converted into DHT within certain tissues in the body (Swerdloff et al., 2017). DHT is an androgen metabolite of testosterone with several-fold higher activity than testosterone. The transformation of testosterone into DHT is mediated by the enzyme 5α-reductase. The tissues in which 5α-reductase is present and testosterone is converted into DHT are limited but most importantly include the skin, hair follicles, and prostate gland. Although DHT is more potent than testosterone, it is thought to have minimal biological role as a circulating hormone (Horton, 1992; Swerdloff et al., 2017). Instead, testosterone serves as the main circulating androgen, and the role of DHT is thought to be mainly via local metabolism and potentiation of testosterone into DHT within certain tissues.
5α-Reductase inhibitors (5α-RIs), such as finasteride (Proscar, Propecia) and dutasteride (Avodart), inhibit 5α-reductase and thereby block the conversion of testosterone into DHT. This results in marked decreases in circulating and within-tissue levels of DHT. Due to the primary role of DHT as a mediator in tissues rather than as circulating hormone, the antiandrogenic efficacy of 5α-RIs is limited. This is evidenced by the fact that they are well-tolerated in cisgender men and do not cause notable demasculinization in these individuals (Hirshburg, 2016). The medical use of 5α-RIs is mainly restricted to the treatment of scalp hair loss in men and women, hirsutism (excessive facial/body hair) in women, and prostate enlargement in men. They might also be useful for acne in women, but evidence of this is very limited (Wiki). Due to their specificity, 5α-RIs are inappropriate as general antiandrogens in transfeminine people. Moreover, DHT levels decrease in tandem with testosterone levels with suppression of testosterone production in transfeminine hormone therapy, and routine use of 5α-RIs in transfeminine people with testosterone levels within the female range is of limited usefulness and can be considered unnecessary (Gooren et al., 2016; Irwig, 2020; Prince & Safer, 2020; Glintborg et al., 2021). In any case, 5α-RIs may be useful in transfeminine people on hormone therapy who have persistent body hair growth or scalp hair loss—as they have been shown to be in cisgender women (Barrionuevo et al., 2018; Prince & Safer, 2020). However, it is notable that evidence of effectiveness in cisgender women is better for androgen receptor antagonists for such indications (van Zuuren et al., 2015). This is intuitive as androgen receptor antagonists block both testosterone and DHT whereas 5α-RIs only prevent conversion of testosterone into DHT. Hence, although 5α-RIs strongly reduce or eliminate DHT and their net effect is antiandrogenic, they do not decrease testosterone levels and in fact increase them.
There are three subtypes of 5α-reductase. Dutasteride inhibits all three subtypes of 5α-reductase whereas finasteride only inhibits two of the subtypes. As a result of this, dutasteride is a more complete 5α-RI than finasteride. Dutasteride decreases DHT levels in the blood by up to 98% while finasteride can only decrease them by around 65 to 70%. As nearly all circulating DHT originates from synthesis in peripheral tissues, these decreases indicate parallel reductions in tissue DHT production (Horton, 1992). In accordance with these findings, dutasteride has been found to be more effective than finasteride in the treatment of scalp hair loss in men (Zhou et al., 2018; Dhurat et al., 2020; Wiki). For these reasons, although both finasteride and dutasteride are effective 5α-RIs, dutasteride may be the preferable choice if a 5α-RI is used (Zhou et al., 2018; Dhurat et al., 2020).
A potentially undesirable effect of 5α-RIs in transfeminine people is that they may increase circulating testosterone levels to a degree in those in whom testosterone production isn’t fully suppressed (Leinung, Feustel, & Joseph, 2018; Aly, 2019; Traish et al., 2019; Irwig, 2020; Glintborg et al., 2021). It appears that DHT adds significantly to negative feedback on gonadotropin secretion in the pituitary gland in people with testes who have low testosterone levels relative to the normal male range (Traish et al., 2019). The therapeutic implications of this for transfeminine people, if any, are uncertain.
Clinical dose-ranging studies have found that lower doses of finasteride and dutasteride than are typically used still provide substantial or near-maximal 5α-reductase inhibition (Gormley et al., 1990; Vermeulen et al., 1991; Sudduth & Koronkowski, 1993; Drake et al., 1999; Roberts et al., 1999; Clark et al., 2004; Frye, 2006; Olsen et al., 2006; Harcha et al., 2014; Kuhl & Wiegratz, 2017). In one study with finasteride for instance, DHT levels decreased by 49.5% at 0.05 mg/day, 68.6% at 0.2 mg/day, 71.4% at 1 mg/day, and 72.2% at 5 mg/day (Drake et al., 1999). Parallel reductions in DHT levels were seen locally in the scalp (Drake et al., 1999). In a study with dutasteride, DHT levels were decreased by 52.9% at 0.05 mg/day, 94.7% at 0.5 mg/day, 97.7% at 2.5 mg/day, and 98.4% at 5 mg/day (Clark et al., 2004). Based on these findings, 5α-RIs can potentially be taken at lower doses to help reduce medication costs if needed. Finasteride tablets can be split to achieve smaller doses. Conversely, dutasteride cannot be split as it is formulated as an oil capsule. However, dutasteride has a long half-life, and instead of dividing pills, it can be taken less frequently (e.g., once every few days) as a means of reducing dosage.
5α-Reductase inhibitors are taken orally in the form of tablets and capsules. Compoundedtopical formulations of finasteride also exist (Marks et al., 2020). However, caution is advised with these preparations as they have been found to be excessively dosed and to produce equivalent systemic DHT suppression as oral finasteride formulations (Marks et al., 2020). Lower-concentration formulations of topical finsteride on the other hand may be more locally selective (Marks et al., 2020).
Table 8: Available forms and recommended doses of 5α-reductase inhibitors for transfeminine people:
GnRH agonists and antagonists (GnRHa), also known as GnRH receptor agonists and antagonists or GnRH modulators, are antiandrogens which work by preventing the effects of GnRH in the pituitary gland and thereby suppressing LH and FSH secretion. Receptor agonists normally activate receptors while receptor antagonists block and thereby inhibit the activation of receptors. Due to a physiological quirk however, GnRH agonists and antagonists have the same effects in the pituitary gland. This is because GnRH is secreted in pulses under normal physiological circumstances, and when the GnRH receptor is unnaturally activated in a continuous manner, as with exogenous GnRH agonists, the GnRH receptor in the pituitary gland is strongly desensitized to the point of becoming inactive. Consequently, both GnRH agonists and GnRH antagonists have the effect of abolishing gonadal sex hormone production. This, in turn, reduces testosterone levels into the castrate or normal female range (both <50 ng/dL or <1.7 nmol/L) in people with testes. GnRHa are like a reversible gonadectomy, and for this reason, are also sometimes referred to as “medical castration”. Provided that an estrogen is taken in combination with a GnRHa to prevent sex hormone deficiency, these medications have essentially no known side effects or risks. For these reasons, GnRHa are the ideal antiandrogens for use in transfeminine people.
GnRHa are widely used to suppess puberty in adolescent transgender individuals. Unfortunately however, they are very expensive (e.g., ~US$10,000 per year) and medical insurance does not usually cover them for adult transgender people. Consequently, GnRHa are not commonly used in adult transfeminine people at this time. An exception is in the United Kingdom, where GnRH agonists are covered for all adult transgender people by the National Health Service (NHS). Another exception is buserelin (Suprefact), which has become available very inexpensively in its nasal spray form from certain Eastern European online pharmacies in recent years (Aly, 2018).
GnRH agonists cause a brief flare in testosterone levels at the start of therapy prior to the GnRH receptors in the pituitary gland becoming desensitized (Wiki). Testosterone levels increase by up to about 1.5- to 2-fold for about 1 week and then decrease thereafter (Wiki). Castrate or female-range levels of testosterone are generally reached within 2 to 4 weeks (Wiki). In contrast to GnRH agonists, there is no testosterone flare with GnRH antagonists and testosterone levels start decreasing immediately, reaching castrate levels within a few days (Wiki; Graph). This is because GnRH antagonists work by blocking the GnRH receptor without initially activating it, and hence desensitization of the receptor is not necessary for their action. If desired, the testosterone flare at the initiation of GnRH agonist therapy can be prevented or blunted with the use of antigonadotropins, for instance estrogens and progestogens, as well as with potent androgen receptor antagonists such as bicalutamide (Wiki).
GnRH agonists must be injected subcutaneously or intramuscularly once per day or once every one to six months depending on the formulation employed (buserelin, goserelin, leuprorelin, triptorelin). Alternatively, they can be surgically implanted once a year (histrelin, leuprorelin) or used as a nasal spray two to three times per day (buserelin, nafarelin). The first GnRH antagonists were developed for use by once-monthly intramuscular or subcutaneous injection (abarelix, degarelix). More recently, orally administered GnRH antagonists such as elagolix and relugolix have been introduced for medical use. They are taken in the form of tablets once or twice daily.
Table 9: Available forms and recommended doses of GnRH agonists for transfeminine people:
a 500 μg 3x/day for the first week then 200 μg/day. b 800 μg 3x/day for the first week then 400 μg 3x/day. c 500 μg 2x/day can be used instead of 400 μg 3x/day but is less effective (70% decrease in testosterone levels (to ~180 ng/dL [6.2 nmol/L]) instead of 90% decrease (to ~50 ng/dL [1.7 nmol/L]) per available studies of buserelin in men with prostate cancer) (Aly, 2018; Wiki).
Table 10: Available forms and recommended doses of GnRH antagonists for transfeminine people:
a First month is 240 mg then 80 mg per month thereafter. b 150 mg 1x/day is less effective than 200 mg 2x/day (which provides full gonadal sex-hormone suppression in cisgender women) (Wiki). c 80–120 mg/day for full gonadal sex-hormone suppression and 20–40 mg/day for substantial but partial gonadal sex-hormone suppression (MacLean et al., 2015; DailyMed).
Other Hormonal Medications
Androgens and Anabolic Steroids
In addition to estrogens, progestogens, and antiandrogens, androgens/anabolic steroids (AAS) are sometimes added to transfeminine hormone therapy. This is when testosterone levels are low (e.g., below the female average of 30 ng/dL [1.0 nmol/L]) and androgen replacement is desired. It has been proposed that adequate levels of testosterone may provide benefits such as increased sexual desire, improved mood and energy, positive effects on skin health and cellulite (Avram, 2004), and increased muscle size and strength (Huang & Basaria, 2017). However, there is insufficient clinical evidence to support such benefits at present, and androgens can produce adverse effects in cisgender women and transfeminine people, for instance acne, hirsutism, scalp hair loss, and masculinization (Wiki). For transfeminine people who nonetheless desire androgen replacement therapy, possible options for androgen medications include testosterone and its esters, dehydroepiandrosterone (DHEA; prasterone), and nandroloneesters such as nandrolone decanoate (ND) (Aly, 2020; Table), among others.
Monitoring of Therapy
Transfeminine people on hormone therapy should undergo regular laboratory monitoring in the form of blood work to assess efficacy and monitor for safety. Total estradiol levels and total testosterone levels should be measured to assess the effectiveness of therapy—that is, whether hormone levels are in appropriate ranges for cisgender females—and determine whether medication adjustments may be necessary. Levels of free testosterone, free estradiol, estrone (E1), dihydrotestosterone (DHT), luteinizing hormone (LH), follicle-stimulating hormone (FSH), and sex hormone-binding globulin (SHBG) can also be measured to provide further information although they’re not absolutely necessary. If progesterone is used as a part of hormone therapy, progesterone levels can be measured to provide insight on the degree of progesterone exposure. In addition to hormone blood tests, transfeminine people can monitor their physical changes with hormone therapy, such as breast development and other aspects of feminization, using various physical and digital measurement methods (e.g., Wiki).
Certain therapeutic situations can result in inaccurate lab blood work results. Monitoring of progesterone levels with oral progesterone using immunoassay-based blood tests can result in falsely high readings for progesterone levels due to cross-reactivity with high levels of progesterone metabolites such as allopregnanolone (Aly, 2018; Wiki). Instead of immunoassay-based tests, mass spectrometry-based tests should be used to determine progesterone levels with oral progesterone (Aly, 2018; Wiki). Conversely, either type of test may be used to measure progesterone levels with non-oral progesterone therapy. High-dose biotin (vitamin B7) supplements can interfere with the accuracy of immunoassay-based hormone blood tests, causing falsely low or falsely high readings (Samarasinghe et al., 2017; Avery, 2019; Bowen et al., 2019; FDA, 2019; Luong, Male, & Glennon, 2019). Transdermal estradiol formulations applied to the arm can result in contamination of blood draws taken from the same arm and can result in falsely high readings for estradiol levels (Vihtamäkia, Luukkaala, & Tuimala, 2004).
Certain cancers are known to be hormone-sensitive and their incidence can be influenced by hormone therapy. Screening for breast and prostate cancer is recommended in transfeminine people (Sterling & Garcia, 2020; Iwamoto et al., 2021). The risk of breast cancer appears to be dramatically increased with transfeminine hormone therapy, perhaps especially with progestogens (Aly, 2020). However, the risk still remains lower than in cisgender women (Aly, 2020). The incidence of prostate cancer is greatly decreased with hormone therapy in transfeminine people as a consequence of androgen deprivation, but the risk is not abolished and prostate cancer can still occur (de Nie et al., 2020). The prostate gland is not removed with vaginoplasty, so transfeminine people who have undergone vaginoplasty will also require monitoring for prostate cancer still. Testicular cancer is not known to be a hormone-dependent cancer and its incidence does not appear to be increased with hormone therapy in transfeminine people (Bensley et al., 2021; de Nie et al., 2021; Jacoby et al., 2021).
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\ No newline at end of file
diff --git a/transfemscience.org/assets/js/hover-refs.js b/transfemscience.org/assets/js/hover-refs.js
index 51622943..7e461154 100644
--- a/transfemscience.org/assets/js/hover-refs.js
+++ b/transfemscience.org/assets/js/hover-refs.js
@@ -125,8 +125,82 @@
const isInReferenceList = referenceLists.some(list => list.contains(link));
if (isInReferenceList) return;
- // We only care if the link HAS an href and it's in our map
+ // We only care if the link HAS an href
if (rawHref && !rawHref.startsWith('#')) {
+
+ // FIRST: Check if this link should show a hover box based on parentheses
+ // Check if the link itself contains parentheses with a year (e.g. "(2005)" or "(2005a)" or "(Aly, 2020)")
+ // We allow other text inside the parens, but it MUST contain a year-like number.
+ const yearParensRegex = /\([^)]*\b\d{4}[a-z]?\b[^)]*\)/i;
+ const textHasParens = link.textContent.includes('(') || link.textContent.includes(')');
+ const textMatchingParensYear = yearParensRegex.test(link.textContent);
+
+ // Check if enclosed in parentheses by walking siblings
+ let isEnclosed = false;
+ // We only scan if:
+ // 1. The text itself doesn't contain matching parens (if it does, we already know if it's valid or not)
+ // OR
+ // 2. The text doesn't contain parens at all (so we look for surrounding ones)
+
+ if (!textMatchingParensYear && !textHasParens) {
+ // If text has parens but didn't match yearParensRegex, it's invalid (e.g. "Kuhl (Citation)")
+ // So we only scan if text does NOT have parens.
+
+ let openParenCount = 0;
+ let foundOpen = false;
+
+ let curr = link.previousSibling;
+ let scans = 0;
+ const MAX_SCANS = 100; // Reasonable lookbehind limit
+
+ while (curr && scans < MAX_SCANS) {
+ if (curr.nodeType === 3) { // Text node
+ const txt = curr.textContent;
+ // Count parens from right to left
+ for (let i = txt.length - 1; i >= 0; i--) {
+ const c = txt[i];
+ if (c === ')') openParenCount--;
+ else if (c === '(') openParenCount++;
+
+ if (openParenCount > 0) {
+ foundOpen = true;
+ break;
+ }
+ }
+ } else if (curr.nodeType === 1) { // Element node
+ const tagName = curr.tagName;
+ // Stop at block boundaries
+ if (/^(DIV|P|BODY|MAIN|SECTION|BLOCKQUOTE|UL|OL|LI|TABLE|BR|HR|H[1-6])$/.test(tagName)) {
+ break;
+ }
+ // Check text content of inline elements
+ const txt = curr.textContent;
+ for (let i = txt.length - 1; i >= 0; i--) {
+ const c = txt[i];
+ if (c === ')') openParenCount--;
+ else if (c === '(') openParenCount++;
+
+ if (openParenCount > 0) {
+ foundOpen = true;
+ break;
+ }
+ }
+ }
+
+ if (foundOpen) break;
+ curr = curr.previousSibling;
+ scans++;
+ }
+
+ if (foundOpen) {
+ isEnclosed = true;
+ }
+ }
+
+ // Only proceed if link has parentheses (in text or structurally enclosed)
+ if (!textMatchingParensYear && !isEnclosed) return;
+
+ // SECOND: Try to find matching reference in urlMap
const exactHref = normalizeRefUrl(rawHref);
const baseHref = exactHref.split('#')[0];
@@ -135,91 +209,25 @@
bestMatchHTML = urlMap.get(baseHref);
}
- // Fallback for unmatched links (e.g. Wiki links, or refs without date/year)
- // User requested to show just the URL, but ONLY if it looks like a ref (in parentheses)
+
+ // THIRD: Fallback for unmatched links - show the URL itself
if (!bestMatchHTML) {
- // Check if the link itself contains parentheses with a year (e.g. "(2005)" or "(2005a)" or "(Aly, 2020)")
- // We allow other text inside the parens, but it MUST contain a year-like number.
- const yearParensRegex = /\([^)]*\b\d{4}[a-z]?\b[^)]*\)/i;
- const textHasParens = link.textContent.includes('(') || link.textContent.includes(')');
- const textMatchingParensYear = yearParensRegex.test(link.textContent);
-
- // Check if enclosed in parentheses or brackets by walking siblings
- let isEnclosed = false;
- // We only scan if:
- // 1. The text itself doesn't contain matching parens (if it does, we already know if it's valid or not)
- // OR
- // 2. The text doesn't contain parens at all (so we look for surrounding ones)
-
- if (!textMatchingParensYear && !textHasParens) {
- // If text has parens but didn't match yearParensRegex, it's invalid (e.g. "Kuhl (Citation)")
- // So we only scan if text does NOT have parens.
-
- let openParenCount = 0;
- let openBracketCount = 0;
- let foundOpen = false;
-
- let curr = link.previousSibling;
- let scans = 0;
- const MAX_SCANS = 100; // Reasonable lookbehind limit
-
- while (curr && scans < MAX_SCANS) {
- if (curr.nodeType === 3) { // Text node
- const txt = curr.textContent;
- // Count parens from right to left
- for (let i = txt.length - 1; i >= 0; i--) {
- const c = txt[i];
- if (c === ')') openParenCount--;
- else if (c === '(') openParenCount++;
- else if (c === ']') openBracketCount--;
- else if (c === '[') openBracketCount++;
-
- if (openParenCount > 0 || openBracketCount > 0) {
- foundOpen = true;
- break;
- }
- }
- } else if (curr.nodeType === 1) { // Element node
- const tagName = curr.tagName;
- // Stop at block boundaries
- if (/^(DIV|P|BODY|MAIN|SECTION|BLOCKQUOTE|UL|OL|LI|TABLE|BR|HR|H[1-6])$/.test(tagName)) {
- break;
- }
- // Check text content of inline elements
- const txt = curr.textContent;
- for (let i = txt.length - 1; i >= 0; i--) {
- const c = txt[i];
- if (c === ')') openParenCount--;
- else if (c === '(') openParenCount++;
- else if (c === ']') openBracketCount--;
- else if (c === '[') openBracketCount++;
-
- if (openParenCount > 0 || openBracketCount > 0) {
- foundOpen = true;
- break;
- }
- }
- }
-
- if (foundOpen) break;
- curr = curr.previousSibling;
- scans++;
- }
-
- if (foundOpen) {
- isEnclosed = true;
- }
+ // Exception: Skip if link text starts with lowercase (e.g. "(see here)" or "(more info)")
+ // These are likely general parenthetical links, not citations
+ const linkText = link.textContent.trim();
+ const firstChar = linkText.charAt(0);
+ if (firstChar && firstChar === firstChar.toLowerCase() && firstChar !== firstChar.toUpperCase()) {
+ return; // Skip lowercase-starting unmatched links
}
-
- if (textMatchingParensYear || isEnclosed) {
- let displayUrl = rawHref;
- if (rawHref.startsWith('/')) {
- displayUrl = 'https://transfemscience.org' + rawHref;
- }
- bestMatchHTML = `
`;
}
+
if (bestMatchHTML) {
link.classList.add('reference-link');
diff --git a/transfemscience.org/feed-posts.xml b/transfemscience.org/feed-posts.xml
index b2b95e82..954fe7cf 100644
--- a/transfemscience.org/feed-posts.xml
+++ b/transfemscience.org/feed-posts.xml
@@ -1 +1 @@
-Jekyll2026-02-06T16:23:17-08:00https://transfemscience.org/feed-posts.xmlTransfeminine ScienceTransfeminine Science is a site for information on hormone therapy for transfeminine people.Transfeminine Science
\ No newline at end of file
+Jekyll2026-02-09T15:58:58-08:00https://transfemscience.org/feed-posts.xmlTransfeminine ScienceTransfeminine Science is a site for information on hormone therapy for transfeminine people.Transfeminine Science
\ No newline at end of file
diff --git a/transfemscience.org/feed.xml b/transfemscience.org/feed.xml
index f7bc6f46..141b255a 100644
--- a/transfemscience.org/feed.xml
+++ b/transfemscience.org/feed.xml
@@ -1,4 +1,4 @@
-Jekyll2026-02-06T16:23:17-08:00https://transfemscience.org/feed.xmlTransfeminine Science | ArticlesTransfeminine Science is a site for information on hormone therapy for transfeminine people.Transfeminine ScienceA Review of Pharmaceutical Interventions for Scalp Hair Loss and Implications for Transfeminine People2025-09-08T18:00:00-07:002025-09-09T00:00:00-07:00https://transfemscience.org/articles/hair-lossA Review of Pharmaceutical Interventions for Scalp Hair Loss and Implications for Transfeminine People
+Jekyll2026-02-09T15:58:58-08:00https://transfemscience.org/feed.xmlTransfeminine Science | ArticlesTransfeminine Science is a site for information on hormone therapy for transfeminine people.Transfeminine ScienceA Review of Pharmaceutical Interventions for Scalp Hair Loss and Implications for Transfeminine People2025-09-08T18:00:00-07:002025-09-09T00:00:00-07:00https://transfemscience.org/articles/hair-lossA Review of Pharmaceutical Interventions for Scalp Hair Loss and Implications for Transfeminine People
@@ -2377,7 +2377,9 @@ Figure 5. Meta-analysis of estradiol concentration-time data from cisgender wome
A letter to the editor commented on and concorded with Rothman and colleagues’ second literature review:
-
Patel, K. T., & Tangpricha, V. (2024). Parenteral Estradiol for Transgender Women: Time to adjust the dose. Endocrine Practice, 30(9), 893–894. [DOI:10.1016/j.eprac.2024.07.005]
+
+
Patel, K. T., & Tangpricha, V. (2024). Parenteral Estradiol for Transgender Women: Time to adjust the dose. Endocrine Practice, 30(9), 893–894. [DOI:10.1016/j.eprac.2024.07.005]
Figure 5. Meta-analysis of estradiol concentration-time data from cisgender women under EEn alone or EEn/DHPA. Fitted data curves from various studies individually and combined into a single-dose curve over 30 days were generated based on an informal meta-analysis of published estradiol concentration-time data from cisgender women under EEn or EEn/DHPA [17]. […]
_in_postmenopausal_women.png){kind=link}
