Hormone Therapy

Q: Are the effects of hormone therapy irreversible?

Several effects are irreversible: breast development with oestrogen, and voice deepening and clitoral enlargement with testosterone. Fertility may be permanently lost. Other effects are partly reversible. Facial and body hair growth caused by testosterone is also permanent, and after removal of the testicles, lifelong oestrogen replacement is required. Breast tissue does not disappear as a result of testosterone.

Q: How long does it take for hormone therapy to have an effect?

Initial effects become noticeable after a few weeks; pronounced changes develop over months to years, with maximum effect after 2-5 years. With testosterone, these include voice deepening, increased hair growth and increased muscle mass; with oestrogen, breast development, fat redistribution and reduced muscle mass. Some of these changes are permanent.

Q: Is hormone therapy lifelong?

In practice, almost always, yes. After removal of the gonads, replacement therapy is medically necessary. Stopping treatment leads to a partial return of the original characteristics and, in some people, severe psychological symptoms. Long-term studies spanning more than ten to twenty years are also scarce. This is unusual for a lifelong treatment.

Q: Does hormone therapy affect fertility?

Yes. Both hormones can permanently eliminate fertility. Oestrogen leads to reduced sperm production and testosterone usually stops menstruation. Sperm damage may be permanent. Anyone who may want children later must therefore make arrangements before starting treatment. Freezing and storing reproductive cells before starting treatment is advisable; see fertility and transition.

Box of Progynova 2 mg (estradiol valerate) with a blister strip and two loose blue tablets. — Photo: Medgirl131, CC BY-SA 4.0, via Wikimedia Commons

In gender treatments, cross-sex hormones are administered to change the body in the direction of the opposite sex. This is not supplementation for a deficiency, as in endocrine disorders, but a pharmacological switch of a healthy hormonal system. The consequences are far-reaching, partly irreversible, and lifelong medication is required in almost all cases.

Feminising hormone therapy

Trans women and some non-binary people receive oestrogen, usually in combination with an anti-androgen (cyproterone acetate, bicalutamide or spironolactone) to suppress testosterone. Effects include breast development, fat redistribution, reduced muscle mass, decreased erectile function and reduced sperm production. The voice and skeleton no longer change; height, shoulder width and hands remain male.

Breast development and sperm damage can be permanent. After removal of the testicles (orchiectomy), lifelong oestrogen replacement is needed to counter bone loss and other consequences of a hormone-free state.

Masculinising hormone therapy

Trans men receive testosterone by injection, gel or patch. The voice deepens, facial and body hair increase, the clitoris enlarges, menstruation usually stops, fat distribution shifts and muscle mass increases. Voice deepening, increased hair growth and clitoral enlargement are permanent. Breast tissue does not disappear as a result of testosterone; a mastectomy is required for that.

With long-term use, the body's own sex hormone production may decrease or stop. After removal of the ovaries, lifelong testosterone replacement is required.

Indication and access

In the Netherlands, adults start hormone therapy after a diagnostic process at a specialised centre (Amsterdam UMC, Radboudumc). Because of enormous waiting times and the expansion of GP-led care and foreign providers, the diagnostic process in practice is sometimes limited. Self-medication via the internet does occur and carries medical risks. See waiting times and care pathway.

Effectiveness: what do we know, and what don't we?

Studies on psychological wellbeing after hormone therapy often show positive results at group level, but the methodological quality is weak across the board: small groups, short follow-up, high dropout rates, no control group, and outcome measures that rely heavily on self-reporting by a motivated group.

The Cass Review (2024) rated virtually all evidence for hormone therapy in young people as 'low' or 'very low quality'. The Swedish SBU and the Finnish COHERE reached similar conclusions. For adults, the evidence is somewhat stronger, but long-term studies (>10-20 years) are scarce. This is medically unusual for a treatment that is continued for life in practice.

Risks

Oestrogen increases the risk of thrombosis (especially when taken orally), affects the cardiovascular risk profile, may strain liver function and may be linked to breast cancer with long-term use. Cyproterone acetate is associated with meningioma. Testosterone causes erythrocytosis (increasing the risk of thrombosis), may adversely affect the cholesterol profile and causes vaginal atrophy. For both hormones, the rule is: lifelong dosing, lifelong monitoring. See side effects of hormone therapy. Which changes remain and which reverse after stopping is explained on gendermolen.nl.

International policy shift

Since 2020, Sweden, Finland, Norway, Denmark and the UK have tightened or restricted access to hormones for minors to research settings. They base this on systematic evidence reviews concluding that the benefits for young people have not been sufficiently demonstrated to justify the risks. NHS England paused new referrals of 16- and 17-year-olds for cross-sex hormones in March 2026. In the Netherlands, the Health Council concluded in June 2026 that transgender care for young people is carefully organised.

Are the effects of hormone therapy irreversible?

How long does it take for hormone therapy to have an effect?

Is hormone therapy lifelong?

Does hormone therapy affect fertility?