Feminising Hormone Therapy

Evidence Note

What is the question?

Feminising hormone therapy (FHT) uses oestrogen — almost always 17β-oestradiol, the same hormone the ovary makes — usually alongside a medication that lowers or blocks testosterone, to bring the body’s hormonal environment closer to the typical premenopausal female range. The question this note summarises is a practical one: what does that reliably change, what can it not change, and how good is the evidence for each?

What does the evidence say?

Oestradiol produces feminising changes in most adults who take it: breast development, softer and less oily skin, a more feminine distribution of body fat, reduced muscle bulk, and finer body hair. The changes are gradual, unfolding over months to years in a fairly consistent order. What hormones do not do is just as clear: they do not raise or lower the voice, they do not remodel the adult skeleton — jaw, brow, height, the bony frame — and they barely touch established facial hair.

Breast development is the best-measured outcome. Prospective studies show most growth happens in the first year and then largely plateaus, that final size is modest on average, and — importantly — that no measurable factor, including the oestradiol level in the blood, predicts how much a given person will develop. A practical rule follows: once testosterone is adequately suppressed, pushing oestradiol higher does not buy more feminisation. It mainly adds risk.

On safety, modern therapy with 17β-oestradiol is considered acceptably safe for most healthy adults when it is monitored, but it is not risk-free. The clearest signal is venous thromboembolism — blood clots — though the alarming historical figures came from an older, far more thrombogenic synthetic oestrogen, ethinylestradiol, that is no longer used; with modern oestradiol the absolute risk is much lower. Heart-attack risk does not appear to be raised. Overall mortality in the long-term cohorts is higher than in the general population, but the pattern points to smoking, comorbidity and the social determinants of health rather than to a direct effect of the hormones themselves.

How strong is the evidence?

It is uneven, and worth reading with that in mind. Breast development rests on objective, prospective cohort data — moderate-to-high confidence for this field. Most of the other physical timelines come largely from expert-consensus tables rather than careful measurement, so they are best treated as indicative. The safety evidence comes from a small number of observational, mostly European cohorts that cannot fully separate the hormones from confounders such as smoking and HIV; there are no randomised trials for the core long-term outcomes. The overall picture is moderate: strongest where outcomes have actually been measured, weaker where figures are extrapolated, and still evolving in the contested areas.

Where are the uncertainties?

Several important questions remain genuinely open. Why breast development varies so widely is unknown — no predictor has been found. Whether progesterone adds meaningful benefit is not established: the first randomised trial has reported encouraging early results at a conference and in a press release, but the peer-reviewed outcome paper is not yet available, so the claim remains preliminary. Which anti-androgen best balances effect against risk has never been settled on the outcomes patients actually care about. And robust long-term data on bone health in people taking modern regimens are thin. Much of the standard caution around clotting and stroke is reasonable, but it is extrapolated from studies of cisgender women rather than demonstrated directly in trans women.

Oestrogen reliably feminises the body. How far it will, no blood level can foretell.

That single fact — dependable direction, unpredictable degree — is the one most worth carrying into any conversation about starting.

What does this mean for practice?

This note is a summary, not a prescribing guide; decisions about starting, and with which medications, belong with a person and their own clinician. But a few conclusions are well enough supported to state plainly.

Adequate testosterone suppression matters more for feminisation than a high oestradiol level, and very high levels add risk without clear benefit. Route matters for clot risk: transdermal oestradiol (patch or gel) carries the lowest thrombogenic profile — firmly established in cisgender women, reasonably extrapolated to trans women — and is often preferred for those at higher risk, including smokers and older people. Stopping smoking is one of the highest-value things anyone on oestrogen can do. Where cyproterone acetate is used, the clear direction of practice is the lowest effective dose, given its prolactin, liver and meningioma signals. Baseline assessment, a discussion of fertility preservation before starting, and ongoing monitoring are all part of safe treatment.

Finally, expectations deserve as much care as prescriptions. Breast growth is usually modest and cannot be predicted in advance; for some, surgery later is a reasonable part of the plan rather than a sign that treatment failed. The body’s response is real, gradual and individual — and that individuality is not a problem to be solved so much as a fact to be planned around.