Masculinising Hormone Therapy

Evidence Note

What is the question?

Masculinising hormone therapy uses testosterone to bring about masculine physical changes and to ease gender dysphoria. This note is the short answer to a broad question: what does it actually do, which changes last, what needs watching — and, crucially, how strong the evidence really is. The full evidence, graded claim by claim, is set out in the companion review; this is the distilled version.

What does the evidence say?

Virilisation is reliable in direction, variable in degree. Most people with androgen-responsive tissue masculinise: a deeper voice, facial and body hair, clitoral growth, more muscle and less fat, oilier skin, and periods stopping. How far and how fast each change goes differs considerably between people, and the widely quoted timelines are clinical estimates rather than precise measurements.

Some changes are permanent, some are not. Voice deepening and clitoral growth are typically permanent; facial and body hair often persist at least in part; established male-pattern scalp-hair loss may not fully reverse. Fat distribution, muscle and menstruation are reversible if testosterone stops. Consent conversations are clearest when they keep the two apart.

The clearest thing to monitor is the blood count. Testosterone raises the haematocrit — the proportion of red cells in the blood — and a minority (about 11% in the largest cohort) rise above the usual threshold, most often in the first year. It is the best-characterised effect and the main routine safety check, managed by adjusting dose or formulation and, occasionally, other measures.

Bone stays stable — with a catch. Bone density is generally maintained or rises slightly, but that protection depends partly on the small amount of oestradiol the body makes from testosterone. The practical point: masculinising does not mean driving oestrogen to zero. A very low oestradiol on testosterone is a warning sign for bone, not a goal.

Testosterone is not contraception, and it does not appear to end fertility. Ovulation can continue even after periods stop, and pregnancy on testosterone is documented — so anyone who could become pregnant and does not want to needs a testosterone-compatible method of contraception. At the same time, menstruation and fertility markers often recover after stopping, and conception after prior testosterone is documented — though population-level chances of a live birth remain uncertain. Testosterone must never be taken during pregnancy: it can harm a developing fetus.

“No periods” does not mean “nothing to watch.” Hysterectomy studies show the lining of the womb can remain active despite the absence of periods, so unexpected bleeding is worth a clinician’s assessment, and cervical screening still applies on the usual schedule.

Mental-health symptoms often improve after starting testosterone — reported honestly. Starting testosterone is consistently associated with less depression and distress. That signal is real but low-certainty by design — mostly short, unblinded studies — and hormone therapy is not a substitute for mental-health care where that is needed.

How strong is the evidence?

Almost all of it is observational: a handful of European cohorts, cross-sectional studies and case series, plus a single small open-label trial on mood. The direction of the main effects — voice change, the blood-count rise — is firm. But the long-term, hard outcomes (heart attacks, strokes, clots, fractures, cancer, overall mortality) are largely unmeasured, so “no clear excess has been found” is reassurance of a limited kind, not proof of safety. Much of the surrounding reasoning is borrowed from cisgender populations. And nearly all of it describes standard masculinising doses — nonbinary and low-dose regimens cannot simply be read across.

Where are the uncertainties?

The largest gaps are long-term. Cardiovascular events, fracture risk, cancer and mortality across decades of testosterone are essentially unstudied, as are population-level fertility outcomes and the question of which formulation is safest. Timelines and risks for nonbinary and low-dose use are thinner still.

Testosterone is neither a contraceptive nor a guarantee of infertility — and more testosterone does not mean more masculine.

What does this mean for practice?

Expect change in a predictable direction but not on a fixed schedule, and be wary of reading someone else’s result as your own target. Above-range testosterone is not known to add masculinisation and may add risk, so the aim is a target range, not “more”. Haematocrit is worth monitoring; oestradiol should not be pushed to zero. If pregnancy is possible and unwanted, use a testosterone-compatible contraceptive rather than relying on testosterone; treat unexpected bleeding as worth checking; keep up cervical screening. And none of this is a reason to start, stop or change how testosterone is taken without a clinician — those decisions are made for the whole person, together.