Testosterone During Transition

Testosterone is prescribed in gender-affirming care to trans men and some non-binary people who want masculinisation. It works quickly, causes broad and partly irreversible physical changes, and requires lifelong dosing and monitoring. A significant proportion of the long-term research on this use is lacking; many safety statements are extrapolated from hormone replacement for hypogonadism.

Methods of administration

• Injections: testosterone enanthate weekly or every two weeks, testosterone undecanoate every 10 to 12 weeks. High peak levels followed by a decline.
• Gel: applied to the skin daily, with more stable levels, but a risk of transferring it to partners and children through skin contact.
• Patches: applied daily, with good dosing accuracy.
• Implants: long-acting and used less frequently.

Effects

• Deepening of the voice — permanent and often occurring within just a few months. See voice training.
• Increased facial and body hair — largely permanent.
• Enlargement of the clitoris — permanent. This is the basis of metoidioplasty.
• Stopping of menstruation — with long-term use, this may become permanent.
• Increased muscle mass, redistribution of fat towards the abdomen, and a stronger body odour.
• Acne, oily skin, and sometimes accelerated androgenic hair loss (in people with a genetic predisposition).

Testosterone does not reduce breast tissue — this requires a mastectomy. Height, the pelvis, and the hands do not change (once puberty has ended). In women who start testosterone before skeletal maturity, height may be affected.

Permanent and reversible effects

Deepening of the voice, clitoral enlargement, and most of the hair growth are permanent. Fat distribution and muscle mass partly return to their previous state after stopping; menstruation often, but not always, returns. More information about the irreversible voice changes caused by testosterone is available at genderzorgen.nl.

Fertility

Testosterone suppresses menstruation and ovulation, but it is not a contraceptive. Pregnancy is possible while using it. The effect of long-term use on fertility is not fully predictable. Freezing and storing eggs or embryos before starting is advisable, although this requires temporary hormone stimulation, which can be psychologically burdensome for some people. See fertility and transition.

Risks and monitoring

• Erythrocytosis: an excessively high concentration of red blood cells, increasing the risk of thrombosis. This requires regular blood tests and sometimes dose adjustment or therapeutic phlebotomy.
• Cardiovascular effects: testosterone can adversely affect the cholesterol profile (HDL decreases and LDL increases). Whether this translates into increased cardiovascular morbidity with long-term use in trans men has not been adequately studied.
• Vaginal atrophy: with long-term use, the vaginal lining becomes thinner and more fragile, causing symptoms and possibly increasing the risk of infections.
• Uterus and ovaries: chronic testosterone exposure of an intact uterus has not been adequately studied. Some clinicians recommend a hysterectomy after several years — a choice with its own irreversible consequences.
• Liver: some methods of administration place increased strain on the liver; monitoring liver function is standard.

Laboratory monitoring at least once a year (haematocrit, liver function, lipids, and hormone levels) is essential. Self-medication without monitoring is medically irresponsible. A full overview is available under side effects of hormone therapy and cardiovascular disease during transition.