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Testosterone during transition

Testosterone is administered in gender care to trans men and some non-binary people who wish for masculinisation. It works quickly, produces broad and partly irreversible bodily changes, and requires lifelong dosing and monitoring. A substantial part of the long-term research for this application is missing; many safety statements are extrapolated from hormone replacement in hypogonadism.

Routes of administration

• Injections: weekly or several times a month (testosterone undecanoate, enanthate). High peak, then decline.
• Gel: applied daily to the skin, more stable levels, risk of transfer to partners and children on skin contact.
• Patches: daily, good dose accuracy.
• Implants: long-acting, less frequent.

Effects

• Voice deepening — permanent, often appears within a few months.
• Increase in facial and body hair — largely permanent.
• Enlargement of the clitoris — permanent.
• Stopping of menstruation — with long-term use this can be definitive.
• Increase in muscle mass, redistribution of fat towards the abdomen, stronger body odour.
• Acne, oily skin, sometimes accelerated androgenic hair loss (with genetic predisposition).

Breast tissue does not diminish with testosterone — for that a mastectomy is needed. Body height, pelvis and hands do not change (already after puberty). In women who start testosterone before skeletal maturity, height can be affected.

Permanent and reversible effects

Voice deepening, clitoral enlargement and largely the body hair are permanent. Fat distribution and muscle mass partly normalise on stopping; menstruation often, but not always, returns.

Fertility

Testosterone suppresses menstruation and ovulation but is not a contraceptive. Pregnancy is possible during use. With long-term use, the influence on fertility is not fully predictable. Cryopreservation of eggs or embryos before starting is advisable, although that requires temporary hormonal stimulation which is psychologically burdensome for some. See fertility and transition.

Risks and monitoring

• Erythrocytosis: too high a concentration of red blood cells, raises thrombosis risk. Requires regular blood checks and sometimes dose adjustment or therapeutic phlebotomy.
• Cardiovascular: testosterone can adversely affect the cholesterol profile (HDL falls, LDL rises). Whether this translates into elevated cardiovascular morbidity with long-term use in trans men is insufficiently researched.
• Vaginal atrophy: with long-term use the vaginal mucosa becomes thinner and more vulnerable, with complaints and possibly an increased risk of infection.
• Uterus and ovaries: chronic testosterone exposure of an intact uterus is insufficiently researched. Some clinicians advise hysterectomy after several years — a choice with its own irreversible consequences.
• Liver: with some routes of administration an increased burden, monitoring of liver function is standard.

At least annual lab monitoring (haematocrit, liver, lipids, hormone levels) is essential. Self-medication without monitoring is medically irresponsible.