Masculinising Hormone Therapy — Evidence Review (ER-007)

Masculinising hormone therapy (testosterone) is widely used in gender-affirming care, yet accurate, consumer-facing accounts of its evidence base are scarce and often oversimplified. This Evidence Review grades what is known — and what remains uncertain — across eight domains: mechanism of action; virilising effects and their timelines; cardiovascular and metabolic effects; bone health; fertility and reproduction; routes and formulations; mental health and quality of life; and the overall quality of the evidence. Each claim is graded for certainty and checked against primary literature, with cisgender-derived data flagged throughout. Key graded positions: virilisation is predictable in direction but variable in degree, with some changes permanent and others reversible; the rise in haematocrit is the best-characterised safety effect and is formulation- and exposure-associated rather than a simple peak-driven phenomenon; hard cardiovascular outcomes remain genuinely uncertain (neither reassurance nor alarm is well supported); bone density is maintained largely through aromatisation of testosterone to oestradiol, so oestrogen should not be suppressed to zero; testosterone is neither reliable contraception nor a cause of permanent infertility, though long-term population fertility outcomes are uncertain; and mental-health benefits are consistent but low-certainty by study design. The review is the trans-masculine counterpart to the platform's Feminising Hormone Therapy review.