Testosterone

Testosterone matters for men's bones, but not quite in the way most people assume. A good share of its bone-protective effect actually works through oestradiol — an oestrogen made when an enzyme called aromatase converts testosterone in the body. Men with conditions that block oestrogen action, even when testosterone itself is normal or high, end up with severely low bone density, which shows this conversion step is essential, not optional.

Testosterone naturally declines with age, typically picking up pace after 50, and bone loss tends to follow. A single total testosterone blood test can be misleading, because a protein called sex-hormone-binding globulin rises with age and can tie up testosterone, hiding a low free level behind a normal-looking total number. Morning samples on two separate days, along with SHBG, give a clearer picture if low testosterone is suspected.

Two situations deserve special attention. Androgen deprivation therapy for prostate cancer deliberately suppresses testosterone to very low levels, which removes oestrogen's bone protection too and can cause rapid bone loss along with loss of lean mass — comparable to what women experience at menopause. Similarly, if a man on testosterone treatment has his oestradiol aggressively suppressed by an aromatase inhibitor, his bones may suffer even if his testosterone number looks good. These are worth raising directly with the prescribing doctor, since decisions about these medications belong with them.