aromatase

Aromatase is the enzyme that turns male hormones into female ones. It is the bridge between the two sex-steroid worlds: wherever it works, it converts androgens such as testosterone into estrogens such as estradiol, so even a man's body makes some estrogen using his own testosterone as raw material.

The enzyme is found in many tissues, notably fat (adipose) tissue, bone, brain, and the gonads. In men, aromatase-made estrogen is not a leftover curiosity but a necessary hormone: it is needed to close the growth plates at the end of puberty and to maintain healthy bone density. The amount of estrogen produced depends partly on body fat, since more fat tissue means more aromatase activity.

Because aromatase governs estrogen production, it is a therapeutic target. Aromatase inhibitors lower estrogen and are used mainly to treat estrogen-driven breast cancer in women. In men, excess aromatase activity (for example with obesity) can raise estrogen enough to cause gynecomastia, and rare mutations that disable the enzyme cause men to keep growing taller and develop thin bones, proving estrogen's quiet but essential role in male physiology.