5-alpha-reductase

5-alpha-reductase is the enzyme that upgrades testosterone into a stronger androgen. Picture it as a small workshop sitting inside certain tissues that takes the body's general-purpose male hormone and sharpens it into a more potent local version, DHT, exactly where extra androgen punch is needed.

The enzyme catalyzes the conversion of testosterone to dihydrotestosterone (DHT). It is concentrated in androgen-sensitive tissues such as the prostate, skin and hair follicles, and the external genitalia. There are two main forms of the enzyme encoded by different genes, with somewhat different tissue distributions, which matters for how selectively a drug can target one process versus another.

Blocking this enzyme is a useful medical strategy. Finasteride and dutasteride inhibit 5-alpha-reductase to lower DHT, shrinking the enlarged prostate of benign prostatic hyperplasia and slowing male-pattern hair loss. People born with a genetic deficiency of the enzyme show how specific its role is: they have normal testosterone and develop muscle and a deep voice at puberty, yet their DHT-dependent external genitalia are underformed at birth.