Glossary
5α-reductase (Steroid 5α-reductase)
The enzyme that converts testosterone into the more potent androgen DHT. Two clinically relevant isoforms exist: type 1 (sebaceous glands) and type 2 (hair follicle and prostate).
Also: 5-alpha-reductase, 5AR, SRD5A
5α-reductase is the enzyme family that converts testosterone into dihydrotestosterone (DHT). Two isoforms matter clinically.
Type 1 dominates sebaceous glands, keratinocytes, sweat glands, liver, and brain. It contributes only modestly to the DHT acting on hair follicles directly.
Type 2 dominates the hair follicle outer root sheath and dermal papilla, the prostate, beard, and genital skin. It is the primary driver of follicle miniaturisation in pattern hair loss.
Finasteride selectively blocks type 2 and lowers scalp DHT by roughly 64 to 70%. Dutasteride blocks both types and lowers scalp DHT by approximately 90%. The choice between them trades wider DHT suppression for a marginally larger hair effect, with similar side-effect profiles.