Dihydrotestosterone is the most potent endogenous androgen — roughly 3-5x stronger at the androgen receptor than testosterone.
Mechanism — Converted from testosterone by 5-alpha-reductase (types 1 and 2). Drives androgen-mediated effects in androgen-sensitive tissue: prostate, scalp follicles, sebaceous glands, libido pathways. Inhibited by finasteride (5AR-2) and dutasteride (both types).
Lab range — Men: 30-85 ng/dL. Women: trace. Important to check on TRT, in male-pattern hair loss workup, and in PCOS evaluation.
Caveats — Suppression via finasteride can cause sexual side effects, including persistent post-finasteride syndrome in a small subset. The risk-benefit for hair preservation is personal.