DHT (dihydrotestosterone)
The androgen chiefly responsible for follicular miniaturisation in genetically susceptible scalp regions. Finasteride works by inhibiting the enzyme that converts testosterone to DHT.
Dihydrotestosterone is the androgen at the centre of common hair loss. It is produced from testosterone by the enzyme 5α-reductase — the type II isoform being the one that matters most in scalp follicles — and it binds the androgen receptor more avidly than testosterone itself, making it the more potent signal wherever it is formed. Much of the DHT acting on follicles is produced peripherally, in tissues expressing the enzyme — including the follicle itself — so scalp biology can differ from what a blood test implies.
In genetically susceptible follicles, sustained DHT signalling drives miniaturisation: growth phases shorten, shafts emerge finer, and terminal hairs regress towards vellus ones over successive cycles. Susceptibility is written regionally into the scalp — frontal and crown follicles respond, occipital donor follicles largely do not — and that regional difference is both why pattern loss has a pattern and why transplantation works at all.
The same molecule illustrates the androgen paradox: DHT miniaturises scalp hair while driving beard and body hair growth, and it is essential to normal male development. It is not a toxin to be eliminated but a signal some follicles are programmed to over-read — which is why susceptibility, not hormone level, is the dominant variable. Men with identical DHT levels bald entirely differently; genetics sets the receiver, not just the broadcast. It is also why routine hormone panels rarely change management in typical male pattern loss.
Clinically, DHT is the field’s most successful drug target. Finasteride inhibits the type II enzyme and lowers scalp and serum DHT substantially, slowing or partially reversing pattern loss; dutasteride, used off-label for hair in most jurisdictions, inhibits both major isoforms. For the clinic, DHT is also the explanatory backbone of honest counselling: it is the reason loss is progressive, the reason medical stabilisation is discussed before surgery, and the reason transplanted occipital hair is expected to persist — resistant follicles keep their programming wherever they grow.