DHT does not attack hair; it shortens the growth phase of follicles that are genetically sensitive to it. Each cycle the follicle produces a finer, shorter hair, until it produces nothing visible. Follicles on the back and sides of the scalp are largely insensitive, which is why they survive โ and why they are the donor area.
Miniaturisation, not loss
A hair follicle cycles: a growth phase lasting years, a brief transition, a resting phase, then a new hair. DHT binds to receptors in sensitive follicles and shortens the growth phase. The hair produced in each successive cycle is thinner, shorter and less pigmented.
This is why early pattern loss looks like thinning rather than bald patches. Hair count barely changes at first; hair calibre does. By the time an area looks bare, the follicles there have been shrinking for years.
Why it follows a pattern
Sensitivity to DHT is not uniform across the scalp. It is concentrated at the frontal hairline, temples and crown, and largely absent at the back and sides. The pattern of loss is a map of that sensitivity, which is why it is so consistent between people โ and why the Norwood scale works as a description at all.
The same principle explains the whole basis of transplantation. Follicles moved from the insensitive donor area keep their insensitivity in their new position. They are not protected by where they end up; they were never susceptible in the first place.
What this means practically
| Observation | What it usually indicates |
|---|---|
| Hair getting finer at the temples | Early miniaturisation, follicles still present |
| Scalp visible under bright light only | Reduced calibre before reduced count |
| Even thinning across the whole scalp | Less typical of pattern loss โ worth investigating other causes |
| Dense back and sides despite a bare crown | The usual pattern, and what makes donor supply possible |
Where treatment fits
Because miniaturisation is gradual, treatment aimed at slowing it works best while follicles still exist. That is the reasoning behind starting medical treatment early and behind the sequencing question in PRP or transplant. Once follicles are gone, only grafts put hair back.
Not all loss is androgenetic. Sudden, diffuse or patchy loss, loss with scaling or scarring, and loss in women often has a different mechanism and needs assessment rather than assumption โ see hair loss in women.