Androgenetic alopecia (AGA) is the most common form of hair loss in both men and women, arising from a combination of genetic predisposition and follicular sensitivity to androgens — particularly dihydrotestosterone (DHT).

How It Develops

In genetically susceptible individuals, activity of the enzyme 5-alpha reductase — which converts testosterone into the more potent androgen DHT — and androgen receptor density are both elevated. DHT-sensitive follicles progressively shrink (miniaturization), the anagen phase shortens, and hairs become thinner and shorter until they are reduced to fine "vellus" hairs.

Different Patterns in Men and Women

In men, loss typically begins at the temples and crown (Norwood scale). In women, the hairline is usually preserved while diffuse thinning develops at the crown with a widening center part (Ludwig scale).

Key point: Follicles in the back and sides of the scalp — the typical donor area — are genetically resistant to DHT. This is the entire scientific basis of hair transplantation: these follicles retain their resistance even after being moved (the principle of "donor dominance").