Hormone levels vary from person to person, and vary within a person at different stages of their life. At the hair follicle, testosterone is converted by an enzyme called 5 alpha reductase into another form of testosterone called dihydrotestosterone or DHT, for short. DHT is the androgen hormone that triggers hair loss in androgenetic alopecia. When hair follicles that are genetically programmed to be receptive to DHT receive the DHT message over a long enough period of time, the follicle begins to produce less hair. Other hormones included in a class known as estrogens, can inhibit or counteract the follicle-shrinking effect of the androgens.
Women generally have more estrogens circulating in their blood than men.
As a result of this higher level of estrogens, even women having a genetic predisposition for pattern hair loss are protected from losing their hair because of the high level of estrogens in their blood. When these women reach menopause however, their estrogen level may decrease sufficiently, and the hair loss message may start to get through to the genetically programmed follicles. Their hair can begin to thin, sometimes dramatically. The specific way that time duration affects pattern hair loss is not completely understood. It is the genetically-programmed receptiveness of the hair follicles, and an adequate amount of time, not the amount of hormones, that cause hair follicle shrinkage, and the hair thinning that results.