How do androgens cause alopecia?

Androgens contribute to hair loss in both men and women. The pathogenesis of this process involves (1) increased sensitivity of affected hairs to the effects of testosterone, and (2) increased levels of circulating androgens (ovarian or adrenal source in women). Dihydrotestosterone seems to be responsible for the miniaturization of hairs in select locations. The androgen testosterone is converted to dihyrotestosterone by the enzyme 5-alpha reductase. Dihydrotestosterone causes a reduction in protein synthesis by inhibiting adenyl cyclase in the hair bulb; the result is shorter and finer hair, referred to as miniaturized hair. As the hair growth cycle proceeds, the hairs continue to become smaller and finer. Androgenetic alopecia may occur at any point in an individual’s life and is believed to be hereditary.

References:

1. Parker, Frank. Skin diseases of general importance. In: Goldman L, Bennett JC, eds. Cecil Textbook of Medicine. 21^st ed. Philadelphia, PA: W.B. Saunders Company; 2000: 2276-98.
2. Bolognia JL, Braverman IM. Skin manifestations of internal disease. In: Harrison’s Principles of Internal Medicine. 14^th ed. New York, NY: McGraw-Hill; 1998: 310-28.