A number of hormones affect hair growth, with thyroid and growth hormones producing a gen eralized increased growth in hair. Estrogens have only minimal effects on hair growth. Androgens are the most important determinant of the type of hair distributed throughout the body. The principal circulating androgen, testosterone, is converted in the hair follicle by 5-alpha reductase to dihydrotestosterone (DHT), which stimulate the dermal papilla to produce a terminal melanized hair. The effect of androgens on hair growth is skin area-specific, due to local variations in androgen receptor and 5-alpha reductase content). While the effect of androgens on hairs (i. e., terminalization of vel-lus hairs) will be more readily apparent in skin areas with a greater numbers of hair follicles, hair follicle density does not correlate with fol-licular sensitivity to androgens. Some areas of the body, termed nonsexual skin (e. g., that of the eyelashes, eyebrows, and lateral and occipital aspects of the scalp), are relatively independent of the effect of androgens.
Other areas are quite sensitive to androgens. In these locations hair follicles are terminalized even in the presence of relatively low levels of androgens. Such areas include the pubic area and the axilla, which begin to develop terminal hair even in early puberty when only minimally increased amounts of androgens are observed. Finally, some areas of skin respond only to high levels of androgens. These sites include the chest, abdomen, back, thighs, upper arms, and face. The presence of terminal hairs in these areas is characteristically masculine, and if present in women is considered pathological, i. e., hirsutism.
Hirsutism is defined as the presence in women of terminal hairs in a male-like pattern. This affects between 5% and 10% of surveyed women. Hirsutism above all else should be principally considered a sign of an underlying endocrine or metabolic disorder, and these patients should undergo a thorough evaluation. The hormonal therapy of hirsutism consists of medications that either suppress androgen production, or block androgen action.
The main purpose of hormonal therapy is to stop new hairs from growing and potentially slow the growth of terminal hairs already present. Although hormonal therapy alone will sometimes produce a thinning and loss of pig mentation of terminal hairs, it generally will not reverse the terminalization of hairs.
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