the size of the hair follicle and diameter of the hair fiber, as well as shift hairs from the growing to resting state (6). The main androgen circulating in the plasma of men is testosterone, whereas the most important androgen in women is andro-stenedione. These androgens are metabolized by the enzyme 5a-reductase, reducing testosterone to dihydrotestosterone (DHT) and androstenedione to testosterone (and some DHT). In balding scalps, DHT production and the level of 5a-reductase activity are increased relative to nonbalding scalps (7). Another important enzyme, aromatase, has recently gained some attention (8,9). Aromatase is specifically located in the outer root sheath of hair follicles. It converts androgens (e.g., testosterone) to estrogens (e.g., estradiol), and there may be a two- to fivefold increase in the amount of aromatase in the scalp of women relative to men (8,9). This finding may explain the different clinical presentation of androgenetic alopecia in men and women.
The clinical presentation of androgenetic alopecia is different for men and women. It may occur as early as 17 years of age in normal males and by 25 to 30 years of age in endocrinologically normal females (6). There is no evidence, however, to suggest that there is an age at which the onset of the balding process is no longer initiated or a threshold age at which the progression of baldness ceases to continue (7,10). Invariably, both men and women see increased shedding of hair, which prompts them to seek out medical advice.
In men, androgenetic alopecia is usually progressive, typically receding from the normal hairline in an M-shaped pattern with an enlarging balding vertex (6). Several classification systems have been used to characterize the balding state of men, the most popular being the Hamilton scale as modified by Norwood (Fig. 1) (11) and the Savin scale (Fig. 2). Women often do not present with a distinct pattern, but rather have diffuse hair loss or thinning of the temporal and parietal areas with retention of the frontal hairline in most cases. Ludwig (12) has described this pattern commonly seen in women (Fig. 3) and this classification is widely used and accepted. Women may present with the typical male patterning of hair loss, with this occurring more frequently in postmenopausal women than premenopausal women (13). In both sexes, concomitant loss of hair in the tem-ple-sideburn areas and nape of neck can be observed as well as occasional increases in body hair.
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