Vellus hairs are soft, fine, and usually short. Examples of vellus hair in women are found on the face, chest, back, etc. Terminal hairs are longer, coarser, and thicker such as those found in the scalp, pubic area and axilla of both men and women. In men, they are also found on the face and body. Thus, increasing numbers of hair grow and fall out at the same time giving the impression of shedding.
Various hormones control hair growth.
Thyroid hormone and growth hormone affect hair growth. The most important one is testosterone . A good example is puberty in which in both sexes, androgens induce the appearance of pubic and axillary hair. Once vellus hair has been transformed into terminal hair, it cannot change back even if androgens disappear. Therefore, hirsutism must be treated promptly to minimize the number of terminal hairs induced by the excess androgens.