The endowment of hair is determined early in gestation. It is genetically determined but is equal in both sexes. Thus, the number of hair follicles/skin area is greater in Caucasians than in Orientals and greater in Mediterranean than in Nordic individuals.
Hair grows from hair follicles within the skin. There are about 50 million hair follicles covering the body of which one fifth is located in the scalp. Only the soles and palms are free of hair follicles. As long as follicles are not destroyed hair continues to grow even if it is plucked, depilated or removed in any other fashion.

There are two types of hair as an adult; vellus hair and terminal hair. Vellus hairs are soft, tine, and usually short. They may be colorless giving the impression of hairless skin. Examples of vellus hair in women can be 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 in the face and body.

Hair growth occurs in cycles, but because hairs in the human are usually not in phase with each other (some are growing, some are resting, some are falling), they give the impression of continuous growth. The duration of this growth phase (also called anagen) determines the maximum length of hair in a particular area. For instance, in the scalp anagen lasts three years with a short resting phase (telogen). Thus 10-15% of the hairs are in the resting phase at any given time. In other areas (for instance, the arm), a short anagen and a tong telogen phase results in short hair of stable non-growing length. Rapid hormonal changes (i.e. pregnancy, birth control pills) may cause hair to synchronize growth cycles. 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 hormone controlling growth are “androgens”, commonly known as male hormones. The most important one is testosterone. Except in the scalp, androgens cause hair to change from vellus to terminal.  A good example is puberty, which in both sexes’ androgens induce the appearance of pubic and axillary hair.  Estrogens or female hormones have the opposite effect.  Once vellus hair has been transformed into terminal hair, it cannot change back even if androgens disappear. Therefore,  hirsutism must he treated promptly to minimize the number of  terminal hair  induced by excess androgens.