In medical practice, once generalized hypertrichosis has been distinguished from hirsutism, it is most often considered a variation of normal, primarily resulting from genetic factors.

Nearly all the skin of the human body except palms of hands and soles of feet is covered with hairs. The density of the hairs (in hair follicles per square centimeter) thickness of the hairs, color of the hairs, speed of hair growth, and qualities such as kinkiness, vary from one part of the body to another, and from one person to another. All of these features have strong genetic determinants, as demonstrated by the heritability of these qualities.

Doctors generally distinguish scalp hair, vellus hair, and androgenic (terminal) hair. Scalp hair is the hair on the head. Its absence is termed "baldness." Vellus hair is the hair on the rest of the body which has not been stimulated and transformed by sex hormones. Androgenic hair is the hair that greatly increases in heaviness and rate of growth with puberty.

Even children are covered with fine vellus hair, varying in density, length, and heaviness.

The hair follicles on much of the body respond to androgens (primarily testosterone and its derivatives). The rate of hair growth increases and the heaviness of the hairs increases. However, different areas respond with different sensitivities. As puberty progresses, the sequence of appearance of sexual (androgenic) hair reflects the gradations of androgen sensitivity. The pubic area is most sensitive, and heavier hair usually grows there first in response to androgens. The following regions also respond to androgens, in order of decreasing sensitivity: axillary and perianal areas, sideburns, above the upper lip, periareolar areas, chin and beard areas, arms and legs, chest, shoulders, buttocks, back, and abdomen.

It is the hair in these areas that appears earlier or grows to excess in disorders of excess androgen (e.g., precocious puberty, late-onset congenital adrenal hyperplasia, and polycystic ovary syndrome).

When the unwanted or excessive hair occurs in other places, and especially in other sequences of appearance, it is rarely due to a disorder of androgen excess. For example, it is not unusual for a young girl to be taken to a pediatric endocrinologist because her mother is distressed by the heaviness of the girl's arm and leg hair, but this condition is never due to a disorder of androgen excess if pubic hair has not appeared.

Most hypertrichosis is genetic, but a small number of unusual systemic disorders can sometimes increase vellus hair. Some drugs (e.g., diazoxide, diphenylhydantoin, and minoxidil) and toxins (e.g., mercury) can induce generalized hair growth as well. Unusual hypertrichosis can also be caused by untreated infection, or by malnutrition. For this reason, it is an occasional sign of anorexia nervosa.

Severe hypertrichosis is quite rare, almost certainly due to unknown genetic defects, and can result in excessive or animal-like hair on both face and body. Unfortunately, some of these people have been displayed in carnival sideshows with names such as "dog-boy" or the "bearded lady." Fedor Jeftichew and Annie Jones are well known examples.

In some cases an area of skin can react to repeated trauma or to some other asymmetric stimulus (such as wearing of a cast) with increased hair growth.

In the vast majority of cases, hypertrichosis is a cosmetic problem. The treatments range from camouflage (e.g., bleaching with hydrogen peroxide), to temporary removal by waxing, or permanent removal by electrolysis or laser destruction of hair follicles.