Androgenetic alopecia, (AGA), is also known as pattern hair loss or pattern baldness. This form of alopecia is, by far, the most common type of hair loss. Occuring in more than 40 million American men and 20 million American women, pattern hair loss is likely to affect almost every individual at some point in their adult life. In men, pattern hair loss occurs when the anterior hairline recedes, often as the crown scalp hair thins. In severely affected men, the pattern may become pronounced to the point that only a thin fringe of hair remains in the back of the scalp. In women, the degree of hair loss is typically less severe and the anterior hairline is preserved.

In susceptible persons, hair follicles contain androgen-sensitive receptors that act like hair loss triggers. In the presence of androgens, genes that shorten the growing phase of the hair are stimulated, and hair follicles shrink and become miniaturized. As the hair successively cycles, the follicles become smaller (leading to shorter, finer hair), and hypopigmented vellus hairs replace fully pigmented terminal hairs. In women, the thinning is diffuse, but more pronounced in the frontal and parietal scalp. Even men with severe androgenetic alopecia almost always retain a thin fringe of hair behind the ears in the back of the scalp.

Alopecia areata (AA) describes discrete lesions of patchy hair loss arising from an autoimmune etiology. AA usually presents as a single oval patch or multiple confluent patches of asymptomatic, well-circumscribed, nonscarring lesions devoid of hair. Severity varies from a small bare patch to loss of hair on the entire scalp. So-called "exclamation point" hairs are a hallmark of the disorder. These hairs are usually located at the periphery of the patch and extend several millimeters above the scalp.

Alopecia areata is diagnosed in approximately 1 to 2 percent of the general population, with men and women equally affected. The condition may be present in persons of any age, but most commonly occurs in those under age 30.

The course of alopecia areata is one of spontaneous remissions and recurrences. Although patients with this disorder are usually otherwise healthy, some have comorbid conditions such as atopy, thyroid disease, or vitiligo. AA has been strongly associated with certain human leukocyte antigen class II alleles.

Immunomodulating agents used in the treatment of alopecia areata include corticosteroids, 5 percent minoxidil, and anthralin ointment. Topical immunotherapeutic agents (e.g., dinitrochlorobenzene, squaric acid dibutyl ester, and diphenylcyclopropenone) are also used, although management regimens for these potent agents are challenging. Dermatology consultation or referral may be necessary. All of these agents stimulate hair growth but do not prevent hair loss. Moreover, they probably do not influence the course of the disease.