Alopecia, or hair loss, usually occurs on the scalp; hair loss elsewhere on the body is less common and less conspicuous. In the nonscarring form of this disorder (noncicatricial alopecia), the hair follicle can generally regrow hair. But scarring alopecia usually destroys the hair follicle, making hair loss irreversible.
The most common form of nonscarring alopecia is male pattern alopecia, which appears to be related to androgen levels and to aging. Genetic predisposition commonly influences the time of onset, degree of baldness, speed with which it spreads, and pattern of hair loss. Women may experience a similar disorder, called androgenetic alopecia, characterized by diffuse thinning over the top of the scalp.
Other forms of nonscarring alopecia include:
physiologic alopecia (usually temporary): sudden hair loss in infants, loss of straight hairline in adolescents, and diffuse hair loss after childbirth
alopecia areata (an autoimmune disorder): generally reversible and self-limiting; usually occurs in young and middle-aged adults of both sexes; also occurs in children
trichotillomania: compulsive pulling, plucking or twisting of one’s own hair; usually occurs in children.
Predisposing factors of nonscarring alopecia also include radiation, many types of drug therapies and drug reactions, bacterial and fungal infections, psoriasis, seborrheic dermatitis (from scratching the affected area), and endocrine disorders, such as thyroid, parathyroid, and pituitary dysfunctions.