Alopecia (hair loss)

[Oralee]

Hair loss, also called alopecia, can occur on any part of the body. Hair loss that occurs on the scalp is generally called baldness. Hair loss is often of great concern to people for cosmetic reasons, but it can also be a sign of a bodywide (systemic) disorder.
Hair grows in cycles (see also Overview of Hair Growth). Each cycle consists of

A long growing phase (anagen) lasting 2 to 6 years
A brief transitional phase (catagen) lasting 3 weeks
A short resting phase (telogen) lasting 2 to 3 months

At the end of the resting phase, the hair falls out (exogen), and the cycle begins again as a new hair starts growing in the follicle. Normally, about 50 to 100 scalp hairs reach the end of resting phase each day and fall out.
Disorders of the growth cycle that lead to hair loss include

Anagen effluvium: Disruption of the growing phase that causes loss of anagen hairs
Telogen effluvium: Many more than 100 hairs a day go into resting phase

Doctors sometimes classify hair loss as focal (confined to one part of the scalp) or diffuse (widespread). Hair loss may also be classified by whether or not scarring exists.