Hair diseases.

A few common causes of hair loss account for the vast majority of cases. A well-directed history and simple physical examination is often sufficient to make a diagnosis. Laboratory testing is often unnecessary. A scalp biopsy can be useful, but only if processed and interpreted correctly. Androgenetic alopecia, alopecia areata, senescent alopecia, telogen effluvium, traction alopecia, trichotillomania, and cosmetic hair damage are common causes of non-scarring alopecia. Discoid lupus erythematosus, lichen planopilaris, and central, centrifugal scarring alopecia are the most common forms of scarring hair loss.