Space-occupying masses in the scalp cause hair loss secondary to the pressure effects causing atrophy of the follicles or by invasion and replacement of the follicle. Traumatic alopecia may occur secondary to acute injury and scarring, conscious or subconscious rubbing or pulling out of hair or because of chronic pressure as for example during prolonged anesthesia. Alopecia areata is of unknown etiology and has many variants. It occurs suddenly in the absence of above mentioned causative factors. The course of alopecia areata is unpredictable, with either regrowth over a few months or complete loss.
Biopsy shows intense lymphocytic infiltrate around hair follicle.
The most common type of hair loss by far in men and women is androgenetic alopecia. The follicles in predisposed regions continue to produce hair of decreasing growth rate until static resulting in eventual baldness. There is no difference in testosterone levels in balding and non-balding men. Women usually have the diffuse variant of androgenetic loss whereas men may have diffuse, patterned or both. Hair regrowth may not occur for several months after therapy is initiated.