Doctors don't know why certain hair follicles are programmed to have a shorter growth period than others. However, several factors may influence hair loss:
Hormones, such as abnormal levels of androgens (male hormones normally produced by both men and women)
Genes,from both male and female parents, may influence a person's predisposition to male or female pattern baldness.
Stress, illness, and childbirth can cause temporary hair loss. Ringworm caused by a fungal infection can also cause hair loss.
Drugs, including chemotherapy drugs used in cancer treatment, blood thinners, beta-adrenergic blockers used to control blood pressure, and birth control pills, can cause temporary hair loss.
Burns, injuries, and X-rays can cause temporary hair loss. In such cases, normal hair growth usually returns once the injury heals.
Autoimmune disease may cause alopecia areata. In alopecia areata, the immune system revs up for unknown reasons and affects the hair follicles. In most people with alopecia areata, the hair grows back, although it may temporarily be very fine and possibly a lighter color before normal coloration and thickness return.
Cosmetic procedures, such as shampooing too often, perms, bleaching, and dyeing hair can contribute to overall hair thinning by making hair weak and brittle. Tight braiding, using rollers or hot curlers, and running hair picks through tight curls can also damage and break hair. However, these procedures don't cause baldness. In most instances hair grows back normally if the source of the problem is removed. Still, severe damage to the hair or scalp sometimes causes permanent bald patches.
Medical conditions. Thyroid disease, lupus, diabetes, iron deficiency, and anemia can cause hair loss, but when the underlying condition is treated the hair will return.
Diet. A low-protein diet or severely calorie-restricted diet can also cause temporary hair loss.