Burns, injuries, and X-rays can cause temporary hair loss. In such cases, normal hair growth usually returns once the injury heals unless a scar is produced. Then, hair will never regrow.
Autoimmune diseasemay 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, eating disorders, and anemia can cause hair loss. Most times, when the underlying condition is treated, the hair will return unless there is scarring as in some forms of lupus, lichen planus or follicular disorders.
Diet. A low-protein diet or severely calorie-restricted diet can also cause temporary hair loss.