Diseases that can cause hair loss, thinning, or breakage include:

• Lupus, in which hair tends to become brittle and may fall out in patches. Short, broken hairs (“lupus hairs”) commonly appear above the forehead. Hair loss is usually not permanent. Some people with lupus also develop a form of lupus called discoid or cutaneous lupus that affects only the skin ("cutaneous" refers to skin). Scars that sometimes develop on the scalp may cause hair loss.
• Thyroid problems, which are a common cause of scattered hair loss. Both an overactive thyroid (hyperthyroidism) and an underactive thyroid (hypothyroidism) can cause hair loss. Hair loss associated with thyroid disease can be reversed with proper treatment.
• Cancer, such as Hodgkin's lymphoma.
• An adult form of muscular dystrophy (myotonic dystrophy).
• Diseases of the pituitary gland.
• Heavy metal poisoning, such as thallium or arsenic poisoning.
• A disease that causes inflammation and scar tissue throughout the body (sarcoidosis).
• Late-stage syphilis.
• HIV (human immunodeficiency virus) infection.
• Any severe ongoing (chronic) illness.