Hair loss (alopecia) is defined as losing enough hair that a person has visibly thin or balding patches. The most common causes of hair loss include:

• Heredity. The most common cause of hair loss in both men and women. A history of hereditary hair loss (e.g., androgenetic alopecia, alopecia areata) from either side of the family greatly increases an individual’s likelihood of experiencing hair loss. Heredity often affects the age at which hair loss begins, speed of progression, pattern and extent. At a predetermined point, heredity triggers the production of an enzyme (complex proteins found in body cells that enable chemical reactions to take place) called 5-alpha reductase. When the male sex hormone testosterone combines with this enzyme, dihydrotestosterone (DHT) is produced. DHT causes both the affected hair follicle (tiny, tubular structure in the skin through which hair grows) and hair to shrink in size, causing hair to thin or permanently fall out.
  
  
• Trauma. A frequent cause of hair loss in women and children. Trauma to hair follicles may be caused by the act of drawing or pulling the hair (traction) for styling purposes (e.g., braids, ponytail), from heat (e.g., blow-drying) or self-induced pulling habits (trichotillomania). Excessive shampooing, brushing or combing can also damage hair, resulting in breakage. Hair loss from trauma is often reversible but may become permanent if trauma continues. An exception is physical trauma to the scalp (e.g., burns), which can cause scarring or damage to hair follicles, often making hair regrowth impossible or unlikely.
  
  
• Hormones. Patterned hair loss in both men and women occurs, in part, because of sensitivity of the affected hair follicles to androgen (male sex hormone) production levels. Various hormonal conditions can also interfere with the normal cycle of hair growth and cause hair loss (e.g., high levels of thyroid hormones). Hormonal changes due to polycystic ovarian syndrome (a condition in which androgens are overproduced by the ovaries) and use of birth control pills can cause hair loss in women.
  
  
• Disease or illness. A variety of common diseases (e.g., cancer, diabetes, kidney or liver disease) may cause hair loss in patients. Skin infections (e.g., ringworm) can invade the hair and skin of the scalp and cause hair loss. However, follicles typically have the potential to regrow hair if it is lost due to disease or illness.
  
  
• Medications. Numerous medications may cause hair loss as a side effect (e.g., chemotherapy drugs, arthritis drugs, high blood pressure drugs, gout drugs, anticoagulants, antidepressants, lithium [a mood stabilizer]). The degree to which hair loss is experienced varies, depending on the drug or medication being administered and the nature of the underlying medical condition.
  
  
• Stress. The body’s response to a physically or emotionally stressful event (e.g., pregnancy, menopause, surgery, high fever, severe flu) may initiate hair loss in some people. Following a stressful event, hair shifts into a resting (telogen) phase prematurely. This reduces the amount of new hairs that are grown, sometimes causing abnormal hair loss. Although, any hair lost in this manner is usually temporary and grows back.
  
  
• Poor nutrition. Poor eating habits or eating disorders (e.g., anorexia nervosa, bulimia) can cause abnormal hair loss, in addition to other more serious medical conditions (e.g., low blood iron levels). People whose bodies do not receive enough vital nutrients such as protein or certain vitamins may experience abnormal thinning or loss of hair, regardless of age. Teenage or adolescent girls with poor body images experience hair loss from poor nutrition most frequently.
  
  
• Chemicals. Hair that is repeatedly treated with chemicals (e.g., dying, bleaching, straightening, perming) may break off or fall out. Hair loss as a result of chemicals is usually reversible but may be permanent in some cases if scarring occurs.