Hair loss, also known as alopecia or baldness, refers to a loss of hair from part of the head or body. Typically at least the head is involved. The severity of hair loss can vary from a small area to the entire body. Typically inflammation or scarring is not present. Hair loss in some people causes psychological distress. Common types include: male-pattern hair loss, female-pattern hair loss, alopecia areata, and a thinning of hair known as telogen effluvium. The cause of male-pattern hair loss is a combination of genetics and male hormones, the cause of female pattern hair loss is unclear, the cause of alopecia areata is autoimmune, and the cause of is telogen effluvium is typically a physically or psychologically stressful event. Telogen effluvium is very common following pregnancy. Less common causes of hair loss without inflammation or scarring include the pulling out of hair, certain medications including chemotherapy, HIV/AIDS, hypothyroidism, and malnutrition including iron deficiency. Causes of hair loss that occurs with scarring or inflammation include fungal infection, lupus erythematosus, radiation therapy, and sarcoidosis. Diagnosis of hair loss is partly based on the areas affected. Treatment of pattern hair loss may simply involve accepting the condition. Intervention that can be tried include the medications minoxidil or finasteride, and hair transplant surgery. Alopecia areata may be treated by steroid injections in the affected area but these need to be frequently repeated to be affective. Hair loss is a common problem. Pattern hair loss by age 50 affects about half of males and a quarter of females. About 2% of people develop alopecia areata at some point in time.

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