Hair loss typically refers to an unwanted reduction in hair, especially scalp hair.
The severity and nature of hair loss can vary greatly; it ranges from male and female pattern alopecia (androgenetic alopecia, also called androgenic alopecia or alopecia androgenetica), alopecia areata, which involves the loss of some of the hair from the head, and alopecia totalis, which involves the loss of all head hair, to the most extreme form, alopecia universalis, which involves the loss of all hair from the head and the body. Treatment for alopecia has limited success. The more hair lost, the less successful the treatment will be.
Male pattern baldness is thought to occur in varying forms in about 66% of adult males at some point in their lives.
Hair loss can also be induced by chemotherapy and radiation. This was the mechanism that caused hair removal in x-ray hair removal.