Male pattern baldness (androgenetic alopecia)
This is the most common cause of hair loss in men, and it's usually inherited, as well as effected by the male hormone, DHT. As many as half of all men experience baldness by the time they reach their 50s. In most men, the hair thins from the forehead first, forming an "M" shape. Then, the hair starts going at the top of the scalp. Eventually, the thinning hair at the front and top meet up, leaving only a ring of hair around the head.
The female version of hair loss can affect about a third of women past menopause. Unlike in men, the front hairline stays put, but the hair thins all over the head, widening the part and often forming a "Christmas tree" pattern. Hair loss in women can be particularly devastating emotionally, due to the emphasis placed on a woman's appearance.
This disorder, which affects an estimated 1.7 percent of the population, can strike at any age, and it can affect hair on both the head and body. Researchers believe that people are genetically predisposed to develop alopecia areata, and then a virus or other environmental event occurs to trigger the hair loss. It's considered an autoimmune disorder, meaning that the body attacks itself. In this case, white blood cells from the immune system attack hair follicles, causing the hair to fall out and the hair growth process to slow. Typically, the hair suddenly falls out from its roots in one circular area, but then grows back after a few months. Variations of this condition include alopecia totalis (the loss of all scalp hair), and alopecia universalis (loss of all scalp and body hair).
Burns, bacterial or fungal skin infections, injury, or X-ray treatments can leave scars on the scalp, which prevent hair from growing in certain patches.
Telogen is the last phase of the hair growth cycle -- the resting phase. At theend of this stage, normally the hair falls out and the cycle begins again. It's normal to lose about 50-100 hairs per day from telogen, but people who have telogen effluvium lose more. Acute telogen effluvium can last about six months. The chronic version of the condition can persist for years. Sometimes telogen effluvium will improve on its own without treatment. In other cases, medication (minoxidil) can help restore hair growth.
Hair shaft defect
Very rarely, people are born with sparse or brittle hair because of a disease that damages the hair shaft. These diseases are typically inherited, and include monilethrix, pili torti and trichothiodystrophy.
Trichotillomania, which comes from the Greek words for hair (trich) and morbid impulse (mania), affects an estimated 1 to 3 percent of people. People who have this condition feel an impulsive need to pull hair from their scalp, eyelashes, eyebrows and beard. Some people do this to relieve tension; others aren't aware they're doing it. Scientists believe the disorder is caused by imbalances in the brain chemicals serotonin and dopamine. It also can be life threatening if people eat their hair, a condition called trichophagia.