Hair loss, especially when severe, often profoundly affects the lives of those afflicted. Severe hair loss evokes not only cosmetic concerns but may also evoke feelings of vulnerability (nakedness), loss of self-esteem and alterations in self-image. Each hair follicle produces hair in phases. There is a growing (anagen) phase, which lasts from 2 to 6 years, a resting (catagen) phase which lasts for about 3 months, and finally the shedding of the hair (telogen) phase which allows the follicle to begin pushing a new hair to the surface.

Most modern medical research leans toward the position that a predominantly male hormone called dehydrotestosterone (DHT), which is converted from testosterone, binds to sites on hair follicles and is a primary instigator of hair loss. DHT appears to make the follicles go into their "resting" phase faster which in turn starts to cause the hairs produced by those follicles to become thinner and thinner with each growth cycle.

Female pattern baldness usually begins about age 30, becomes noticeable around age 40, and may be even more noticeable after menopause. It is usually an overall thinning rather than a bald area on top of the head, though women may have a receding hairline, too. It is thought that about 15% of American women have such hair loss. As in males, hair follicles simply shut down, with hormones playing some role in the process.

By far the most common form of hair loss in women is due to hormone changes. It is estimated that around 30% of Caucasian females are affected before menopause. Women rarely lose their hair due to a genetic predisposition; instead it is usually caused by hormonal imbalance and other cyclical shifts. Other causes include, harsh commercial shampoos, perms, hair color, bleach, blow-drying, improper combing/brushing, high fevers, and scalp infections.

One plausible theory to explain some of the difference between men and women is based on the angle of follicles and resultant sebum build-up. The angle at which hairs come out of a female scalp allow the sebum (oil produced by the scalp) to "run off" whereas the angle of hairs on a male scalp (straight up) can lead to oil blockage.

Most doctors agree that if you have a oily scalp with thinning hair, frequent shampooing is advised. shampooing can reduce surface sebum, which contains high levels of testosterone and DHT that may reenter the skin and affect the hair follicle.

Specific foods or vitamins don't regrow hair, although good nutrition is essential for healthy hair.

Telogen effluvium is a form of nonscarring alopecia characterized by diffuse hair shedding, often with an acute onset. A chronic form with a more insidious onset and a longer duration also exists. Telogen effluvium is a reactive process caused by a metabolic or hormonal stress or by medications. Generally, recovery is spontaneous and occurs within 6 months. The interval between the inciting event in telogen effluvium and the onset of shedding corresponds to the length of the telogen phase, between 1 and 6 months.