Hair loss is not caused by poor circulation, clogged hair follicles, frequent shampooing, the wearing of hats or helmets, or the presence of mites. Relatively uncommon causes of hair loss, which are reversible with treatment or a change in lifestyle, include thyroid disease, iron deficiency, high fever, stress, trauma, surgery or general anesthesia, "crash diets," childbirth, and certain medications. There are also certain dermatologic scalp disorders that can result in temporary or permanent hair loss.

The overwhelming majority of male hair loss is caused by hereditary "androgenetic alopecia", known as male pattern baldness. This condition affects an estimated 40 million men in North America alone.

There are several variables that, combined together, can cause male pattern baldness: genes, hormones and age. Hair loss genes are a necessary component of hair loss and can be passed on by either parent. This genetic transfer is not predictable and might skip a generation or affect one or two siblings but not all siblings within a family. T he common belief that baldness is genetically transferred only from the mother's side of the family is incorrect. The genetics of male pattern baldness is complicated and not completely understood but is felt to involve more than one gene.

The presence of the hair loss gene is not enough for baldness to occur, as other variables such as hormones and age play a role. The hair loss may begin any time after puberty as levels of testosterone start to rise. The first change is usually recession in the temporal areas, which is seen in almost all mature Caucasian males, including men whose hair loss will not progress beyond the temporal regions.

When testosterone is chemically altered by the enzyme 5-alpha reductase in the skin, it becomes a more potent hormone, known as dihydro-testosterone(or DHT). DHT acts by binding to special receptor sites on the cells of the hair follicles. The effect is not immediate, however. Susceptible hair follicles must continually be exposed to DHT over a period of time for hair loss to occur.

The age at which exposure to DHT finally causes the follicles to react depends on the individual's genetic composition and levels of testosterone. Once the effect begins to manifest itself, the length of the anagen (growing) cycle of the hair shaft decreases, and the telogen (resting) phase of the hair shaft lengthens. Genetically susceptible hair follicles that were producing healthy, terminal hairs will begin to produce thinner, shorter, more brittle hairs with weaker shafts ("miniaturization"). As the condition continues, the follicles produce finer and finer hairs, until they become almost invisible, short, vellus hairs, or die out altogether.

Hair loss does not occur at a consistent rate or all at once, but rather in cycles. People who are losing their hair experience alternating periods of slow and rapid hair loss and sometimes experience periods of stability. Many of the factors that cause the rate of loss to accelerate or decelerate are unknown.

The genetic disposition to hair loss, however, generally affects only the top of the head and the crown, while the hair follicles that reside in the horseshoe shaped area around the circumference of the head are generally not susceptible. Hairs in this area, especially at the back of the head, are not genetically sensitive to DHT, which makes this area an optimal donor site for hair transplantation into balding areas.

Hair transplantation transfers non-balding hairs (i.e. those that are not genetically sensitive to DHT) to bald areas, where, due to their unresponsiveness to DHT, they retain their non-balding qualities. These hairs provide a permanent correction for the previous hair loss.