On average, there are 100,000 to 150,000 hairs on the human scalp. The hairs grow from hair roots, or follicles (saclike structures under the skin). Blood vessels at the base of each follicle provide the nourishment necessary for hair growth. Hair growth in each root occurs in a cycle independent of the other roots. At any time about 90 percent of the hairs on the scalp are in the growth phase, while the other 10 percent are in the resting phase. The growth phase lasts an average of four to five years, after which the follicle enters the resting phase, which lasts about two months to four months. At the end of the resting phase, the hair falls out naturally and is replaced by a new hair. Consequently, some hair loss is a normal part of the hair growth cycle. In fact, on a typical day, about 50 to 150 scalp hairs are lost. Baldness (or alopecia) results when hair loss occurs at an abnormally high rate; when hair replacement occurs at an abnormally slow rate; or when normal hairs are replaced by thinner, shorter ones.
What is male pattern baldness?
About 95 percent of all cases of hair loss are the result of androgenetic alopecia (also known as male pattern baldness in men). Androgenetic alopecia occurs much more frequently in men than in women. It affects roughly 40 million men in the United States. Approximately 25 percent of men begin balding by age 30; two-thirds begin balding by age 60.
While some types of hair loss are easily reversible, male pattern baldness is more permanent. It occurs in a characteristic pattern on the scalp: hair loss usually begins at the temples and at the top of the head toward the back, causing a receding hairline and a bald spot. Hair loss may continue until the two sections become joined, leaving a horseshoe-shaped area of hair on the sides and back of the head. Balding may begin at any age after puberty, even in the middle teens, and can range from partial loss to complete baldness. Male pattern baldness progresses slowly and is not associated with redness, itching, or pain.
What causes male pattern baldness?
The causes of male pattern baldness are thought to be complex and are not completely understood. However, as suggested by its medical name (androgenetic alopecia), male pattern baldness seems to involve both hormonal (androgen) and genetic factors. Many different types of hormones play roles in the regulation of scalp hair, but the hormones with the largest effect are the androgens. Testosterone and its more potent derivative dihydrotestosterone (DHT) are responsible for increasing the size of hair follicles in areas such as the beard and underarm during puberty, but can also cause hair follicles in the scalp to decrease in size later in life. These contrasting responses to DHT might be explained by genetic differences in the individual hair follicles. Similarly, in men who are balding, genetically determined characteristics of hair follicles in the scalp may cause the follicles to be more likely to degrade in the presence of androgens. Hair follicles become miniaturized, and the hair consequently becomes thinner and shorter. The growing phase of the growth cycle becomes progressively shorter, which means more hairs are shed. Although the follicles still have an adequate blood supply, they continue to shrink, and some eventually die.
Whatever the exact causes of male pattern baldness may be, it is a hereditary trait. There are multiple genetic factors that influence male pattern baldness. A tendency toward baldness in the men on either the mother's or father's side of a man's family indicates a genetic predisposition to baldness. The speed, pattern, time of onset and degree of balding are all influenced by heredity. Generally, the earlier the onset of balding, the more extensive the degree of hair loss will eventually be.
Contrary to popular myth, balding is not caused by wearing hats. Massaging or brushing the scalp will not help regrow hair, and excessive cleaning of the scalp will not "unclog" follicles and allow hair growth. While extreme psychological stress may contribute to a form of hair loss known as alopecia areata, normal everyday stress does not contribute to balding and is not a cause of androgenetic alopecia.
What are some other types of hair loss?
Aside from male pattern baldness, some of the most common types of hair loss include:
- Alopecia areata, which leads to patchy hair loss on the scalp. While the causes of alopecia areata are not completely understood, the hair loss is thought to be the result of the body's immune system attacking the hair follicle. Alopecia areata often resolves without treatment.
- Traction alopecia, which results from long-term pulling on the hair. This type of hair loss can be caused by certain hairstyles, such as tight braids. The hair loss is usually reversible once the cause of this pulling is eliminated.
Certain diseases, such as diabetes, systemic lupus erythematosus and disorders of the thyroid gland can also cause baldness. Sudden hair loss may be an early warning sign for some diseases and should be reported to a doctor.
Hair loss has many other causes, including illness, poor nutrition, skin damage, some medications, and certain medical treatments such as anticancer chemotherapy and radiation therapy. Ringworm, a fungal infection, may sometimes be the cause of balding in children, but is generally not a problem in adults.