Hair loss usually develops gradually and may be patchy or diffuse (all over). Roughly 100 hairs are lost from your head every day. The average scalp contains about 100,000 hairs.

Each individual hair survives for an average of 4-1/2 years, during which time it grows about half an inch a month. Usually in its 5th year, the hair falls out and is replaced within 6 months by a new one. Genetic baldness is caused by the body's failure to produce new hairs and not by excessive hair loss.

Both men and women tend to lose hair thickness and amount as they age. Inherited or "pattern baldness" affects many more men than women. About 25% of men begin to bald by the time they are 30 years old, and about two-thirds are either bald or have a balding pattern by age 60.

Typical male pattern baldness involves a receding hairline and thinning around the crown with eventual bald spots. Ultimately, you may have only a horseshoe ring of hair around the sides. In addition to genes, male-pattern baldness seems to require the presence of the male hormone testosterone. Men who do not produce testosterone (because of genetic abnormalities or castration) do not develop this pattern of baldness.

Some women also develop a particular pattern of hair loss due to genetics, age, and male hormones that tend to increase in women after menopause. The pattern is different from that of men. Female pattern baldness involves a thinning throughout the scalp while the frontal hairline generally remains intact.

Baldness is not usually caused by a disease, but is related to aging, heredity, and testosterone. In addition to the common male and female patterns from a combination of these factors, other possible causes of hair loss, especially if in an unusual pattern, include:

• Hormonal changes (for example, thyroid disease, childbirth, or use of the birth control pill)
• A serious illness (like a tumor of the ovary or adrenal glands) or fever
• Medication such as cancer chemotherapy
• Excessive shampooing and blow-drying
• Emotional or physical stress
• Nervous habits such as continual hair pulling or scalp rubbing
• Burns or radiation therapy
• Alopecia areata -- bald patches that develop on the scalp, beard, and, possibly, eyebrows. Eyelashes may fall out as well. This is thought to be an immune disorder.
• Tinea capitis (ringworm of the scalp)

Habif TP. Clinical Dermatology. 4th ed. St. Louis, Mo: Mosby, Inc. 2004:844-845.

Rakel P, ed. Conn’s Current Therapy 2005. 57th Ed. Philadelphia, PA: WB Saunders; 2005: 897.