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)

Hair loss from menopause or childbirth often returns to normal 6 months to 2 years later.

For hair loss caused by illness (such as fever), radiation therapy, or medication use, no treatment is necessary. The hair will usually grow back when the illness has ended or the therapy is finished. A wig, hat, or other covering may be desired until the hair grows back.

For hair loss due to heredity, age, and hormones, the topical medication Rogaine (minoxidil) can be helpful for both male and female pattern baldness. Expect to wait 6 months before you see results. The oral medication Propecia (finasteride) is effective in some men. This medicine can decrease sex drive. When either medication is stopped, the former baldness pattern returns.

Hair transplants performed by a physician is a surgical approach to transferring growing hair from one part of the head to another. It is somewhat painful and expensive, but usually permanent.

Hair weaves, hair pieces, or changes of hair style may disguise hair loss. This is generally the least expensive and safest approach to hair loss. Hair pieces should not be sutured to the scalp because of the risk of scars and infection.

Call your doctor if:

• You are losing hair in an atypical pattern.

• You are losing hair rapidly or at an early age (for example, teens or twenties).

• You have any pain or itching associated with the hair loss.

• The skin on your scalp under the involved area is red, scaly, or otherwise abnormal.

• You have acne, facial hair, or menstrual irregularities.

• You are a woman and have male pattern baldness.

• You have bald spots on your beard or eyebrows.

• You have been gaining weight or have muscle weakness, intolerance to cold temperatures, or fatigue.

A careful medical history and examination of the hair and scalp are usually enough to diagnose the nature of your hair loss.

Your doctor will ask detailed questions such as:

• Are you losing hair only from your scalp or from other parts of your body as well?

• Is there a pattern to the hair loss like a receding hair line, thinning or bald areas on the crown, or is the hair loss throughout your head?

• Have you had a recent illness or high fever?

• Do you dye your hair?

• Do you blow dry your hair? How often?

• How often do you shampoo your hair?

• What kind of shampoo, hair spray, gel, or other product do you put on your hair?

• Have you been under unusual stress lately?

• Do you have nervous habits that include hair pulling or scalp rubbing?

• Do you have any other symptoms like itching, flaking, or redness of your scalp?

• What medications do you take, including over the counter drugs?

Diagnostic tests that may be performed (but are rarely needed) include:

• Microscopic examination of a plucked hair

• Skin biopsy (if skin changes are present)

Ringworm on the scalp may require the use of an oral drug, such as griseofulvin, because creams and lotions applied to the affected area may not get into the hair follicles to kill the fungus.

Treatment of alopecia areata may require topical or injectable steroids or ultraviolet light.