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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) 

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.

Diagnostic tests that may be performed (but are rarely needed) include: Microscopic examination of a plucked hair
Skin biopsy (if skin changes are present)
 

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.  

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