
Hair loss usually develops gradually and may be patchy or diffuse (all over).
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.
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.
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:
Alopecia areata -- bald patches that develop on the scalp, beard, and, possibly, eyebrows. Eyelashes may fall out as well.
Autoimmune conditions such as lupus
Burns
Certain infectious diseases such as syphilis
Chemotherapy
Emotional or physical stress
Excessive shampooing and blow-drying
Fever
Hormonal changes -- for example, thyroid disease , childbirth, or use of birth control pills
Nervous habits such as continual hair pulling or scalp rubbing
Radiation therapy
Tinea capitis (ringworm of the scalp)
Tumor of the ovary or adrenal glands