Baldness runs in families. The hair loss usually begins on the sides, near the front, or on the top of the head toward the back. The hair loss can begin at any age, even in the middle teen years. Usually, this condition causes the hair to thin in the front, on the sides, or on the crown. It rarely progresses to total hair loss.
Many cancer drugs cause baldness.
It may also result from an underactive thyroid gland or pituitary gland or even from pregnancy. The hair may fall out as long as 3 or 4 months after the illness or other condition. Usually, the hair loss is temporary, and the hair grows back. Alopecia areata is a condition in which hair is lost suddenly in a particular area, usually in the scalp or beard. Rarely, all body hair may be lost, a condition called alopecia universalis.