Excessive hair loss caused by heredity is by far the most common form of hair loss. Anecdotal evidence has long suggested that patterned hair loss in men (androgenetic alopecia) is inherited as a dominant trait from the maternal side of the family. However, the genetic predisposition for patterned hair loss in both men and women is likely attributed to a combination of multiple genes, the influence of androgens (male sex hormones) and age.

Hair loss does not pose any health risks in and of itself, although it may rarely indicate an underlying medical condition or disease (e.g., lupus). Problems or difficulties experienced due to hair loss are often psychological, but can affect a person’s quality of life.

More commonly, hair loss may cause individuals to become introverted, have low self-esteem or feel tense or unattractive.  This is particularly true in men or women who begin to lose hair in their teenage years or early 20s (premature male or female pattern baldness), because these feelings tend to be more troubling in younger patients. Although treatment for hair loss is not necessary, patients who are emotionally impacted by the condition may benefit from seeking treatment.

A typical hair shaft has three layers:

• Cuticle. The outermost layer and the main hair structure.
  
  
• Cortex. Middle layer that gives strength to the hair shaft. The cortex also determines the color and texture of the hair.
  
  
• Medulla. The innermost or core of the hair. The body and strength of the hair are determined within the medulla.

Hair is composed of a protein called keratin and is produced in hair follicles (tiny, tubular structures through which hair grows) located in the outer layer of the skin (epidermis). Each person is born with a set number of follicles. The follicle holds the lower part of the hair shaft in place and contains blood vessels that cause new hair to grow. The growth of hair from these follicles is cyclical and happens in three stages:

• Growth (anagen) phase. May last two to three years for any given hair. On a normal scalp, about 80 to 90 percent of hair follicles are in the growth phase at any given time.
  
  
• Involutional (catagen) phase. Reduction in size of the hair. Approximately 1 to 3 percent of hair on the scalp is undergoing involution at any given moment. The involutional phase is a transitional period the hair undergoes before being shed. Typically, it lasts about two or three weeks.
  
  
• Resting (telogen) phase. Shedding of hair from the scalp. Following involution, hairs go through a resting phase for about two to three months. At the end of this time, the hairs are shed and the growth cycle (anagen) is reinitiated. On a normal scalp, approximately 5 to 10 percent of hairs on the scalp are in the resting phase at any given time, with about 75 hairs shed every day.

The hair growth cycle varies from one person to another. A full head of hair has about 100,000 hairs, with the exception of redheads, who have fewer hairs, and blondes, who have more. Up to 30 percent of hair may be lost by middle age as a normal part of the aging process. Male pattern hair loss, for example, is not typically noticeable until at least half of the hairs in a designated area have fallen out. The condition also causes remaining hairs to thin, making the appearance of hair loss more defined.

Hair loss becomes abnormal when it is being shed faster than it can be regrown, when it is thinner than before or when hair comes out in patches (alopecia areata). Abnormal hair loss may happen to anyone, though its frequency may vary according to race. Women experience hair loss nearly as frequently as men do. However, they tend to lose less hair due to hormonal differences and different balding patterns. Women may also be able to disguise their hair loss more effectively than men through differences in hair length and styling.