Male pattern hair loss (androgenetic alopecia) is a hereditary condition distinguished by a receding hairline and the disappearance of hair from the top of the head (crown), eventually balding a part (“bald spot”) or the entire top of the scalp. The lower back and sides of the scalp are usually unaffected. The condition progresses gradually throughout a man’s lifetime. Generally, the earlier male pattern baldness begins, the greater the total hair loss over the course of a man’s lifetime. Male pattern baldness is the most common type of hair loss in humans, affecting millions of men worldwide. 

Hair is composed of a protein called keratin and is produced in hair follicles located in the outer layer of the skin (epidermis). Each person is born with a set amount of follicles, which hold the lower part of the hair shaft in place and contain blood vessels that stimulate new hair growth. The process 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 one to three 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 is reinitiated. On a normal scalp, approximately five to 10 percent of hairs are in the resting phase at any given time, with about 75 hairs shed every day.
  
  

  

A full head of hair has about 100,000 hairs. An exception is redheads, who have fewer hairs, and blondes who have more. However, red and blonde hair tends to be thicker and to provide better coverage.

Up to 30 percent of hair may be lost by middle age as a normal part of the aging process. Male pattern hair loss 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. 

At a genetically predetermined point, heredity triggers the production of an enzyme (a complex protein found in body cells) called 5-alpha reductase. When the androgen (male sex hormone) testosterone combines with this enzyme, dihydrotestosterone (DHT) is produced. DHT causes both the affected follicle and hair to shrink in size. Over time, this process miniaturizes follicles to the point that hair permanently falls out without chance of regrowth. A reduction of the levels of these chemicals in follicles works to help reverse the shrinking process and is the goal of certain medical treatments for male pattern hair loss (e.g., finasteride).

Men with male pattern hair loss typically experience a gradual transition over several years from thick, colored terminal hairs to soft, non-colored vellus hairs (“peach fuzz”). The onset of male pattern hair loss shortens the anagen phase while the telogen phase remains unchanged, causing hair to fall out faster than it can be regrown.

Many men experience a receding hairline (around the temples) in an M-shaped hair pattern, often progressing to partial or total baldness across the top of the scalp. Male pattern hair loss affects most men to some degree by age 60. The condition can occur in men of all racial and ethnic backgrounds. It is most common in Caucasians, with the lowest rates of incidence occurring in Native Americans. Men of Asian decent are also impacted less often.  

Other than possible emotional or psychological effects, male pattern hair loss poses no health risks. One exception, however, may be that a decrease in hair volume allows potentially damaging ultraviolet radiation to reach the scalp. Male pattern hair loss may cause some individuals to become introverted, have low self-esteem or feel unattractive or tense in public places. This is particularly true in men who begin to lose hair in their teenage years or early 20s (premature male pattern baldness), as these feelings tend to be more troubling in younger patients. Although treatment is not necessary, patients who are emotionally impacted by the condition may benefit from seeking treatment options.