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Norwood ScaleMale Pattern Hair Loss (MPHL) is very common. About 50% of men 40 to 50 years of age are affected. Male Pattern Hair Loss describes a progressive thinning, or miniaturization, of scalp hair. The Norwood Scale can be used to categorize typical hair loss patterns in men. Typical hair loss is divided into 7 categories and sub-categories. Note that a Norwood 1 indicates no hair loss.

Male Pattern Hair Loss is sometimes referred to as Androgenic Alopecia or Androgenetic Alopecia. The term Androgenic combines the words androgen and gene, because both androgens and a genetic predisposition are thought to play a role in MPHL. Androgens are hormones, such as testosterone. Miniaturization is a process by which hair becomes smaller and finer with each growth cycle until finally, it stops growing altogether. A progressive increase in dihydrotestosterone (DHT) is widely accepted as the cause of miniaturization. Men who are genetically susceptible to MPHL have increased levels of Type II 5 alpha-reductase, an enzyme that converts testosterone into DHT.

All people have DHT but only some will suffer from hair loss, because of a genetic predisposition for hair follicles with an overabundance of androgen receptors to which DHT attaches. Over time, DHT causes a gradual miniaturization and finally the death of these susceptible hair follicles. This action is largely inherited. Although anti-androgen drugs may slow or halt hair loss, they are really only effective at regrowing hair in the crown.

Male Pattern Hair Loss can be halted by hair loss drugs such as Propecia (Finasteride).