Nearly all men have some baldness by the time they are in their 60s. However, the age the hair loss starts is variable. About three in ten 30 year olds, and half of 50 year olds are quite bald. Some women also develop a similar pattern of hair loss.

What causes male pattern baldness?

Hair is made in hair follicles which are like tiny pouches just under the skin surface. A hair normally grows from each follicle for about 3 years. It is then shed, and a new hair grows from the follicle. This cycle of hair growth, shedding, and new growth goes on throughout life. The following is thought to occur in men as they gradually become bald.

• Affected hair follicles on the scalp gradually become smaller than normal.
• As the follicle shrinks, each new hair is thinner than the previous one.
• Before falling out, each new hair grows for much less time than the normal 3 years or so.
• Eventually, all that remains is a much smaller hair follicle and a thin stump of hair that does not grow out to the skin surface.

Male hormones are involved in causing these changes. The level of testosterone, the main male hormone, is normal in men with baldness. Hair follicles convert testosterone into another hormone called dihydrotestosterone. For reasons that are not clear, affected hair follicles become more sensitive to dihydrotestosterone, which causes the hair follicles to shrink. It is also not clear why different hair follicles are affected at different times to make the balding process gradual.

There is some genetic (hereditary) factor involved.

It is also not clear why only scalp hairs are affected, and not other areas such as the beard or armpits.

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