Androgenetic alopecia is characterized by progressive, patterned hair loss from the scalp. Recently the pathogenesis and genetic basis of the hair loss have been better understood, as has the distress experienced by men who have lost their hair. The transition of some terminal hairs into vellus hairs is a universal physiological secondary sexual characteristic.
However it is now known that it is more specifically the male hormone dihydrotestosterone (DHT) which is converted from the enzyme testosterone by the enzymes 5 alpha reductase which contributes to androgenetic alopecia in those who are genetically predisposed. It is interesting to note that individuals with a deficiency in 5 alpha reductase do not develop androgenetic alopecia. This is because the body is unable to convert testosterone into dihydrotestosterone.
When androgenetic alopecia occurs large active hair follicles in specific areas begin to change to smaller less active ones that shrink slightly with each new growth cycle. The enzyme 5 alpha reductase is thought to be the major cause of this. Under the action of the enzyme the male hormone testosterone become dihydrotestosterone. This causes the hair shafts to narrow producing progressively finer hairs with each new growth cycle until eventually the hairs become transparent and stops emerging. If an individual has androgenetic alopecia the overall levels of testosterone may be normal however the activity of 5 alpha reductase is greater than normal which results in increased amounts of dihydrotestosterone in the hair follicle.