The most common form of baldness seen is male pattern baldness, in which the hair recedes from the front of the head, and a bald patch develops on top. The trigger for this type of baldness, which is also known as androgenetic alopecia, is currently believed to be an enzyme, 5-alpha reductase, that converts the hormone testosterone into dihydrotestosterone (DHT), which inhibits hair growth. The age at which the enzyme appears, if it does at all, is genetically determined.
Female pattern baldness, in which hair-loss is distributed evenly instead of forming a bald patch or a receding hairline, is less common. It is believed to result from a decrease in estrogen, a hormone that normally counteracts the balding effect of testosterone, which normally occurs in women's blood.
Hair loss is a frequent side-effect of chemotherapy, in all ages and both sexes.
Some mycotic infections can cause massive hair loss.
Other forms of baldness: