Thinning hair in men as a part of male pattern baldness is caused by a combination of genetics and male hormones. As these hormones increase from the beginning of puberty onward, some men begin to experience a progressive loss of hair density on top of the scalp. This is due to the hair follicles' genetic sensitivity to a naturally occurring chemical related to testosterone. Over time, the chemical causes follicles at the top of the head to begin to grow smaller hairs, and eventually they may produce no new hairs at all.
Male hair loss is the result of the interactions between the hormone dehydrotesterone (DHT), genetics, and the hair follicles at the top of the head. Due to inherited factors that are determined at the time of birth, some follicles are programmed to be vulnerable to the harmful effects of DHT. Over time, DHT accumulating in the follicles acts as a signal to stop normal hair growth cycles. DHT is naturally made from testosterone, a hormone responsible for the development of male secondary sexual characteristics in puberty. This explains in part why thinning hair in men doesn't usually begin until the mid-twenties.
The role of sex hormones in male pattern baldness helps to explain why some types of medication can prevent further hair loss. These types of medication work by reducing an enzyme that converts testosterone to the chemical DHT. DHT acts to miniaturize the hair shaft and reduce the maximum number of growth cycles that follicles will undergo during an individual's lifetime. The medications lower the amount of DHT circulating in the blood, thereby lowering the amount that reaches susceptible follicles.
These medications can help most men protect their hairlines or even regain some scalp hair. Two drugs of this type, finasteride and dutasteride, can be prescribed to treat thinning hair in men. While they were originally developed to treat another health condition caused by DHT accumulation, a number of test subjects regrew hair lost to pattern baldness. New medications could selectively protect the hair follicles from DHT, potentially increasing their effectiveness.
Numerous incorrect myths have been used to explain thinning hair in men, including poor hygiene or certain styling habits, among many others. Hair loss can occur due to medication, chemotherapy, or serious malnutrition, but this is nearly always sudden or rapid. In these cases, normal growth resumes when the cause of hair loss is removed or corrected. On the other hand, progressively thinning hair in men is only due to the interaction between inherited genetics and hormones.