Androgenic alopecia (also known as androgenetic alopecia or alopecia androgenetica) is a common form of hair loss in both male and female humans, chimpanzees, and orangutans. In male humans in particular, this condition is also commonly known as male-pattern baldness. Hair is lost in a well-defined pattern, beginning above both temples. Over time, the hairline recedes to form a characteristic "M" shape. Hair also thins at the crown of the head, often progressing to partial or complete baldness.
The pattern of hair loss in women differs from male-pattern baldness. In women, the hair becomes thinner all over the head, and the hairline does not recede. Androgenetic alopecia in women rarely leads to total baldness.
A variety of genetic and environmental factors are likely play a role in causing androgenetic alopecia. Although researchers are studying risk factors that may contribute to this condition, most of these factors remain unknown. Researchers have determined that this form of hair loss is related to hormones called androgens, particularly an androgen called dihydrotestosterone (DHT). Androgens are important for normal male sexual development before birth and during puberty. Androgens also have other important functions in both males and females, such as regulating hair growth and sex drive.
Large studies in 2005 and 2007 stress the importance of the maternal line in the inheritance of male pattern baldness. German researchers name the androgen receptor gene as the cardinal perquisite for balding. They conclude that a certain variant of the androgen receptor is needed for AGA to develop. In the same year the results of this study were confirmed by other researchers. This gene is recessive and a female would need 2 X chromosomes with the defect to show typical male pattern alopecia. Seeing that androgens and their interaction with the androgen receptor are the cause of AGA it seems logical that the androgen receptor gene plays an important part in it's development.
Other research in 2007 suggests another gene on the X chromosome, that lies close to the androgen receptor gene, is an important gene in male pattern baldness. They found the region Xq11-q12 on the X-chromosome to be strongly associated with AGA in males. They point at the EDA2R gene as the gene that is mostly associated with AGA.
Other genes involved with hair loss have been found. One of them being a gene on chromosome 3. The gene is located at 3q26. This gene is also involved in a type of baldness associated with mental retardation. This gene is recessive .
Another gene that might be involved in hair loss is the P2RY5. This gene is linked to hair structure. Certain variants can lead to baldness at birth while another variant cause “wooly hair”.
5-alpha-reductase is responsible for converting free testosterone into DHT. The genes for 5alpha-reductase are known. The enzymes are present predominantly in the scalp and prostate. Levels of 5alpha-reductase are one factor in determining levels of DHT in the scalp and drugs which interfere with 5alpha-reductase (such as finasteride, which inhibits the predominant type 2 isoform ) have been approved by the FDA as treatments for hair loss.
Sex hormone binding globulin (SHBG), which is responsible for binding testosterone and preventing its bioavailability and conversion to DHT, is typically lower in individuals with high DHT. SHBG is downregulated by insulin.
Increased levels of Insulin Growth Factor-1 (IGF-1) have been correlated to vertex balding
High insulin levels seem the likely link between metabolic syndrome and baldness. Low levels of SHBG in men and non-pregnant women are also correlated with glucose intolerance and diabetes risk, though this correlation disappears during pregnancy.
Daily, vigorous aerobic exercise (as opposed to short workout periods designed to raise androgen levels and build muscle or more sporadic exercise) has been shown to reduce baseline insulin levels as well as baseline total and free testosterone, significantly lowering baseline DHT. It has been suggested that weight training may have a detrimental effect on hair by increasing testosterone levels; however, there is at least one study that indicates a decline in free testosterone as result of weight training.