What is the connection between testosterone and hair growth?

The testosterone is a steroid hormone included in a group called androgens. Found in both males in females, it is produced by testicles in males and ovaries in females. Adrenal glands also produce the hormone in small amounts.
Testosterone is the primary male sex hormone. It plays an important role in the development of testis and prostate. The hormone is responsible for various body changes we undergo in the adolescent years, related with sexual maturation. The characteristics such as deepening of voice, growth of body hair and the development of interest in sex are all triggered by testosterone.
In the years of puberty, the body starts to produce more androgens, mostly testosterone. The produced testosterone circulates around the body and gets attached to the androgen receptors in the hair follicles. The hair have a growth cycle composed of three phases namely anagen – the active growth stage, catagen – the transitional stage and telogen – the dying stage. This cycle repeats itself throughout our lifetimes, which is the reason for continuous hair growth. In the case of body hair, the telogen phase is longer than anagen phase, opposed to the cycle of scalp hair where anagen phase is far longer.
There are two types of hair on our bodies, vellus and terminal. Vellus hair are finer and lighter than the terminal hair, with less visibility. They cover the human body from birth itself. Terminal hair, on the other hand, are darker, thicker and visible. The follicles are sensitive to androgens, the sensitivity depending on the genes. Since the hair follicles are sensitive to androgens, the raising level of the hormones causes the formation of thick hair. The androgens transform vellus hair to terminal hair. The transformation depends upon the sensitivity of the hair follicles to the androgens. Normally, pubic hair is the first to appear, in both men and women. Since androgenic levels in males are about 7 times that in females, the males tend to have more body hair than females. The distribution of the androgenic hair throughout the body depends on the follicles’ sensitivity to testosterone.
As the people enter their middle age, the terminal hair follicles may begin to revert into vellus follicles. Though the scientists have no clear picture about it, it is supposed to be the decrease in circulation of the hormones. This results in the slow death of hair follicles and the reduction in body hair.

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