Testosterone is the primary male sex hormone and androgen in males. In humans, testosterone plays a key role in the development of male reproductive tissues such as testicles and prostate, as well as promoting secondary sexual characteristics such as increased muscle and bone mass, and the growth of body hair. It is associated with increased aggression, sex drive, dominance, courtship display, and a wide range of behavioral characteristics. In addition, testosterone in both sexes is involved in health and well-being, where it has a significant effect on overall mood, cognition, social and sexual behavior, metabolism and energy output, the cardiovascular system, and in the prevention of osteoporosis. Insufficient levels of testosterone in men may lead to abnormalities including frailty, accumulation of adipose fat tissue within the body, anxiety and depression, sexual performance issues, and bone loss. Excessive levels of testosterone in men may be associated with hyperandrogenism, higher risk of heart failure, increased mortality in men with prostate cancer, male pattern baldness. Testosterone is a steroid from the androstane class containing a ketone and a hydroxyl group at positions three and seventeen respectively. It is biosynthesized in several steps from cholesterol and is converted in the liver to inactive metabolites. It exerts its action through binding to and activation of the androgen receptor. In humans and most other vertebrates, testosterone is secreted primarily by the testicles of males and, to a lesser extent, the ovaries of females. On average, in adult males, levels of testosterone are about seven to eight times as great as in adult females. As the metabolism of testosterone in males is more pronounced, the daily production is about 20 times greater in men. Females are also more sensitive to the hormone. In addition to its role as a natural hormone, testosterone is used as a medication to treat hypogonadism and breast cancer. Since testosterone levels decrease as men age, testosterone is sometimes used in older men to counteract this deficiency. It is also used illicitly to enhance physique and performance, for instance in athletes. The World Anti-Doping Agency lists it as S1 Anabolic agent substance "prohibited at all times". Source: Wikipedia (en)