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is the primary male sex hormone and an . In male humans, plays a key role in the development of male reproductive tissues such as testes and prostate, as well as promoting secondary sexual characteristics such as increased muscle and bone mass, and the growth of body hair. In addition, is involved in health and well-being, and the prevention of osteoporosis. Insufficient levels of in men may lead to abnormalities including frailty and bone loss.

is a from the androstane class containing a keto and hydroxyl groups at the three and seventeen positions 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, is secreted primarily by the testicles of males and, to a lesser extent, the ovaries of females. On average, in adult males, levels of are about 7 to 8 times as great as in adult females. As the metabolism of in males is greater, 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, is used as a medication, for instance in the treatment of low levels in men and breast cancer in women. Since levels decrease as men age, is sometimes used in older men to counteract this deficiency. It is also used illicitly to enhance sique and performance, for instance in athletes.