Testosterone is the primary sex hormone and anabolic steroid in males. In male humans, testosterone 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, testosterone in both sexes is involved in health and well-being and in the prevention of osteoporosis. Insufficient levels of testosterone in men may lead to abnormalities including frailty and bone loss.

In addition to its role as a natural hormone, testosterone is used as a medication in the treatment of hypogonadism in men and breast cancer in women.