Testosterone is an anabolic steroid synthesized primarily by the testes in males, the ovaries in females, and adrenal glands in both sexes. Testosterone is synthesized from androstenedione, a product of dehydroepiandrosterone (DHEA) and progesterone, both of which are products of pregnenolone and cholesterol.

At puberty, and throughout most of the reproductive years, approximately 10 – 20 times more testosterone is synthesized in males than in females. In males at puberty, the much higher level of testosterone is responsible for the development of male external genitalia and secondary hair patterns, stimulation of spermatogenesis, stimulation of anabolic activity leading to increased muscle mass and behavioral changes. In pubescent females, testosterone effects are more subtle but equally important for proper musculoskeletal development, general anabolic activity, and libido. In both sexes, testosterone enhances aerobic metabolism and increases protein synthesis.

Testosterone decreases with age in both men and women. Testosterone replacement has been used to treat some postmenopausal symptoms, especially lack of libido in women who have received chemotherapy. It has also been used effectively in the treatment of anemia and the weakness and muscle wasting syndrome associated with AIDS. Recent research on the affects of testosterone on aging demonstrates a gain in lean body mass and a possible decline in bone loss when used in elderly patients.

Since testosterone can have significant side effects (acne, hirsutism, deepening voice, and clitorimegaly) measurement of levels can help define a deficiency and allow titration of therapy without risking significant toxicity.