testosterone (tĕstŏs´tərōn), principal androgen, or male sex hormone. One of the group of compounds known as anabolic steroids, testosterone is secreted by the testes (see testis) but is also synthesized in small quantities in the ovaries, cortices of the adrenal glands, and placenta, usually from cholesterol. Testosterone is necessary in the fetus for the development of male external genitalia; increased levels of testosterone at puberty are responsible for further growth of male genitalia and for the development and maintenance of male secondary sex characteristics such as facial hair and voice changes. Testosterone also stimulates protein synthesis and accounts for the greater muscular development of the male (see metabolism).
An abnormally low testosterone level in men, known as hypogonadism, is treated with testosterone, but it is not clear if testosterone is a safe or effective treatment for so-called low testosterone, such as the lower levels of testosterone typically found in older men. In men with lower testosterone, the level can often be raised by increasing exercise, improving diet, and reducing weight to the recommended range. For many years, synthetic steroids similar to testosterone have been used by athletes with the goal of improving performance, but medical research has shown that these drugs may have a wide range of harmful side effects and their use is now typically banned (see anabolic steroid).