testosterone, 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). For many years, synthetic steroids similar to testosterone have been used by athletes with the goal of improving performance, although recent medical research has shown that these drugs may have a wide range of harmful side effects.

Masculinizing sex hormone produced by the testes. It is responsible for the development of the male sex organs and secondary sex characteristics (e.g., facial hair, masculine musculature, deep voice, and male-pattern baldness). Testosterone can be manufactured by modifying other, less expensive steroids. It is used in the treatment of hypogonadism, undescended testes (cryptorchism), and certain cancers.

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