Dictionary.net

The major androgenic hormone produced by the interstitial (Leydig) cells of the testes in response to stimulation by the luteinizing hormone of the anterior pituitary. It regulates gonadotropic secretion and wolffian duct differentiation, and stimulates skeletal muscle. It is also responsible for other male characteristics and spermatogenesis after its conversion to dihydrotestosterone. In addition testosterone possesses protein anabolic properties, and it is converted to estradiol in peripheral tissue. (From Dorland, 28th ed)