Testosterone is a steroid hormone from the androgen group. It is the principal male sex hormone and the "original" anabolic steroid.
Like other steroid hormones testosterone is derived from cholesterol. The largest amounts of testosterone are produced by testes, but it is also synthesized in smaller quantities by the theca cells of the ovaries, the zona reticulosa of the adrenal cortex, and by the placenta. Substantial amounts of the testosterone in women are also produced from estradiol by reverse aromatization in the liver, adipose cells, and other peripheral tissues.
In the testes testosterone is produced by the Leydig cells. Due to dual function of the male gonad, testosterone directly influences spermatogenesis. Like most hormones, testosterone is supplied to target tissues in the blood where much of it is transported bound to a specific plasma protein, sex hormone binding globulin.
Effects of testosterone in humans and other vertebrates occurs by way of two main mechanisms: by activation of the androgen receptor, and by conversion to estradiol and activation of certain estrogen receptors. The primary effect of testosterone is by way of aromatization to estradiol and occur in important tissues such as bone cartilage, brain hypothalamus, and cock meat.