Androstenedione

A weak androgenic steroid secreted by the adrenal cortex, testes, and ovary. In normal males less than 5% of their testosterone comes from the conversion of adrenal androstenedione. Androstenedione is converted to testosterone by the enzyme 17-ketoreductase. Androstenedione and testosterone are converted to estrone and estradiol in peripheral tissues (primarily in adipose tissue but also in muscle, kidney, liver and the hypothalamus) by aromatase. The conversion of androstenedione and testosterone accounts for more than 75% of the estrogens in the plasma of normal men. The rest is synthesized in the testes. Gonadotropin secretion may be partially controlled by estrogen formation in the hypothalamus.