ANDROGENS & ANABOLIC STEROIDS
In humans, the most important androgen secreted by the testis is testosterone. The pathways of synthesis of testosterone in the testes are similar to those previously described for the adrenal gland and ovary (Figures 39–1 and 40–2).
In men, approximately 8 mg of testosterone is produced daily. About 95% is produced by the Leydig cells and only 5% by the adrenals. The testis also secretes small amounts of another potent androgen, dihydrotestosterone, as well as androstenedione and dehydroepiandrosterone, which are weak androgens. Pregnenolone and progesterone and their 17-hydroxylated derivatives are also released in small amounts. Plasma levels of testosterone in males are about 0.6 mcg/dL after puberty and appear to decline after age
Testosterone is also present in the plasma of women in concen-trations of approximately 0.03 mcg/dL and is derived in approxi-mately equal parts from the ovaries and adrenals and by the peripheral conversion of other hormones.
About 65% of circulating testosterone is bound to sex hormone-binding globulin. SHBG is increased in plasma by estrogen, by thyroid hormone, and in patients with cirrhosis of the liver. It is decreased by androgen and growth hormone and is lower in obese individuals. Most of the remaining testosterone is bound to albu-min. Approximately 2% remains free and available to enter cells and bind to intracellular receptors.