Chapter: Basic & Clinical Pharmacology : The Gonadal Hormones & Inhibitors
Androgens & Anabolic Steroids
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.
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