Endogenous Testosterone

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Testosterone is an anabolic/androgenic hormone. Although it is normally thought as a male hormone, it is naturally produced in both men and women. Its anabolic properties include the maintenance and growth of muscle and bone tissue (Sutton, Coleman, Casey, & Lazarus, 1973). Increases in muscular size appear to be due to the effect of testosterones upon protein synthesis (Griggs, Kingston, Jozefowicz, Herr, Forbes, & Halliday, 1989). Testosterone increases skeletal muscle size through hypotrophy (Boone, Lambert, Flynn, Michaud, Rodriguez-Zayas, & Andres, 1990); not by hyperplasia (Mooradian, Morely, & Korenman, 1987). Testosterone has been reported to enhance glycogen synthesis, reabsorption of sodium in the kidneys, and secretion of sebaceous glands (Mooradian, Morely, & Korenman, 1987). Testosterone is associated with bone growth, calcium retention, sodium reabsorption, and increased metabolic rate.

The negative feedback loop of the hypothalamic-pituitary-testicular axis regulates the production of endogeneous testosterone. The hypothalamus secretes gonadotropin releasing hormone (GnRH), which stimulates luteinizing hormone (LH) and follicle stimulating hormone (FSH) from the anterior pituitary. LH can then stimulate the membrane receptors of the Leydig cells in the testes to stimulate the production of testosterone (Kraemer, 1988).

Resting testosterone values for mature males range from 14.0 to 28.0 nmol/L (Kraemer, 1988). Approximately 97 to 99 percent of testosterone is transported in the blood bound to plasma proteins such as sex hormone binding globulin (SHBG) and albumin. The remaining 1 to 3% is the biologically active, free testosterone. Testosterone circulates in the blood approximately 15 to 30 minutes until it is either bound to receptors or metabolized into inactive products by the liver and subsequently excreted through the urine.

Testosterone can be converted to estradiol through aromatization in adipose tissue, certain brain tissue, and other specific tissues (Mooradian, Morely, & Korenman, 1987). Testosterone is also converted to dihydrotesterone in the gonads and certain peripheral tissues (Di Pasquale, 1992a; Mooradian, Morely, & Korenman, 1987). In normal men, plasma levels of dihydrotestosterone are approximately 55 ng/100 ml. The production of dyhydrotestosterone is 1/20th of the production of testosterone. Dihydrotestosterone is related to facial hair, genetic balding, prostate growth, and the development of male external genitalia. Dihydrotestesterone does not aromatize and has less of an antigonadotropic effect than testosterone. In high levels, dihydrotestosterone can assume the function of testosterone, and testosterone can assume the function of dihydrotestosterone (Di Pasquale, 1992a).

See production of testosterone in testes. Also see BPA and other Hormone Disruptors.


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