Testosterone physiological effects

Steroid hormones bear a remarkable structural similarity to one another (see Fig. 29-1). The precursor for steroid biosynthesis is cholesterol. Consequently, all of the steroid hormones share the same basic chemical configuration as their parent compound. This fact has several important physiologic and pharmacologic implications. First, even relatively minor changes in the side chains of the parent compound create steroids with dramatically different physiologic effects. For instance, the addition of only one hydrogen atom in the sex steroid pathway changes testosterone (the primary male hormone) to estradiol... 

Male and female hormones are produced primarily in the gonads. Androgens are synthesized in the testes in the male. In the female, the ovaries are the principal sites of estrogen and progestin production. As discussed in Chapter 29, small amounts of sex-related hormones are also produced in the adrenal cortex in both sexes, which accounts for the fact that small amounts of hormones of the opposite sex are seen in females and males that is, low testosterone levels are seen in females, and males produce small quantities of estrogen. However, under normal conditions, the amounts of sex-related hormones produced by the adrenal cortex are usually too small to produce significant physiologic effects. 

In adult males, testosterone is the principal androgen produced by the testes.39,40,144 Testosterone is synthesized by Leydig cells located in the interstitial space between the seminiferous tubules (Fig. 30-1). The seminiferous tubules are the convoluted ducts within the testes in which sperm production (spermatogenesis) takes place. Testosterone produced by the Leydig cells exerts a direct effect on the seminiferous tubules, as well as systemic effects on other physiologic systems (see Physiologic Effects of Androgens ). 

Although the various steroid hormones have remarkably different physiological effects, their stractures are quite similar. For example, the only difference between testosterone and progesterone is the substituent at C-17, and the only difference between 5a-dihydrotestosterone and estradiol is one carbon and six hydrogens, but these compounds make the difference between being male and being female. These examples illustrate the extreme specificity of biochemical reactions. 

The overall physiological effects of endogenous androgens are contributed by testosterone and its... 

Androstenolone. d ydroepiandrosterone, 3fi-hy-dmxyattdrosl-S-ene-17-dione an androgen less potent than testosterone, but with similar physiological effects. It is an intermediate in the biosynthesis of testosterone in the adrenal cortex. For structure and biosynthesis, see Androgens. 

Compared to the greatly differing physiological effects, the chemical differences at first appear minor. But too often one overlooks the fact that the side chains of amino acids are after all quite different. And if one were to compare the complete structural formulas of the peptides, then the differences would indeed appear much greater than they are, for example, among progesterone, corticosterone, and testosterone. 

Some of the physiological effects of testosterone are to increase muscle mass and decrease body fat. Derivatives of testosterone called anabolic steroids that enhance these effects have been synthesized. Although they have some medical uses, anabolic steroids have been used in high dosages by some athletes in an effort to increase muscle mass. Such use is banned by most sports organizations. 

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