The concentration of hormones in the blood primarily reflects the rate of secretion by the endocrine glands. Hormones do not generally accumulate in the blood because they are rapidly removed by target organs and by the liver. The half-life of a hormone— the time required for the plasma concentration of a given amount of the hormone to be reduced to half its reference level—ranges from minutes to hours for most hormones (thyroid hormone, however, has a half-life of several days). Hormones removed from the blood by the liver are converted by enzymatic reactions into less active products. Steroids, for example, are converted into more water-soluble polar derivatives that are released into the blood and excreted in the urine and bile.
The effects of hormones are very dependent on concentration. Normal tissue responses are produced only when the hormones are present within their normal, or physiological, range of concentrations. When some hormones are taken in abnormally high, or pharmacological, concentrations (as when they are taken as drugs), their effects may be different from those produced by lower, more physiological, concentrations. The fact that abnormally high concentrations of a hormone may cause the hormone to bind to tissue receptor proteins of different but related hormones may account, in part, for these different effects. Also, since some steroid hormones can be converted by their target cells into products that have different biological effects (as in the conversion of androgens into estrogens), the administration of large quantities of one steroid can result in the production of a significant quantity of other steroids with different effects.
Anabolic steroids are synthetic androgens (male hormones) that promote protein synthesis in muscles and other organs. Use of these drugs by bodybuilders, weightlifters, and others is prohibited by most athletic organizations. Although administration of exogenous androgens does promote muscle growth, it can also cause a number of undesirable side effects. Since the liver and adipose tissue can change androgens into estrogens, male athletes who take exogenous androgens often develop gynecomastia—an abnormal growth of femalelike mammary tissue. High levels of exogenous androgens also inhibit the secretion of FSH and LH from the pituitary, causing atrophy of the testes and erectile dysfunction. The exogenous andro-gens also promote acne, aggressive behavior, male pattern baldness, and premature closure of the epiphyseal discs (growth plates in bones), stunting the growth of adolescents. Female users of exogenous androgens display masculinization and antisocial behavior. In both sexes, the anabolic steroids raise blood levels of LDL cholesterol (the "bad cholesterol") and triglycerides, while lowering the levels of HDL cholesterol (the "good cholesterol"), thus predisposing users to increased risk of heart disease and stroke.
Pharmacological doses of hormones, particularly of steroids, can thus have widespread and often damaging side effects. People with inflammatory diseases who are treated with high doses of cortisone over long periods of time, for example, may develop osteoporosis and characteristic changes in soft tissue structure. Contraceptive pills, which contain sex steroids, have a number of potential side effects that could not have been predicted in 1960, when "the pill" was first introduced. At that time, the concentrations of sex steroids were much higher than they are in the pills presently being marketed.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.