[6] Methods for Monitoring in Vivo Steroid Hormone Production and Interconversion Rates

The production rate of a steroid is defined as the mass of that hormone that enters the blood per unit time from all sources. For an individual steroid, this is equal to that secreted from endocrine glands plus that formed in peripheral tissue from prehormones. Prehormones may be defined as substances that are converted peripherally to more potent compounds, which then contribute significantly to an overall biological effect. The peripheral conversion can be in any tissue including target organs. Androgens and estrogens are notable examples of groups of steroids that may interconvert to alter their biologically active blood pools. For example, several relatively inactive androgens may be converted in a variety of tissues to testosterone and dihydrotestosterone that may be active locally at the site of production or reenter the blood and contribute to the overall total androgen pool. Under certain conditions, androgens, such as testosterone and androstenedione, may be prehormones for the blood estrogens, estrone, and estradiol, which interconvert with one another. This chapter presents the techniques for studying the in vivo interconversion and production rates of steroids in blood and in tissue. Although testosterone and androstenedione are used as typical examples, it should be emphasized that the methods are general and may be applied to any group of interconverting compounds.