Evaluation of Rates of Secretion and of Interconversion of Steroid Hormones
The amount of a steroid produced per day is usually evaluated by methods which measure the disposition of an isotopically labeled tracer of the hormone (injected intravenously) by following the decline in the specific activity of the hormone in blood  or by determining the cumulative specific activity of a urinary metabolite of the hormone [2,3,4]. In the application of these methods it has been assumed that they provide a measure of the dilution of the tracer by the endogenously secreted hormone. In this article, situations will be described in which the measurement of the dilution of the injected tracer of the hormone would not be sufficient to evaluate its rate of secretion. Such situations may arise when the hormone is produced not only by secretion but also by peripheral conversion from other precursors, or when the produced hormone does not mix completely with the injected tracer and does not have the same metabolic fate. In these situations it is convenient to correlate the fate of the administered isotope with that of the endogenously secreted hormone by considering models consisting of interrelated “compartments” or “pools.” A pool is defined by both a compound and the space in which it is distributed. A compound or a tracer entering a pool is assumed to mix immediately with the entire pool. The calculation of rates of entry of material into each pool or rates of transfer from one pool to another is dependent on the chosen models. Some conclusions obtained from tracer analysis of open systems of multiple pools at the steady state are presented in this article where it will be shown that the simultaneous administration of more than one tracer can make possible the evaluation of the secretory rates of a hormone in situations in which. that information cannot be obtained by administering only one tracer of the hormone. The mathematical proofs of statements made in this article are given in a paper in the Journal of Clinical Endocrinology and Metabolism .
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