Quantitative Aspects of Iodine Metabolism. The Exchangeable Organic Iodine Pool, and the Rates of Thyroidal Secretion, Peripheral Degradation and Fecal Excretion of Endogenously Synthesized Organically Bound Iodine

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Abstract
Methods are pre-sented for the determination of the exchangeable organic I pool and the rates of thyroidal secretion, peripheral degradation, and fecal excretion of endogenously synthesized organically bound I. These determinations require only assays of radioactivity following the giving of I131 and measurements of the protein bound I concentration (PBI) in the plasma. The distribution and degradation of exogenously synthesized organically bound I were studied in 3 subjects. The exchangeable thyroidal pool of or-ganic I does not differ markedly in untreated hyperthyroidism and euthyroidism but may be strikingly re''duced following abla-tion therapy or treatment with tapazole. The apparent extra-thyroidal space of distribution of biosynthesized organic I averages about 9 1 or 10-20% of body weight. At normal serum PBI levels, approximately 40-110 [mu]g of I are degraded daily in euthyroid subjects and in patients with hyperthyroidism main- tained in a euthyroid state clinically by treatment with tapazole. The rate of degradation increases with the square of the PBI concentration over a greater than fivefold range in concentra-tion. There was good agreement between the values obtained for thyroidal secretion and the sum of degradation plus fecal loss. Fecal excretion of organic I amounts to approximately 10-15% of the quantity degraded in all subjects. Data for exogenous and endogenous I balance are derived from the assumption of steady state conditions. The kinetics of distribution and degradation of organic I are analyzed and discussed with particular reference to the significance of plasma PBI131 concentrations and the characteristics of the biologic decay curve of thyroidal radio-activity. Certain therapeutic implications of these studies are considered.