A Model of Iodine Kinetics in the Dog

Abstract

A composite mathematical model of iodine kinetics in the dog was developed by combining representative models of the component subsystems: iodide, T3, T4 and the thyroid gland. Blocking of thyroidal uptake with stable iodine permitted the study of each subsystem independently. The iodide subsystem includes early and late distributional phases in addition to urinary and fecal excretion pathways. The T3 and T4 subsystems are each represented by two metabolic compartments and pathways for deiodination and fecal excretion. The thyroid is represented by two compartments, corresponding to rapid and slow secretory phases. Both tracer and steady state tracee data were used in the calculations. Rates of exchange, coupling coefficients, masses, concentrations and spaces of distribution were calculated by the SAAM computer program. The kinetics of hormone distribution and turnover and thyroidal iodine turnover are more rapid in the dog than in man. The only not able exception is the rate of urinary excretion of iodide. In addition, the dog does not conserve either peripheral hormone or intrathyroidal iodide very efficiently, compared to man. There is considerable fecal wastage of both T3and T4 and the thyroid secretes a large fraction of the iodide derived from MIT and DIT. Both factors contribute to the dog's requirement of a higher intake of iodine than man, relative to body weight. Most of the exchange rates in the iodide, T3 and T4 subsystems were reduced (30–40%) by thyroid blockingdoses of stable iodide, but there were few significant changes in volumes of distribution. (Endocrinology95:1078, 1974)