Transport and accumulation of radioiodine by thyroid glands of normal male guinea pigs

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Abstract
Experimental data concerning T/S (ratio of concentration of radioiodide in thyroids to that in serum) during equilibration following single injections of propylthiouracil and radioiodide were obtained from male guinea pigs and were compared with predictions based on an open two-compartmental model of thyroid function. Unidirectional clearance, C/m, of radioiodide by the thyroid from the blood was 1.05 [mu]/mg per min. in recently weaned animals and 1.76 in adult animals; exit rate constant, KTB, was 0.015/min. These constants are lower than those of normal rats and mice. Data concerning dependence of T/S on serum iodide concentration indicated that Km was 0.38 mg I-/100 ml serum. Experimental data obtained over the 3-hr. interval when binding of radioiodide to thyroprotein was allowed were compared with predictions based on an open 2-compartmental model. The fraction of thyroidal radioiodide bound to thyroprotein, Kb, was not constant contrary to expectation from the model; Kb was 0.057/min. up to 30 min. and was 0.024 from 1-3 hr. It was estimated that approximately 50% of the radioiodide leaves the thyroid of the guinea pig without being bound to thyroprotein.