Thyroid Function in Rats and Chickens. Equilibration of Injected Iodide with Existing Thyroidal Iodine in Long-Evans Rats and White Leghorn Chickens1

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Abstract
Some aspects of thyroidal metabolism of iodine (I127 and I131) were compared in Long-Evans rats on low iodine or on iodide supplemented diet, as well as in White Leghorn chickens on iodide supplemented diet. Supplementation of a low iodine diet caused a large increase in circulating iodide127 in both rats and chickens; it produced a significant increase in circulating PBI127 of rats but only an insignificant increase in circulating PBI127 of chickens. Thyroid content of iodine127 in all animals was increased 3- to 4-fold by the iodide supplement. Rates of trapping of iodide127 calculated from specific activities of serum iodide and thyroidal I131 uptake at early times after injection of radioiodide were essentially the same in thyroids of rats on the 2 kinds of diet; in thyroids of chickens on iodide supplemented diet the rate was 5-fold higher than in the rats. Iodide supplementation of the diet caused a striking change in the relative proportions of iodoamino acids in thyroidal iodoprotein, but the distributions were similar in rats and chickens when they were maintained on diets with the same or similar iodine content. At early times after injection of I131, MIT accounted for a higher percentage of thyroidal label than DIT in animals on low iodine diet; the reverse was true when the diet of the animals was supplemented with iodide. In each case, the relative distribution of thyroidal I131 became identical with that of I127; the absolute specific activities of iodine in the iodoamino acids of the iodoprotein became equal and remained that way for as long as observations were made. The results are interpreted to signify that intrathyroidal iodination and deiodination reactions occur continually and lead to randomization of thyroidal iodine, both of iodotyrosines and iodothyronines; they are consistent with, but not conclusive proof of, the functional homogeneity of the major portion of thyroidal iodoprotein. (Endocrinology74: 212, 1964)