Variations in the Response of the Thyroid Gland of the Rat to Different Low-Iodine Diets: Correlation with Iodine Content of Diet

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Abstract
Large variations are frequently encountered in the thyroidal responses of rats to commercially available low-I diets. These variations were correlated with differences in the I content of the diets in this study. A method was developed for measuring I content of low-I diets which was sufficiently sensitive to discriminate between a diet containing 15-20 ng of I/g and one containing 30-40 ng of I/g. Large differences were observed between various commercial low-I diets in their ability to induce goiter and to affect other indices of thyroid function, and these differences could be correlated with differences in the I content. The most severe I deficiency occurred in rats that were fed a Remington diet containing 15-20 ng of I/g. After 3 mo. on this diet, thyroid weight increased about 7-fold, thyroid 127I concentration was reduced to about 0.5% of control values, serum thyroxine (T4) was reduced to < 0.25 .mu.g/100 ml, the ratio of labeled triiodothyronine to labeled T4 was increased to 5.2, and that of labeled monoiodotyrosine to labeled diiodotyrosine was increased to 4.2. Much smaller changes were observed in the rats on the other low-I diets, which ranged from 30-40 to about 100 ng of I/g. A daily supplement of only 0.2 .mu.g iodide/day administered to rats on a Remington low-I diet produced significant changes in thyroidal responses. Iodine deficiency alone is probably sufficient to explain the effects of the Remington low-I diet, and it is probably not necessary to postulate the presence of a goitrogen.