EFFECTS OF DIETARY IODINE ON THE UTILIZATION OF RADIOACTIVE IODIDE BY THE RAT

Abstract

Sprague–Dawley rats were fed on diets ranging from 40 μg I/kg to 3885 μg I/kg. Single doses of iodide-131 were injected intraperitoneally into each of the rats. In vivo measurements of radioisotope levels were made at intervals for 11 to 15 days over the neck and thorax. Thyroidal I131 curves were obtained by using a fraction of the thoracic counts to correct for the extrathyroidal component of the neck counts. Animals on low-iodine diets concentrated I131 in their thyroids more rapidly and to greater peak values, had lower protein-bound iodine (I127) concentrations, and lower total thyroidal iodide (I127) content than did rats in the high-iodine groups. An attempt was made to compensate the thyroidal counts for the continuing decrease in the concentration of iodide-131 in the plasma. From this attempt was derived the "thyroidal index", a parameter which may be related to the rate of exchange of the total thyroidal iodine stores. Biological half-life values (I131 in thyroid gland) for the low-iodine groups were larger than those for the high-iodine animals. The hypothesis is advanced that, at least for the conditions reported here, the biological half-life does not adequately reflect thyroidal activity; exchange of iodine between the rat and its environment is considered to be the more important factor in controlling the numerical value of this parameter.