Effect of variations in acute and chronic iodine intake on the accumulation and metabolism of [35S]methimazole by the rat thyroid gland

Studies were performed to ascertain the effect of various levels of chronic iodine intake and varying doses of iodide [0.002–100 μmoles KI/100 g body weight (BW)] given acutely on the rat thyroid metabolism of [ 35 S]methimazole ([ 35 S]MMI, 8.76 μmoles/kg BW). Variations in both acute and chronic iodine intake were associated with as much as four-fold changes in thyroid levels of total 35 S and unchanged [ 35 S]MMI. Chronic low iodine intake resulted in a considerable reduction in the thyroid uptake of [ 35 S]MMI (40% decrease) from high or normal chronic iodine intake. Unlike [ 35 S]PTU studies, the effect of increasing acute iodide dosage produced a biphasic response in the thyroid uptake of [ 35 S]MMI only in low chronic iodine intake. In these animals 0.1 μmoles KI/100 g BW produced the maximum uptake of [ 35 S]MMI (300% increase) but had no effect on high or normal chronic iodine intake. In these latter groups of rats, thyroidal total 35 S increased to plateau levels with increasing acute iodide dosage in the range of 0.1–1 μmoles/100 g BW which were unaffected by increased iodide up to 100 μmoles/100 g BW. In low chronic iodine intake rats also, the thyroid 35 S level seen at 1 μmole/100 g was unaffected by increased iodide dosage up to 100 μmoles/100 g. The steady thyroid 35 S levels seen in this acute iodide dose range in low, normal and high chronic iodine rats were 100, 70 and 110% respectively greater than their control values. Unlike [ 35 S]PTU studies, in general, an increase in thyroid total 35 S achieved by varying acute or chronic iodine intake was found to be associated with a large increase in the percentage thyroid 35 S occurring as free inorganic sulphate with a consequent effect on thyroid unchanged [ 35 S]MMI. In chronic low iodine intake animals treated with acute radioidide, in agreement with [ 35 S]PTU studies, no direct correlation was found between thyroid uptake or oxidation of [ 35 S]MMI and thyroidal total iodine, the accumulation or organification of acute [ 125 I]iodide, the occurrence of the Wolff-Chaikoff effect or saturation of thyroid iodide transport.