Calcium and metabolism of I¹³¹ in rats and homogenates of rat thyroid

¹ Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, Tennessee

The goitrogenic effect of calcium was studied by the use of I¹³¹ in intact rats and in rat thyroid slices. Rats receiving excess dietary calcium had larger thyroids than controls. Enlargement of the thyroid was accompanied by increased content of inorganic iodine and decreased content of organic iodine. In vitro studies showed that thyroid enlargement was associated with greater total iodine uptake. Addition of calcium to the incubation medium decreased the total iodine uptake of thyroid slices. The reduction occurred as a consequence of interference with the conversion of inorganic iodine to organic iodine. No decrease in inorganic iodine content was observed. The ratios of bound iodine to free iodine were constantly smaller in the thyroid groups receiving calcium supplement. The results of the in vitro studies suggest that calcium interference is due, at least in part, to a direct effect on thyroid tissue.