Accumulation and localization of radiocalcium in the rat thyroid gland

To determine the rate of accumulation of radiocalcium (45Ca) in the thyroid gland, the concentration of radiocalcium was measured in the blood serum and thyroid gland at various time intervals up to 7 days after a single ip injection of 45Ca in rats. The ratio of the 45Ca concentration in thyroid gland to serum (T:S) increased with the time interval after injection. Thyroid activity was varied by hypophysectomy or by feeding a low iodine diet with or without added thiouracil. The more active the gland, the faster it equilibrated with the serum 45Ca and the less was the T:S in the steady state. The equilibration was much slower than for radioiodide, taking several days in normal control rats and hours in very active glands. Autoradiographs showed that the 45Ca concentration was higher in association with the follicular epithelium than with the follicular lumen at 1 h or less after injection. However, at 24 h, the concentration of 45Ca was higher in the lumen than elsewhere in the gland. To determine rate constants describing the equilibration, the data in the normal gland were fitted with a model that took account of the differing rates of equilibration in cell and lumen. The presence of calcium in the lumen indicates that each time the follicular epithelial cell endocytosed stored luminal colloid before secretion, it also took in an appreciable amount of calcium.