THE EFFECTS OF VARYING AMOUNTS OF STABLE IODIDE ON THE FUNCTION OF THE HUMAN THYROID*

The effect of addition of varying amounts of stable iodine to doses of radioiodine was studied in 64 euthyroid persons and in 46 with disturbed thyroid function. The addition of 1 to 2 mg. of stable iodine, as potassium iodide, reduced the uptake of radioiodine to a significantly greater degree in patients with Graves' disease than in euthyroid patients. The difference in response to the added load of iodine suggests that a relatively brief and precise test of thyroid function, an “iodide inhibition test,” may be evolved from these studies. The concentration of serum inorganic iodide was found, by indirect means, to be considerably less than 0.5 μg. per 100 ml. in most instances, regardless of thyroid function. Calculation of the rate of accumulation of iodine by the thyroid revealed that, when the serum iodide level was raised to about 6 μg. per 100 ml., the normally functioning thyroid gland accepted the load and accumulated more than ten times the normal amount of stable iodide per unit time. In contrast, the hyperactive gland was capable of only limited response to such an added load. The elevated concentration of iodide in the serum after administration of iodide resulted in the cessation of secretion of labeled hormone in thyrotoxic patients, but little change occurred in euthyroid patients.