A STUDY OF THE MECHANISM OF THE INHIBITION OF THE THYROID GLAND INDUCED BY INGESTION OF THYROID SUBSTANCE*†

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Abstract
The effect of the intramusc. admn. of varying doses of the thyroid-stimulating hormone of the pituitary gland upon the function of the thyroid gland was studied in 18 normal subjects. 10 mg. of TSH was the dose which was minimally effective in increasing the uptake of I131 by the thyroid gland and the level of the serum PBI, these having been detd. to be the most sensitive indices of thyroid functions. Ingestion of 120 mg. of thyroid daily for 12 or more days resulted in a decrease in the thyroidal uptake of I131 to hypo-thyroid levels, unaccompanied by any increase in the level of the serum PBI and with only a slight increase in the BMR. Both the serum PBI level and the mean uptake of I131 by the thyroid gland, which had been inhibited by ingestion of thyroid substance could be increased to hyperthyroid levels by the admn. of 10 mg. or 60 mg. of TSH. After similar doses of TSH, the increment in the uptake of I131 by the thyroid gland, which had been inhibited by ingestion of thyroid substance, was greater than the increment in the uptake in the control studies. The total collection of I131 in the former case, however, was less than the total uptake in the latter. Both the increase in, and the highest levels reached by, the serum PBI in response to TSH injns. were the same in the control and the thyroid-inhibited periods. Rebound hypo-function of the thyroid gland was noted 14 days following an injn. of 60 mg. of TSH. Rebound hypothyroidism was not noted following admn. of 10 mg. of TSH. A 2d dose of TSH administered during this rebound phase evoked not only a greater increment in, but also a greater total collection of I131 than had been noted following the original dose of TSH. The increase in the level of PBI was the same after each of the 2 injns. of TSH. The data lend support to the view that ingestion of thyroid substance decreases the function of the thyroid gland primarily through pituitary inhibition. To a minor degree, the ingested thyroid substance appears to decrease the responsiveness of the thyroid gland to administered TSH.