EFFECT OF THYROTROPHIN ON THE METABOLISM OF IODIDE131IN THE THYROID GLAND

Abstract

Iodide¹³¹ and PBI¹³¹ in the arterial and thyroid venous blood obtained by eathcterization in 28 dogs was measured to study the effect of TSH on the release and uptake of iodide¹³¹. When the glandular organic iodine was labeled by the injection of I¹³¹ prior to the experiments, release of iodide¹³¹ by TSH was far more intense than that by KSCN, even if the T/S ratio was decreased to less than 50% of the initial value by the injection of KSCN. On the contrary, the release of iodide¹³¹ by TSH was only temporary and definitely smaller than that by KSCN when all of the I¹³¹ in the thyroid was kept in the form of iodide by the administration of methyl mercaptoimidazole. The ratios of iodide¹³¹ to PBI¹³¹ in the thyroid venous output were compared with the ratios of iodotyrosine¹³¹ to iodothyronine¹³¹ in the thyroid gland. Whether TSH was given or not, both ratios were equal when methyl mercaptoimidazole was injected to prevent the reutilization of iodide; when methyl mercaptoimidazole was omitted, the latter was greater than the former. The rate of organic binding of iodide¹³¹ was calculated from the output of iodide¹³¹ and PBI¹³¹, and the ratio of iodotyrosine¹³¹ to iodothyronine¹³¹. It was shown that the organic binding of iodide¹³¹ was accelerated within a few minutes after the injection of TSH. From these experiments, it is concluded that in the physiologic state, iodide is always produced from thyroglobulin in the thyroid, but it is not discharged from the gland because the capacity for organic binding of iodide exceeds the iodide production. Both proteolysis of thyroglobulin and organic binding of iodide are accelerated within a few minutes when 50 to 2000 U.S.P. milliunits/dog of TSH is given. However, in the initial phase of its action the rate of iodide production from iodotyrosine exceeds the rate of organic binding of iodide, and this results in the release of iodide from the thyroid gland in addition to PBI. The thyroid gland starts to accumulate iodide several hours later, since the increased proteolysis of thyroglobulin terminates, as judged from the output of PBI, while the increased rate of organic binding continues.