Effect of Thyrotropin on Thyroid Clearance of Iodide and Pertechnetate: Comparative Observations at Normal and High Plasma Iodide Concentrations

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Abstract
The effect of TSH [thyrotrophin] on thyroid clearance of circulating I- at physiologic and at high concentrations of plasma I- was studied in the dog by a method permitting rapid sequential sampling of arterial and thyroid venous blood. Two hr. before cannulation of an inferior thyroid vein, a tracer dose of radioiodide (I131 or I125) was injected intravenously, either carrier-free ("normal I- group") or with a substantial quantity (2-10 mg) of stable I~ ("high I" group"); in many experiments 99mTcO4 - (radiopertechnetate) was given together with the labeled I". Intravenous injection of 10 U of TSH induced a prompt decline in the glandular clearance of pertechnetate in both normal and high iodide dogs. I" clearance by the thyroid was unaffected by TSH in the normal I" group. In the I" loaded animals TSH induced an initial decline in I" clearance, followed in less than 20 min. after injection by a 50% increase over pre-TSH values. The results suggest that TSH promptly enhances thyroidal organification of I, and that this may be among the earliest of the glandular responses to TSH. The glandular efflux of pertechnetate after TSH injection resembles the prompt release of trapped I" induced by TSH when organic binding of I- is blocked, and may be analogous to the initial decline in I" clearance observed after TSH in the high I- animals. Since under appropriate experimental conditions TSH induced glandular efflux of trapped anion I-, pertechnetate) in the absence of antithyroid drugs, tills response to TSH is not dependent upon the presence of agents which block organification of I-.