THE IODIDE TRAPPING AND BINDING FUNCTIONS OF THE THYROID

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Abstract
A method is presented for the kinetic analysis of thyroidal iodide trapping and binding in vivo. Radio-iodide transfer between thyroid and plasma in both directions was determined in a group of subjects under conditions in which organic I binding was effectively blocked. In the absence of block of iodide binding, thyroidal plasma clearance of iodide131 is essentially constant and the thyroid/plasma iodide131 ratio is too small to be measured (by the in vivo techniques employed here) in hyperthyroid as well as euthyroid subjects. Thyroidal plasma iodide clearance in the unblocked gland is equal to that volume flow of plasma whose iodide content is transferred to the thyroid trap in the blocked gland. This indicates that iodide trapped by the thyroid is bound before significant loss back to the plasma can occur. Further confirmation of the efficiency and rapidity of binding of trapped iodide by the untreated gland is presented in experiments demonstrating the absence of discharge, by thiocyanate, of any significant fraction of accumulated thyroidal radioactivity even a few minutes after the injection of radioactive I. Quantitative experimental data presented favor the interpretation of an inactivation of the thyroid trap by thiocyanate in preference to that of a simple substitution of thiocyanate for iodide. The effect of different dosage regimens of l-methyl-2 mercapto-imidazole on iodide binding and on the character of the thyroidal I131 uptake curves is presented.