THYROIDAL IODIDE TRAPPING AS INFLUENCED BY SERUM IODIDE LEVELS AND THYROTROPHIN12

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Abstract
Thyroidal and serum iodide concns. and their ratios were examined in hypophysectomized, normal, hypophysectomized-thyrotrophin (TSH) treated and propylthiouracil (PTU) fed rats which received graded doses of I131-labelled stable iodide following the blocking of thyroid hormone biosynthesis with a single dose of PTU. Appropriate increase of the serum iodide levels caused the thyroid:serum iodide gradient to drop to 1:1 or below. The magnitude of the serum iodide concn. required to abolish the gradient was dependent on the state of thyrotropic stimulation. Thyroidal iodide concns. rose progressively as serum iodide levels were elevated. When correction was made for the iodide which entered the thyroidal "iodide space" (estimated at 40% of the thyroid volume in hypophysectomized rats) by diffusion, the capacity of the gland for "trapped" iodide was found to be limited and profoundly influenced by TSH.