Intrathyroidally Generated Iodide: The Role of Propylthiouracil-Sensitive Processes in Its Production*

Thyroid radioiodide to serum radioiodide concentration (*T/*S) ratios 1 h after 131I administration were 3 times higher in rats pretreated with propylthiouracil (PTU) than in rats with binding thyroids. Although indications of a preponderance of transported iodide in the thyroid early after 131I injection were found, a reduction of the 1-h *T/*S ratio in TSH-treated rats by the inhibitor of iodotyrosine dehalogenation, mononitrotyrosine, points to an early contribution of intrathyroidally generated (internal) iodide to total thyroid iodide. In rat thyroids labeled 72 h earlier, there was no rise in electrophoretically separated radioiodide 2 h after the administration of PTU alone, but the elevation of thyroid iodide due to TSH was augmented by concurrently given PTU. On the assumption that PTU affects transported components of thyroid iodide identically whether they are labeled 1 or 72 h earlier, we conclude that PTU must severely depress the generation of internal iodide. This effect is no longer evident if TSH is given simultaneously. Further, our results are consistent with the hypothesis that internal iodide passes through a stage in which it is not available for organic binding before it mixes with transported iodide. TSH appears to facilitate the transfer of internal iodide into the pool which it shares with external iodide.