Intrathyroidally Generated Iodide: Its Measurement and Origins*

Thyroid iodide was separated from organic iodine by gel electrophoresis of thyroid lobes 72 h after their labeling with radioiodine. Iodide thus measured accounted for 0.233% of total thyroid iodine, on the average. The lower and upper limits of the contribution of iodide due to a method artifact were established, respectively, by a second electrophoresis of thyroid lobes from which iodide had already been removed and by discharging transported iodide with perchlorate and blocking formation of iodide from organic precursors in vivo with 100 μmol mononitrotyrosine (MNT) and 20 mg propylthiouracil (PTU). They were thus found to be 0.042% and 0.064% of total thyroid iodine. In experiments in which iodide (corrected for 0.05% of the label taken to represent artifactitious iodide) averaged 0.131% of the total thyroid iodine, MNT lowered its level by 50%, perchlorate by 23%, and PTU by 15%. TSH (maximally effective dose, 1 IU) elevated thyroid iodide concentration within 1 h after its injection. Prior or concurrent administration of MNT reduced the thyroid iodide content of TSH-treated animals to a level as low as that in MNT-treated controls. The effect of MNT lasted for at least 6 h after a single injection. We conclude that thyroid iodide measured with our technique has several components: 1) an inevitable method artifact, 2) perchlorate- dischargeable transported iodide, and 3) intrathyroidally produced iodide, with a larger MNT-suppressible fraction (derived from iodotyrosines) and a smaller PTU-suppressible component (apparently originating from iodothyronines). Most of the iodide produced in response to TSH is the result of iodotyrosine dehalogenation.