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Acute and Chronic Effects of Iodide on Thyroid Radioiodine Metabolism in Iodine-Deficient Rats¹

Department of Pharmacology, University of Texas Southwestern Medical School Dallas, Texas 75235

This study was supported by Grant A-3612 from the USPHS.

Abstract

Work of previous investigators has established that the distribution of ¹³¹I among the iodoamino acids of the thyroids of rats on an adequate iodine intake is not seriously affected by a previous acute injection of NaC10₄, whereas it is markedly altered by acute administration of propylthiouracil (PTU). In iodinedeficient rats, we observed that the administration of NaC104 shortly before injection of carrierfree ¹³¹I resulted in a striking further increase in the usual high values for MIT*/DIT/, and in marked decreases in T₄/ and T₃/. Similar changes were seen after injection of PTU. The effect of NaC104 was completely abolished by administration of 20 /µg of stable iodide together with the ¹³¹I. Similarly, in iodine-deficient rats not receiving NaC10₄, injection of graded doses of iodide carrier (up to 5 µg) with ¹³¹I led to progressively greater values for T₄/ and DIT/, and to progressively lower values for MIT/ and T₃/. These results indicate that the degree of incorporation of stable iodide into thyroglobulin is an important factor in determining the distribution of newly entering iodide among the iodoamino acids of the thyroid. Equilibrium labeling experiments with iodine deficient rats demonstrated that as daily iodine intake increased from 0.3 to 5.3 µg/day, there was a progressive increase in the amount of stable MIT, DIT, T₃ and T₄ in the thyroid, while MIT/DIT and T₃/T₄ progressively diminished. Not only the T₄ level, but also the molecular ratio, T₄/DIT (coupling efficiency), was reduced in thyroids of extremely iodine-deficient rats. This suggests that extreme iodine deficiency reduces thyroid hormone production not only by limiting the availability of iodine, but also by impairing the efficiency of the coupling reaction. A relatively small increase in iodine intake restored coupling efficiency to maximal values, and also resulted in a marked decrease in MIT/DIT. (Endocrinology 83: 279, 1968)

Footnotes

¹ Presented in preliminary form at the Fortyninth Meeting of The Endocrine Society, Bal Harbour, Florida, June 15-17, 1967 (Abstract 134).

² Career Research Awardee, USPHS.

Received November 27, 1967.

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