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Iodine Metabolism in the Thyroid: A Comparison of Whole and Sliced Lobes^1,2,

Detroit, Michigan

ABSTRACT

Thyroids from rats fed a low iodine diet were incubated as whole lobes and sliced lobes in buffer containing iodide ¹³¹I. Total uptake of iodide ¹³¹I and the fraction of thyroidal ¹³¹I bound to protein was determined. The distribution of labeled iodoamino acids was observed following chromatography of pancreatic digest of the thyroid. The values for iodide ¹³¹I/diiodotyrosine-¹³¹I lower in whole lobes as compared to sliced thyroid lobes. When the iodide concentration of the buffer was increased, whole thyroid lobes from rats fed an iodine deficient diet were more sensitive to the inhibitory effect of iodide on protein binding of iodine than was true for sliced lobes. This difference in sensitivity appeared to be related to the greater quantity of stable iodide concentrated by whole thyroid lobes. Neither whole lobes or sliced thyroid lobes from rats maintained on an iodine deficient diet showed any response to TSH. Whole thyroid lobes from rats on a stock laboratory diet demonstrated an increase in iodide ¹³¹I uptake and the fraction of ¹³¹I bound to protein when incubated in the presence of TSH. It is suggested that in rats on a low iodine diet, endogenous TSH secretion has effected maximal stimulation of some step in the intrathyroidal iodine cycle so that the gland is insensitive to TSH in the buffer.

FOOTNOTES

¹ This study was supported in part by USPHS Research Grant No. AM 07000, The Michigan Heart Association, and the Woman's Hospital Research Fund.

² A preliminary report of this work has been published (Federation Proc. 23:203, 1964).

³ Department of Medicine, Wayne State University School of Medicine and The Woman's Hospital, Detroit, Michigan.