Metabolism of Intrathyroidal Iodine in Normal Men1

Abstract

Thyroid samples of 11 euthyroid subjects who underwent neck surgery were investigated after successive labeling with 131I and 125I. The aim of the study was to obtain a kinetic estimation of the intrathyroidal metabolism in the normal human thyroid gland. Investigations included the separation of the iodinated compounds by column and paper chromatographies, the purification of thyroglobulin (19 S) by gradient centrifugation and the estimation of the stable and labeled iodine content of the various compounds. Distribution of stable iodine in pancreatin hydrolysates was found: 16.1% as iodide, 32.7% as MIT, 33.4% as DIT, 16.2% as T4 and 7.6% as T3. Specific activity of DIT is equal to the SA of MIT, even for the shortest time intervals. On the contrary, the rise of the T4 SA is a very slow process (“apparent” renewal rate of T4 pool: .06/day). Iodination of thyroglobulin (i.e., .18%) is found too low to permit an equal distribution of MIT, DIT and T4 at the level of each simple molecule of thyroglobulin. A heterogeneous iodine content of the substrate has to be postulated. On the other hand, present investigations emphasize some methodologic difficulties related to the analytical procedures employed: the occurrence of a marked deiodination and a loss of iodothyronines, and the presence of unhydrolyzed iodinated compounds in the thyroid digests. One of these compounds (X2) accounts for 10% of whole iodine of the digests and is found to contain large amounts of DIT and T4, but no MIT.