INTRATHYROIDAL IODINE METABOLISM IN THE RAT. 2 THE IODOTHYRONINES

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Abstract
Incorporation of I131 into iodothyronines in the rat thyroid gland has been studied by 2-dimensional paper chromatography. In contrast with the results obtained in this laboratory and elsewhere by 1-dimensional chromatography, it was found that the ratio of [I131] tri-iodothyronine to [I131] thyroxine did not diminish as the time elapsing between administration of I131 and the death of the animal was lengthened. The ratio was approximately the same 5-15 min. after the injection as at 24 hr. The reason for the discrepancy between 1-dimensional and 2-dimensional chromatographic analysis was that 1-dimensional analysis failed to separate tri-iodothyronine from an unknown iodine-containing compound with faster turnover. A total of 4 unknown iodine-containing compounds was detected in the tyroid hydrolysates. These results are compatible with the view that much of the thyroidal tri-iodothyronine is formed by the coupling of mono- and di-iodotyrosine. The formation of minor quantities of tri-iodothyronine by deiodination of thyroxine is not precluded.