Intrathyroidal iodine metabolism in the rat. The iodotyrosines

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Abstract
Estimations have been made of the I131 incorporated into monoiodotyrosine, di-iodotyrosine and iodothyronines in the rat thyroid gland at intervals from 1 min. to 24 hr. after injection of [I131] iodide. In confirmation of work in other laboratories, the percentage of the total radioactivity in the iodothyronines increased during the period studied, from amounts that could barely be measured at 1 min. to about 20% at 24 hr. The ratio of labelled monoiodotyrosine to labelled di-iodotyrosine remained almost constant during this period. Attempts have been made to duplicate the finding reported in the literature of ratios of mono- to di-[I131]iodotyrosine which changed with time, by working with rats of several strains. All these experiments have resulted in approximately constant ratios. Administration of sodium iodide or thiouracil resulted in altered but nevertheless approximately constant ratios. These results are not consistent with a simple precursor-product relationship between the thyroidal mono- and di-iodotyrosine. If di-iodotyrosine is formed from monoiodotyrosine both substances must be heterogeneous with respect to turnover.