RADIOACTIVE IODINE AS AN INDICATOR OF THE METABOLISM OF IODINE IV. THE DISTRIBUTION OF LABELED THYROXINE AND DIIODOTYROSINE IN LIVER, MUSCLE AND SMALL INTESTINE1

Abstract

The deposition of radioiodide, radiothyroxine and radiodiiodo-tyrosine was measured in liver, muscle and small intestine with radioactive iodine as indicator. The character of the uptake and loss of total radioiodine by these 3 tissues resembled that of plasma. The distribution of each tissue''s I among the 3 fractions, iodide, thyroxine and diiodotyrosine, changed with time but at each time interval the distribution resembled that observed in the plasma. Almost all of the initially labeled iodide in these tissues is replaced by labeled organic I in 26 hrs. by the animal whose thyroid glands had been made hyperactive by injs. of the thyro-trophic hormone. In the normal animal more than half of the initially labeled iodide is still in the inorganic form at 25 hrs. The % of each tissue''s I found as thyroxine increased with time. The diiodotyrosine fraction as a % of each tissue''s total I remained low and relatively uninfluenced by time. The significance of these findings in relation to the I cycle is discussed.