Distribution of Organic125I and127I in the Rat Thyroid Gland During Equilibrium Labeling as Determined by Autoradiography1

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Abstract
Autoradiographs of organic radioiodine in rat thyroids were obtained at various times during the equilibration of thyroidal iodine with dietary I labeled with I125, Organic radioiodine was almost entirely located in colloid at all times. Most lumens were homogeneously labeled. At early times during the equilibration and concentration of organic radioiodine in such lumens was higher the smaller the follicle. As the equilibration progressed all concentration became similar in almost all lumens, independent of follicle size. Concentration became independent of size first among smaller follicles and then among progressively larger ones. At 99 days the specific activity in the thyroid had closely approached that in the diet; at that time, therefore, the concentration of radioiodine closely approached being proportional to the concentration of stable I. Thus, the autoradiographs were interpreted as showing that organic stable I was located almost entirely in colloid, that the luminal concentration of organic stable I was independent of follicle size, and that at early times the specific activity of organic radioiodine was greatest in the smallest follicles.