'Subcellular' localization of radioiodine in the thyroid follicle of the rat following the administration of low and high doses of 125-I

The 'subcellular' localization of radioiodine was observed in the thyroid follicles of rats following the injection (i.p.) of 75 μc to 1000 μc of radioiodide (125I). Twenty-four hours after the injection of the smaller doses (75, 150 and 250 μc), the radioiodine was distributed evenly throughout the follicular lumen. Twenty-four hours after the injection of the large dose (1 mc) of radioiodide, there were two histological patterns of follicular radioiodine localization. In the majority of follicles (about 85%) the radioiodine was distributed uniformly throughout the lumen. In the other follicles (15%) the activity was not uniformly distributed but was primarily localized along the cell-colloid border. At this interface, the radioactivity was entirely localized in the colloidal region.

A possible explanation is that the follicles with an intra-colloidal ring of radioiodine are more sensitive to the early effects of radiation and could be reflecting some cellular biochemical alteration.