Kinetics of Isotopic Equilibration of Iodine in Individual Rat Thyroid Follicles

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Abstract
Estimates of the relative concentration of radioiodine in individual follicles at various stages of isotopic equilibration were made by microdensitometry of auto radiographs. Data at 1 hr after a single injection of radioiodide were used to obtain clearances of blood radioiodide by single follicles. For central follicles clearances appeared proportional to the surface area of the lumen. The observed size independence of the concentration at isotopic equilibrium indicated that clearance of luminal iodoprotein was proportional to the surface area of the lumen also. These results indicated that average uptake and release of radioiodine per cell are independent of follicle size. The analysis of the size distribution of radioiodine concentration in follicles at 4 days indicated that the fraction of iodine released per day varied inversely with the lumen diameter. Clearance of colloid was approximately 115 Mμ3 per cell per day. Results were similar for peripheral follicles except that clearances and exit rate constants were about half those for central follicles. In addition to the size—dependent equilibrating component there was also a slowly equilibrating size—independent component in the gland possibly amounting to as much as 30% of thyroid iodine. This component appeared to be present in all follicles but formed a larger fraction of the iodine in peripheral follicles than in central follicles. Estimates also indicate that slow diffusion of iodoprotein in the lumen of the follicle does not significantly retard the equilibration of the whole gland. The results indicate that the kinetics of equilibration and the release of iodine from the whole gland are likely to be complex. (Endocrinology92: 894, 1973)

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