Thyroidal Iodine Heterogeneity: “Last Come, First Served” System of Iodine Turnover

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Abstract
Rats were given 125I-labeled iodine of constant specific activity in the diet over a 22- to 30-day course. Though the thyroid iodine achieved only about one half the specific activity of the diet, this 125I could be considered to represent “old” iodine in the gland. A single dose of 131I was given then which formed “new” thyroidal iodine. Further significant thyroid iodine uptake was blocked by propylthiouracil started several hours after the 131I dose. Thyroid iodine disposal was followed by serial measurement of blood PB131I/PB125I ratios or of the chromatographically separated thyroid iodinated amino acids. The ratio of 131I/125 I in all the iodinated amino acids and blood PBI fell with time, indicating that the 131I tracer did not mix uniformly with the 125I-labeled iodine and was disposed of first by the thyroid. The thyroid thus degrades or secretes recently formed organic iodine before it does the older, and essentially handles iodine in a “last come, first served” manner. (Endocrinology74: 973, 1964)