Thyroidal Thyronine and Non-Thyronine Iodine Secretion in Euthyroid Subjects

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Abstract
Thyroidal non-thyronine iodine secretion was quantified in 22 euthyroid adult Arkansas subjects by measuring, simultaneously, thyroidal iodine accumulation A, thyroxine iodine turnover H4 and triiodothyronine iodine turnover H3. Non-thyronine iodine secretion was calculated as A-H4-H3. The estimate of A was based on the measurement of the specific activity of urinary iodine and the ratio of thyroidal to renal radioiodine clearance rates with appropriate corrections for biological and chemical losses of iodine and diurnal variation in renal radioiodine clearance. Estimates of H4 and H3 were derived from simultaneous studies of T4 and T3 kinetics and measurements of plasma T4 and T3 concentrations. Plasma T3 was measured using a double column chromatographic method with a final protein-binding quantification and by radioimmunoassay. Mean iodine intake was 564 μg and ranged from 217 to 1201 μg daily (male mean, 710 μg and female mean, 432 μg daily). Mean A was 128 μg and ranged from 26 to 278 μg daily (male mean, 153 μg and female mean 104 μg). Mean values for H4 and H3 were 58 and 13 μg 1/day (male means 64 and 16 μg: female means 53 and 10 μg daily). Finally, mean thyroidal non-thyronine iodine secretion was 56 μg/day (male mean 72 ng and female mean 42 μg daily). These data suggest that, at a mean-iodine intake level of 500–600 μg/day, non-thyronine iodine secretion (presumably iodide) approximates thyroxine iodine secretion and exceeds T3 iodine secretion 8–9 times.

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