Comparison of Thyroidal Iodide Accumulation and Thyroxine Secretion in Euthyroid Subjects

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Abstract
Fractional thyroidal and renal radioiodine clearance and renal 127I excretion were measured in 152 euthyroid subjects. Thyroidal 127I accumulation (A) was calculated from the basic formula of Stanbury et al. with short-term thyroidal and extrathyroidal radioiodine clearance measurements substituted for 48-hr thyroidal radioiodine uptake. Appropriate corrections were included for diurnal variations in renal radioiodine clearance, lag time in urine collections, and losses of iodine in uncollected urine and in sweat and stools. Thyroxine 131I degradation (H4) was measured by whole body counting techniques and compared with calculated thyroidal 127I accumulation in 29 subjects on a mean calculated daily iodine intake of 387 μg. Significant negative correlation of thyroidal radioiodine uptake and renal iodine excretion was observed (p < 0.001). The correlation and regression coefficients of these indices were, however, low (0.33 and 0.238). In addition, mean daily thyroidal 127I accumulation exceeded daily thyroxine 127I degradation by 38 μg. Regression of H4 on A revealed near independence of these parameters; while thyroxine iodine degradation remained rather constant, thyroidal iodine accumulation appeared to increase progressively with increasing iodine intake. It is concluded that the assumption of equal thyroidal iodine uptake and daily thyroxine iodine secretion at equilibrium is not valid. Although significant negative correlation of thyroidal radioiodine uptake with iodine intake was observed, this was not complete (slope of 0.238 rather than 1.0), and progressively increasing thyroidal nonthyroxine iodine secretion appears to occur with increasing iodine intake. The physiologic significance of these observations is discussed.

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