Thyroid Function in Chronic Excess Iodide Ingestion: Comparison of Thyroidal Absolute Iodine Uptake and Degradation of Thyroxine in Euthyroid Japanese Subjects

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Abstract
Thyroidal absolute iodine uptake (AIU) and the degradation of thyroxine were compared in euthyroid Japanese subjects (9 males and 6 females) whose diet customarily included moderate to large quantities of foods rich in iodine. AIU was measured a) from thyroidal clearance of 131I and serum inorganic 127I, and b) from thyroidaly 131I uptake and specific activity of urinary iodine. Values for both methods agreed well and increased as serum inorganic 127I or urinary 127I increased, while the degradation of thyroxine measured in the same subjects did not differ greatly with differing serum inorganic 127I or urinary 127I. Invariably, values for AIU were higher than those for the degradation of thyroxine. Protein-bound 131I in normal thyroid tissue collected at surgery (5 subjects) was more than 85% of total thyroidal 131I when 131I was injected 2 hr before thyroidectomy. These results suggest that thyroid glands of Japanese subjects on a diet rich in iodine organify more 127I than they secrete as thyroid hormone and the excess is secreted as nonhormonal iodine.

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