Thyroid iodine content and serum thyroid hormone levels in autoimmune thyroiditis: effect of iodide supplementation

The relationship between thyroid iodine content (TIC) measured by x-ray fluorescence and serum TSH, T4, and T3 levels was investigated under iodide supplementation (0.5 mg/day for 1 to 9 mo). In five euthyroid control patients, whose TIC ranged from 2.5 to 14 mg, the TIC increased from 1.5 to 4 mg after 4 wk of treatment and had a tendency to plateau when the treatment was pursued. No significant changes in serum T4, T3, and TSH levels have been observed in these control subjects. Fourteen patients with autoimmune thyroiditis with low TIC (0-5 mg) were also studied. In six patients, the TIC increased significantly (3-10 mg over initial value after 3-7 mo of treatment). In parallel, there was a significant increase in serum T4 levels (35-150% over initial value) while T3 levels were modified in only two patients. In five patients serum TSH level decreased and was two- to seven-fold lower than before treatment whatever was its initial value; however, the spectrum of changes varied among patients from slight increase to a complete normalization of hormonosynthesis. In the eight other patients, iodide supplementation aggravated the thyroid disorders during the first months of treatment. The thyroid hormone blood levels dropped significantly in six patients (percent decrease below initial value: 20-100%) and was unchanged in the two others. An increase in the TSH blood level (X2-6) was observed in all patients except one. Concomitantly, the iodine stores were progressively depleted in three patients, unchanged in three and increased in two. When iodide treatment was pursued, an escape from this organification block was observed in two patients.