[Clinical significance of the measurement of the urinary concentration of iodine in differentiating silent thyroiditis from Graves' disease].

In order to differentiate silent thyroiditis (SLT) from Graves' disease, the usefulness of the measurement of the urinary concentration of iodine was evaluated in this study. The subjects employed were 39 patients with SLT and 40 patients with Graves' disease. Patients were advised to avoid any iodine-containing food or medication for a week before the examination. The urinary concentration of iodine (UI) and the serum concentration of thyroid hormones were determined. UI was calculated from the amount of iodine in the spot urine by multiplying it by the ratio of iodine to creatinine. Since the UI value thus obtained was significantly well correlated with the UI value for 24 hour urine, the former value was used instead of the latter value thereafter. Mean UI value in the patients with SLT was 482.4 +/- 296.4 mu g/day and that in the patients with Graves' disease was 169.8 +/- 75.2 mu g/day, the former value being significantly higher than the latter (p < 0.0001). A strong and significant correlation between UI and the serum concentration of FT4 or T3 (TT3) was found in the patients with SLT (r = 0.76, p < 0.0001 and r = 0.54, p < 0.02), but not in those with Graves' disease (r = 0.34, p = 0.07 and r = 0.24, p = 0.14) Mean UI/FT4 ratio and mean UI/TT3 ratio was significantly higher in patients with SLT than those with Graves' disease and the overlaps in the ratios between these two groups were very slight. These results indicate that both the ratios of UI/FT4 and UI/TT3 were useful parameters to differentiate SLT from Graves' disease. The higher UI value observed in the patients with SLT was thought to be due to the increase in the amount of inorganic iodine which was liberated from the iodinated material leaked from the damaged thyroid tissue by the deiodinating mechanism in the peripheral tissues.