High diagnostic value of a radioiodine uptake test with and without iodine restriction in Graves' disease and silent thyroiditis.

The necessity of iodine restriction before radioiodine uptake (RAIU) testing for differentiation of thyrotoxicosis is controversial. The present study was undertaken to investigate the effects of iodine restriction on the RAIU value, and the necessity of iodine restriction in differentiating between Graves' disease (GD) and silent thyroiditis (ST). We investigated 415 patients, 277 of whom were patients with GD who had undergone iodine restriction before RAIU [GD(+)], 66 were patients with GD who did not undergo iodine restriction [GD(-)], 61 were patients with ST who had undergone iodine restriction [ST(+)], and the remaining 11 were patients with ST who did not undergo iodine restriction [ST(-)]. The RAIU value of the GD(+) group, 47.6% +/-14.4% (mean +/-standard deviation [SD]), was significantly higher than that of the GD(-) group, 42.4% +/- 17.6% (p = 0.03). However, the areas under the curves from the receiver operator characteristics analyses for the comparison between groups GD(+)/ST(+), GD(+)/ST(-), GD(-)/ST(+) and GD(-)/ST(-) were 0.99967, 0.99967, 0.98436, and 0.98485, and very high, respectively. High diagnostic value of the RAIU test was confirmed, but not affected by the presence of iodine restriction in the differentiation between GD and ST, therefore, iodine restriction before the RAIU test was unnecessary.