Lifelong thyroid hormone replacement is indicated in patients with hypothyroidism as a result of Hashimoto's thyroiditis. However, previous reports have shown that excess iodine induces hypothyroidism in Hashimoto's thyroiditis. This study investigated the effects of iodine restriction on the thyroid function and the predictable factors for recovery in patients with hypothyroidism due to Hashimoto's thyroiditis. The subject group consisted of 45 patients who had initially been diagnosed with hypothyroidism due to Hashimoto's thyroiditis. The subjects were divided randomly into two groups. One group was an iodine intake restriction group (group 1) (iodine intake: less than 100 micro g/day) and the other group was an iodine intake non-restriction group (group 2). The thyroid-related hormones and the urinary excretion of iodine were measured at the baseline state and after 3 months. After 3 months, a recovery to the euthyroid state was found in 78.3 % of group 1 (18 out of 23 patients), which is higher than the 45.5% from group 2 (10 out of 22 patients). In group 1, mean serum fT4 level (0.80 +/- 0.27 ng/dL at the baseline, 0.98 +/- 0.21 ng/dL after 3 months) and the TSH level (37.95 +/- 81.76 micro IU/mL at the baseline, 25.66 +/- 70.79 micro IU/mL after 3 months) changed significantly during this period (p < 0.05). In group 2, the mean serum fT4 level decreased (0.98 +/- 0.17 ng/dL at baseline, 0.92 +/- 0.28 ng/dL after 3 months, p < 0.05). In the iodine restriction group, the urinary iodine excretion values were higher in the recovered patients than in non-recovered patients (3.51 +/- 1.62 mg/L vs. 1.21 +/- 0.39 mg/ L, p=0.006) and the initial serum TSH values were lower in the recovered patients than in the non-recovered patients (14.28 +/- 12.63 micro IU/mL vs. 123.14 +/- 156.51 micro IU/mL, p=0.005). In conclusion, 78.3% of patients with hypothyroidism due to Hashimoto's thyroiditis regained an euthyroid state iodine restriction alone. Both a low initial serum TSH and a high initial urinary iodine concentration can be predictable factors for a recovery from hypothyroidism due to Hashimoto's thyroiditis after restricting their iodine intake.