Hypothyroidism with a serum concentration of thyroid-stimulating hormone (TSH) above 40 microU/ml was noted in 3 (3.2%) of 93 patients on regular hemodialysis or continuous ambulatory peritoneal dialysis. These 3 patients had no history of thyroid disease and were receiving no medication known to influence thyroid function. They had habitually eaten iodine-rich foods and showed an enlarged thyroid gland with a preserved radioactive iodine uptake and a markedly elevated serum inorganic iodine (II) level. In all 3 patients, both thyroidal microsomal antibody and thyroglobulin antibody titers measured by hemagglutination methods were less than 100, and TSH-binding inhibitory immunoglobulin was negative. Moreover, histologically no lymphocytic infiltrations were observed. With only iodine restriction, serum TSH level markedly decreased from 44.6 to 3.6 microU/ml in case 1, from 90.6 to 3.2 microU/ml in case 2 and from 43.2 to 9.4 microU/ml in case 3 in parallel with decreases in the serum II level. These results suggest that at least in an area like Japan, where the daily intake of iodine is high, iodine-induced hypothyroidism may be induced in patients undergoing regular dialysis treatment even in the absence of apparent underlying thyroid disease.