Malignant thyroid tumors after iodine-131 therapy: a retrospective cohort study.

We studied the incidence of malignant thyroid tumors after 131I therapy in 2727 patients with hyperthyroidism and in 273 euthyroid patients with cardiac disease. The patients were all adults, with a mean age of 57 years. The 131I therapy was given between 1951 and 1965. The mean follow-up period was 13 years for the hyperthyroid patients (15 years for the 85 per cent surviving for more than five years) and six years for the cardiac patients (12 years for the 41 per cent surviving for more than five years). The incidence of malignant thyroid tumors was based on a search of the Swedish Cancer Registry for the occurrence of such tumors in any of the 3000 patients. At present there is no increased incidence of malignant thyroid tumors after 131I therapy (four cases observed versus 3.2 cases expected).