Mortality in women treated for hyperthyroidism.

To evaluate the late effects of exposure to radioiodine (131I), a retrospective cohort study was conducted of women treated for hyperthyroidism at the Mayo Clinic, Rochester, Minnesota, from 1946 through 1964. Cause of death was determined in 1005 women treated with 131I and in 2141 women surgically treated. No increased risk of total mortality was observed for the 131I-treated women (relative risk = 1.0). There were no increased risks for the major causes of mortality (cancer, cardiovascular-renal disease or cerebrovascular lesions) in the 131I-treated women; in addition, there were no increased risks for site-specific cancer mortality. Several hypotheses are presented to suggest reasons for the lack of an association between 131I exposure and increased cancer mortality.