Background: Radioiodine is used increasingly as first-line treatment for hyperthyroidism, but concerns remain about subsequent risk of cancer, especially in those treated at a young age. We investigated cancer incidence and mortality in patients treated with radioiodine for hyperthyroidism. Methods: We did a population-based study in 7417 patients treated in Birmingham, UK, between 1950 and 1991. We compared details of all cancer diagnoses and deaths in 1971-91 from the UK Office for National Statistics with data on cancer incidence and mortality for England and Wales specific for age, sex, and period. Findings: During 72,073 person-years of follow-up, 634 cancer diagnoses were made, compared with an expected number of 761 (standardised incidence ratio [SIR] 0.83 [95% CI 0.77-0.90]). The relative risk of cancer mortality was also decreased (observed cancer deaths 448, expected 499; standardised mortality ratio [SMR] 0.90 [0.82-0.98]). Incidence of cancers of the pancreas, bronchus, trachea, bladder, and lymphatic and haemopoietic systems was lowered. Mortality from cancers at all these sites was also reduced but findings were significant only for bronchus and trachea. There were significant increases in incidence and mortality for cancers of the small bowel (SIR 4.81 [2.16-10.72], SMR 7.03 [3.16-15.66]) and thyroid (SIR 3.25 [1.69-6.25], SMR 2.78 [1.16-6.67]), although absolute risk of these cancers was small. Interpretation: The decrease in overall cancer incidence and mortality in those treated for hyperthyroidism with radioiodine is reassuring. The absolute risk of cancers of the small bowel and thyroid remain low, but the increased relative risk shows the need for long-term vigilance in those receiving radioiodine.
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