Cancer incidence and mortality in patients treated either with RAI or thyroidectomy for hyperthyroidism

Context: Some previous studies have suggested increased cancer risk in hyperthyroid patients treated with radioactive iodine (RAI). It is unclear whether the excess cancer risk is attributable to hyperthyroidism, its treatment, or the shared risk factors of the two diseases. Objective: The objective was to assess cancer morbidity and mortality in hyperthyroid patients treated with either RAI or surgery. Patients: We identified 4334 patients treated surgically for hyperthyroidism in Finland during 1986-2007 from the Hospital Discharge Registry and 1814 patients treated with RAI for hyperthyroidism at Tampere University Hospital. For each patient, three age-and gender-matched controls were chosen. Informationoncancer diagnoses was obtained from the Cancer Registry. The follow-up began 3 months after the treatment and ended at cancer diagnosis, death, emigration, or the common closing date (December 31, 2009). Results: The overall cancer incidence was not increased among the hyperthyroid patients compared to their controls (rate ratio [RR], 1.05:95%confidence interval [CI], 0.96-1.15). However, the risk of cancers of the respiratory tract (RR, 1.46: 95% CI, 1.05-2.02) and the stomach (RR, 1.64: 95% CI, 1.01-2.68) was increased among the patients. The overall cancer mortality did not differ between the patients and the controls (RR, 1.08: 95% CI, 0.94-1.25). The type of treatment did not affect the overall risk of cancer (hazard ratio for RAI vs thyroidectomy, 1.03: 95% CI, 0.86-1.23) or cancer mortality (hazard ratio, 1.04: 95% CI, 0.91-1.21). Conclusions: In this cohort of Finnish patients with hyperthyroidism treated with thyroidectomy or RAI, the overall risk of cancer was not increased, although an increased risk of gastric and respiratory tract cancers was seen in hyperthyroid patients. Based on this large-scale, long-term follow-up study, the increased cancer risk in hyperthyroid patients is attributable to hyperthyroidismandshared risk factors, not the treatment modality.