Cancer Risks of Patients with Graves' Disease Who Received Antithyroid Drugs as Initial Treatment: A Nationwide Population-Based Analysis.

Background: Population-based studies that examine the associations between hyperthyroidism and cancer risk have yielded inconsistent results. It remains unclear whether the risks of different cancers increase in patients with Graves' disease (GD) who received antithyroid drugs (ATDs) as initial treatment. We aimed to determine whether cancer risk increases in patients with GD, compared with controls. Methods: This nationwide retrospective cohort study utilized data from the National Health Information Database of South Korea. We included 29,502 patients aged >20 years with GD, who received ATDs as initial treatment, and 57,173 age- and sex-matched controls. The primary outcome was the incidence of various types of cancers. Hazard ratios (HRs) with confidence intervals (CIs) for cancer risk were estimated using Cox proportional hazards models. We also analyzed HR by follow-up period since the diagnosis of GD, accounting for surveillance effect. Results: The risk of biliary tract and pancreatic cancers (HR: 1.41, CI: 1.24-1.60), thyroid cancer (HR: 15.51, CI: 12.29-19.57), prostate cancer (HR: 1.48, CI: 1.28-1.71), and ovarian cancer (HR: 1.31, CI: 1.13-1.52) was elevated in the GD group than in the control group even after the first year of follow-up was excluded. The increased risk of these cancers persisted after a follow-up period of more than 5 years. The risk of thyroid cancer in patients with GD was higher during the initial follow-up period (1 to <2 years) (HR: 19.35, CI: 7.66-48.87) compared with that in the follow-up period exceeding 2 years. The cancer risk estimates remained significant after excluding patients with GD who underwent subsequent radioactive iodine therapy. Conclusion: In this large-scale population-based study, GD was associated with increased risks of biliary tract and pancreatic, prostate, ovarian, and thyroid cancers. The increased risk of thyroid cancer, particularly during the initial follow-up period, may be a surveillance effect.