Graves' disease and the risk of Parkinson's disease: a Korean population-based study.

Two European cohort studies have suggested that Graves' disease is associated with the development of Parkinson's disease, although the results were limited and controversial. We evaluated whether patients with Graves' disease had an increased risk of developing Parkinson's disease according to treatment modality. We included 65 380 Graves' disease patients and 326 900 healthy controls matched according to age and sex, using the Korean National Health Insurance database. The primary outcome was the incidences of Parkinson's disease amongst Graves' disease patients and controls. Subgroup analyses of Graves' disease patients were performed according to anti-thyroid drug treatment, radioactive iodine therapy and surgery. The cumulative dose and duration values of anti-thyroid drug were calculated for each patient and categorized into highest, middle and lowest tertiles. Amongst 65 380 Graves' disease patients, 301 Parkinson's disease cases were diagnosed during 453 654 person-years of follow-up. Relative to the controls, and regardless of age, sex or comorbidities, the Graves' disease patients had a 33% higher risk of developing Parkinson's disease (hazard ratio: 1.33, 95% confidence interval: 1.17-1.51). Most Graves' disease patients (96%) had received medical therapy, and increased risks of Parkinson's disease were observed in the various subgroups for cumulative dose and treatment duration. This study revealed that Graves' disease was an independent risk factor for developing Parkinson's disease, and that the risk remained elevated regardless of demographic factors or treatment duration/dosage of the anti-thyroid drug. Clinicians should be aware that Graves' disease patients have an increased risk of developing Parkinson's disease, even though Graves' disease patients are often relatively young.