Association Between Thyroid Diseases and Parkinson's Disease: A Nested Case-Control Study Using a National Health Screening Cohort.

BACKGROUND Although the dopaminergic system is interconnected with the hypothalamic-pituitary-thyroid axis, few studies have explained the causal relationship between thyroid disease and Parkinson's disease (PD). OBJECTIVE The goal of this study was to investigate the association between thyroid diseases and PD in Korean residents. METHODS The Korean National Health Insurance Service-National Sample Cohort, which includes individuals aged ≥40 years, was assessed from 2002 to 2015. A total of 5,586 PD patients were matched by age, sex, income, and the region of residence with 22,344 control participants at a ratio of 1:4. In the PD and control groups, previous histories of levothyroxine treatment, goiter, hypothyroidism, thyroiditis, and hyperthyroidism were investigated. RESULTS The rates of levothyroxine treatment for more than 3 months, hypothyroidism, and hyperthyroidism were higher in the PD group than the control group (3.2%, 3.8%, and 2.8% vs. 2.5%, 2.9%, and 1.9%, respectively, p <  0.05). The adjusted odds ratios (ORs) in model 2, which was adjusted for all potential confounders, for hypothyroidism and hyperthyroidism in the PD group were 1.25 (95% confidence interval (CI) 1.01-1.55, p = 0.044) and 1.37 (95% CI 1.13-1.67, p = 0.002), respectively. In subgroup analyses, the association between hypothyroidism and PD was maintained in men older than 70 years and the association between hyperthyroidism and PD was maintained in women younger than 70 years. CONCLUSION Both hyperthyroidism and hypothyroidism were associated with higher risk of PD, particularly for women younger than 70 years and men older than 70 years, respectively.