No causal relationship between thyroid function and Parkinson's disease: A bidirectional Mendelian randomization study

Background Parkinson's disease (PD) is the second most prevalent degenerative disease globally. While observational studies have demonstrated a correlation between thyroid function and PD, the causal relationship between these two factors remains uncertain.Methods A bidirectional Mendelian randomization (MR) analysis was performed to explore the causal relationship between thyroid function (free thyroxine [FT4], thyroid-stimulating hormone [TSH], hyperthyroidism, and hypothyroidism) and PD. GWAS summary-level statistics of thyroid function and PD were obtained from publicly available GWAS databases. The inverse variance weighted method was the main MR approach to assess causal associations. In addition, two additional MR methods (MR-Egger regression and weighted median) were performed to supplement the IVW. Furthermore, various sensitivity tests were performed to verify the reliability of the MR findings: (i) Heterogeneity was examined by Cochrane's Q test. (ii) Horizontal pleiotropy was assessed by the MR-Egger intercept test and MR-PRESSO global test. (iii) The robustness of MR results was estimated using the leave-one-out method.Results Various MR results showed that FT4, TSH, hyperthyroidism, and hypothyroidism did not causally affect PD (P > 0.05). Likewise, PD did not causally affect FT4, TSH, hyperthyroidism, and hypothyroidism (P > 0.05). Cochrane's Q test indicated that MR analysis was not affected by significant heterogeneity (P > 0.05). MR-Egger intercept test and MR-PRESSO global test indicated that MR analysis was not affected by a remarkable horizontal pleiotropy (P > 0.05). The leave-one-out method demonstrated the stability of MR results.Conclusion MR analysis did not support a causal relationship between thyroid function and PD.