Background: Epidemiological evidence hints at a connection between thyroid dysfunction and chronic spontaneous urticaria (CSU), but the causality is obscure. This study applies a two-sample Mendelian randomization (MR) method to explore the causality between them. Methods: We utilized bidirectional MR to explore the causality between CSU and thyroid autoimmunity in Europeans and East Asians. The SNPs examined in this study were sourced from the Open GWAS database, the ThyroidOmics Consortium, the FinnGen Consortium, and BioBank Japan. In the forward MR analysis, we considered thyroid dysfunction, AITD, and thyroid function indicators—FT3, FT4, TT3, ln (FT3/FT4), ln (TT3/FT4), TSH and TPOab—as exposures, with CSU as the outcome in a cohort of 398,763 individuals. Conversely, in the reverse MR analysis, CSU served as the exposure, and thyroid function phenotypes were the outcomes. The primary analytical method was IVW, complemented by sensitivity analyses to evaluate pleiotropy and robustness. Results: IVW analysis indicates that genetic predisposition to thyroid disorders is linked to increase CSU risk in Europeans, with significant odds ratios noted for Graves’ disease, hyperthyroidism, and Hashimoto’s disease. Reverse MR suggests possible causal links between a genetic propensity for CSU and a decrease in hyperthyroidism and the ln (FT3/FT4) ratio. No evidence was found linking thyroid function phenotypes with CSU in East Asians. Conclusion: Our research indicates that genetic susceptibilities to thyroid disorders are associated with a higher risk of CSU in European populations, with no observed link in East Asian populations. These findings may offer new avenues for future investigation into the relationship between thyroid autoimmunity and CSU.