Objective:To explore the causal relationship between body mass index (BMI) and hypothyroidism using the two-sample Mendelian randomization model.Methods:A large-scale anthropometric genome-wide association study published in the GIANT database was used to select single nucleotide polymorphisms (SNPs) which were statistically significantly associated with BMI as an instrumental variable ( P<5×10 -8, linkage disequilibrium r 2<0.1). The causal relationship between BMI and hypothyroidism was determined by the inverse variance weighted (IVW), weighted median method and the MR-Egger method, respectively. A heterogeneity test, gene pleiotropy test, and sensitivity analysis were performed to evaluate the stability and reliability of the results. Results:A total of 89 SNPs related to BMI were screened out as instrumental variables. IVW analysis suggested that for every standard deviation increase in BMI, the risk of hypothyroidism increased by 0.9% (odd ratio ( OR)=1.009, 95% confidence interval ( CI): 1.006-1.012, P<0.001). Similar results were obtained with the weighted median method ( OR=1.007, 95% CI: 1.002-1.011, P=0.003) and the MR-Egger method ( OR=1.008, 95% CI: 1.001-1.015, P=0.006). The MR-Egger analysis showed that genetic pleiotropy did not bias the results (intercept=0.000 1, P=0.776), the one-by-one exclusion method did not show that a single instrumental variable SNP had a significant impact on the results, and the difference was not statistically significant ( P>0.05). Conclusion:Mendelian randomized analysis showed a positive causal relationship between BMI and hypothyroidism.