There was a significant association of MTHFR A1298C with hypothyroidism in subgroup analyses stratified by thyroid function (C vs. A/allele model: OR = 0.63, 95% CI: 0.44-0.92, P = 0.02; CC vs. AC+AA/recessive model: OR = 0.42, 95% CI: 0.22-0.79, P = 0.007; CC vs. AA/homozygote model: OR = 0.43, 95% CI: 0.25-0.85, P = 0.02) ( Figure 3B and Supplemental Figures ), but no relationship was found between MTHFR A1298C and hyperthyroidism ( Table 3 ). Here, MTHFR is linked to hyperthyroidism.