Objective:To investigate the relationship between serum 25-hydroxy vitamin D[25(OH)D]deficiency and thyroid autoi-mmunity among middle-aged and elderly population. Methods:A total of 2374 middle-aged and elderly patients were enrolled in this study. Serum 25(OH)D and thyroid autoantibody were determined. The levels of 25(OH)D and the prevalence of 25(OH)D deficiency were compared between the positive thyroid autoantibody group and the negative thyroid autoantibody group. Binary logistic regression was conducted to investigate the relationship between the levels of 25(OH)D and the prevalence of 25(OH)D deficiency and thyroid autoantibody. Results:Females had higher prevalence of 25(OH)D deficiency and positive thyroid autoantibody than males. The serum 25(OH)D level in the positive TPOAb group were significantly lower than those in the negative TPOAb group,and the prevalence of 25(OH)D deficiency(25(OH)D< 50 nmol/L) were significantly higher in the positive thyroid autoantibody group compared with those in the negative thyroid autoantibody group. Binary logistic regression analysis revealed that 25(OH)D< 50 nmol/L was a risk factor for positive TPOAb and positive TPOAb and/or TgAb after controlling for age,gender,body mass index (BMI),serum FT3 and FT4(OR = 1.313 and OR = 1.287,respectively). Conclusion:Patients with positive thyroid autoantibody have a high prevalence of 25(OH)D deficiency,and 25(OH)D< 50 nmol/L was a risk factor for positive thyroid autoantibody.