Thyroid autoimmunity and metabolic syndrome: a nationwide population-based study

Objective: Recent studies have reported that thyroid hormone levels are associated with metabolic syndrome (MetS) even in euthyroid subjects. However, the association between thyroid autoimmunity and MetS is uncertain. This study aimed to investigate the relationship between thyroid autoimmunity and MetS in a large cohort study of euthyroid subjects. Methods: A total of 4775 participants aged >= 19 years from the Korea National Health and Nutrition Examination Survey VI (2013-2015) with anti-thyroid peroxidase antibody (TPOAb) results and normal thyroid functions were included in this study. Subjects were grouped according to thyroid autoimmunity (positivity of TPOAb). We estimated the odds ratios (ORs) for MetS according to TPOAb positivity using logistic regression models, adjusted for potential confounders. Results: Of the study subjects, 25% (n = 1206) were diagnosed with MetS. Subjects with MetS showed higher median TPOAb levels (6.3 vs 6.8 IU/mL, P < 0.001) and higher positivity of TPOAb (5 vs 7%, P = 0.002) than those without MetS. There was a significant difference in prevalence of MetS depending on the TPOAb positivity (25% vs 33%, P = 0.002). Subjects with TPOAb positive had a significantly greater risk of abdominal obesity (OR 1.675, 95% CI: 1.302-2.154, P < 0.001), low high-density lipoprotein cholesterol (OR: 1.603, 95% CI: 1.244-2.066, P < 0.001) and elevated blood pressure (OR: 1.418, 95% CI: 1.099-1.829, P = 0.007) as compared to those with TPOAb negative. Positivity of TPOAb was a significant risk factor for MetS even after adjusting for confounding variables including age, sex, household income, education, smoking, alcohol consumption, walking activity, thyroid-stimulating hormone and free thyroxine (OR: 1.389, 95% CI: 1.048-1.841, P = 0.022). Conclusion: In euthyroid subjects, thyroid autoimmunity is associated with MetS. Further large longitudinal studies are needed to clarify causality.