Association Between Thyroid Function Indicators and Metabolic‐Associated Fatty Liver Disease: Effect Modification by Iodine Nutritional Status

Background The relationship between thyroid function indicators and metabolic dysfunction‐associated fatty liver disease (MAFLD) has yielded contradictory results. This study aimed to explore the association between thyroid function indicators and MAFLD, as well as the modifying effect of iodine nutritional status. Methods This cross‐sectional analysis utilized nationally representative data from the 2009–2012 National Health and Nutrition Examination Survey (NHANES). Different multivariate logistic regression models were constructed to investigate the association between thyroid function indicators and MAFLD. Mediation analyses were performed to explore the mediating roles of MAFLD diagnosis‐related variables. Lastly, we analyzed the relationship between thyroid function indicators and MAFLD across different iodine nutritional statuses. Results After adjusting for covariates unrelated to MAFLD diagnosis, elevated FT3 and FT3/FT4 and decreased FT4 levels increased the odds of MAFLD across three adjusted models (all p < 0.05). Besides, FT3 and FT3/FT4 may be associated with MAFLD through potential pathways involving ALT levels, HDL_C levels, or BMI ( p for indirect effect < 0.05). Moreover, the association between thyroid function and MAFLD may vary depending on iodine nutritional status. Specifically, FT4 exhibited an inverse relationship under iodine‐deficient conditions, whereas elevated FT3 and FT3/FT4 levels were positively associated with MAFLD in iodine‐adequate populations. Under iodine‐excessive conditions, TSH and FT3/FT4 showed significant involvement in MAFLD ( p < 0.05). Conclusion Thyroid function indicators are associated with MAFLD. This association may potentially be mediated by ALT, HDL_C, and BMI and appears to vary depending on iodine nutritional status, though this modifying effect warrants further validation.