Visceral Fat Metabolic Indices and Thyroid Function: A Stratified Analysis of Non-Linear Associations and Population Modifiers Using NHANES Data

Background This study was designed to investigate the relationship between visceral fat metabolic score (METS-VF), lipid accumulation product (LAP), visceral adiposity index (VAI) and thyroid function. Methods Utilizing data from the National Health and Nutrition Examination Survey (NHANES) 2007-2012, participants were excluded if they lacked data on METS-VF, LAP, VAI or thyroid function, or were under 18 years of age. Multiple linear regression, smooth curve fitting, and subgroup analyses were performed to determine the independent relationship between lipid accumulation and thyroid function. Results After full covariate adjustment, all three visceral adiposity indices showed significant positive associations with FT3 (LAP: β=0.028, VAI: β=0.024, METS-VF: β=0.026; all P<0.001), FT3/FT4 ratio, TT3, TT4, and TgAb. LAP and VAI demonstrated inverse associations with FT4 (β=-0.218 and −0.183, respectively; both P<0.001), while VAI and METS-VF were positively associated with TSH (β=0.149, P=0.041; β=0.167, P=0.025). Quartile analyses confirmed dose-dependent relationships, with Q4 participants showing elevated FT3, FT3/FT4, TT3, TT4, and reduced FT4 compared to Q1. RCS analyses revealed distinct non-linear patterns: LAP exhibited non-linearity with FT3, TSH, TT3, and TT4 (all P-nonlinear<0.05) but linear inverse associations with FT4. VAI displayed reverse L-shaped curves for FT3, TSH, and TT3 with plateaus at higher levels, while TT4 showed an inverted U-shape. METS-VF demonstrated non-linear increases for FT3 and TT3, linear associations with TSH and TT4, and an inverted U-curve for FT4. Stratified analyses identified age, race, and smoking as consistent modifiers of FT3/FT4 associations across all indices (interaction P<0.05), with stronger effects in younger/older adults, males, White participants, and high-income groups. TT3 and TT4 modification patterns varied by index. Thyroid autoantibodies showed minimal associations across all indices. Conclusion Visceral lipid accumulation is closely associated with thyroid dysfunction, and this association exhibits significant non-linear characteristics, which are modulated by factors such as age, race, and lifestyle habits. These findings provide new perspectives for the early identification and intervention of obesity-related thyroid dysfunction.