Ethnicity-Specific Thyroid Hormone Sensitivity Indices and Their Association With Metabolic Syndrome in Euthyroid South Asian Adults

Background: Metabolic syndrome (MetS) is a combination of interrelated cardiometabolic abnormalities that include abdominal obesity, dyslipidemia, high blood pressure, and insufficient glucose control. Recent studies indicate that sensitivity to altered thyroid hormone, even in euthyroid subjects, can be a contributor to metabolic risk. The present study assessed the relationship between thyroid hormone sensitivity indices, namely the Free Triiodothyronine/Free Thyroxine (FT3/FT4) ratio, the Thyroid-Stimulating Hormone Index (TSHI), the Thyrotropin T4 Resistance Index (TT4RI), and MetS among euthyroid South Asians. Methods: This cross-sectional observational study included 553 euthyroid adults aged 18 to 60 years, evaluated using demographic, anthropometric, and biochemical measurements. We calculated thyroid hormone sensitivity indices (FT3/FT4 ratio, TSHI, and TT4RI) using standard formulas, and defined prevalent MetS according to the IDF South Asian criteria. We conducted Spearman correlation analyses, group comparisons, ROC analysis, and binary logistic regression. Results: Metabolic syndrome was present in 115 (20.8%) participants. TSHI and TT4RI were significantly correlated with several metabolic components, including waist circumference, blood pressure, triglycerides, fasting glucose, and BMI, and were negatively correlated with HDL cholesterol (p<0.001). TT4RI was significantly associated with prevalent MetS after adjustment for relevant covariates (OR=1.18, 95% CI: 1.02-1.37, p=0.027). Both indices showed modest discriminatory performance. Conclusion: MetS was associated with decreased thyroid hormone sensitivity, particularly higher TT4RI levels, in euthyroid South Asian adults. TSHI and TT4RI may serve as complementary markers associated with prevalent metabolic syndrome; however, these findings should be interpreted cautiously and require validation in prospective longitudinal studies.