Thyroid Hormone Resistance Indices and Their Correlations With Insulin Resistance in Chinese Euthyroid Subjects

<b>Aim:</b> This study aimed to analyze the relationship between sensitivity to thyroid hormones and insulin resistance (IR) in people with different levels of body mass index (BMI). <b>Methods:</b> We included 32,478 euthyroid participants, and they were divided into three groups according to body mass index (BMI): normal weight (<i>n</i> = 20,200), overweight (<i>n</i> = 10,178), and obesity (<i>n</i> = 2100). We used the Thyroid Feedback Quantile-based Index (TFQI), the TSH index (TSHI), and the Thyrotroph T4 Resistance Index (TT4RI) to represent thyroid hormones sensitivity and used Triglyceride-Glucose (TyG) Index, the Atherogenic Index of Plasma (AIP), and The Metabolic Score for Insulin Resistance (METS-IR) to represent IR. <b>Results:</b> In the BMI < 25 kg/m² group, linear regression showed that TFQI, TSHI, and TT4RI were all positively correlated with TyG (<i>β</i> = 0.096, 0.089, and 0.089, respectively, all <i>p</i> < 0.001). Logistic regression showed that the odds ratios (ORs) for having high TyG in the highest quartiles of the TFQI, TSHI, and TT4RI were 1.430, 1.537, and 1.518, respectively (all <i>p</i> < 0.001), compared with the lowest quartiles of the TFQI, TSHI, and TT4RI. In the 25 ≤ BMI < 29.9 kg/m² group, TFQI, TSHI, and TT4RI were positively associated with TyG (<i>β</i> = 0.071, 0.067, and 0.066, respectively, all <i>p</i> < 0.001) according to linear regression. The ORs for having high TyG in the highest quartiles of the TFQI, TSHI, and TT4RI were 1.269, 1.363, and 1.353, respectively (all <i>p</i> < 0.001) by logistic regression. In the BMI ≥ 30 kg/m² group, positive correlations were found between the three thyroid indices and TyG (<i>β</i> _TFQI = 0.097, <i>β</i> _TSHI = 0.084, <i>β</i> _TT4RI = 0.083, all <i>p</i> < 0.01) by linear regression. The ORs for having high TyG in the highest quartiles of the TFQI, TSHI, and TT4RI were 1.298 (<i>p</i>=0.101), 1.454 (<i>p</i>=0.05), and 1.455 (<i>p</i>=0.05), respectively. <b>Conclusions:</b> Reduced sensitivity to thyroid hormones was associated with high levels of IR in euthyroid adults of normal weight and overweight. In obese patients, no significant correlation was found between sensitivity to thyroid hormones and IR.