[Correlation analysis between serum free triiodothyronine and C-peptide-based insulin resistance index in euthyroid...

Objective: To explore the correlation between serum free triiodothyronine (FT3) and C-peptide-based insulin resistance index (HOMA2 IR-CP) in euthyroid adults. Methods: A cross-sectional study. The clinical data of euthyroid adult participants who underwent physical examination in the Second Medical Center of Chinese PLA General Hospital from January to December in 2019 were retrospectively analyzed. According to the HOMA2 IR-CP level, the participants were divided into HOMA2 IR-CP>2.18 group (n=3 463) and HOMA2 IR-CP≤2.18 group (n=8 204). Univariate Pearson correlation analysis and multivariate logistic regression analysis were used to analyze the correlation between FT3 and HOMA2 IR-CP. The interaction model was used to analyze the interaction between FT3 and related factors, and the dose-response relationship between continuity variable FT3 and HOMA2 IR-CP was explored by using restricted cubic spline plots. Results: A total of 11 667 euthyroid adult participants aged (50.7±10.0)years were recruited according to the inclusion and exclusion criteria, with 7 756 males and 3 911 females. The proportion of males, body mass index, systolic blood pressure, glycated hemoglobin A1c, fasting plasma glucose, triglyceride, hemoglobin, alanine aminotransferase and FT3 levels in HOMA2 IR-CP>2.18 group were significantly higher than those in HOMA2 IR-CP≤2.18 group (all P<0.05). The levels of low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, and free thyroxine in HOMA2 IR-CP>2.18 group were lower than those in HOMA2 IR-CP≤2.18 group (all P<0.001). Univariate Pearson correlation analysis showed that FT3 was associated with HOMA2 IR-CP (r=0.21, P<0.001). Multivariate logistic regression analysis suggested an association between FT3 and HOMA2 IR-CP after adjusting for confounding factors(P_{for trend}<0.001). Subgroup analysis showed an association between FT3 and HOMA2 IR-CP in different subgroups of gender, age and glucose metabolism status (P_{for trend}<0.05). Multiplication interaction analysis suggested that there was an interaction between FT3 and age (P_interaction<0.001). Restricted cubic spline model analysis demonstrated that the correlation between FT3 and HOMA2 IR-CP was linear (P_overall<0.001, P_nonlinear=0.479). Conclusions: There is a correlation between serum FT3 and HOMA2 IR-CP in euthyroid adults. With the increase of FT3 level, insulin resistance increases gradually.