Associations of Free Triiodothyronine With Sarcopenia Among Patients With Type 2 Diabetes: Cross-Sectional and Sex-Stratified Analyses

Abstract

Objective: Sarcopenia is prevalent in Type 2 diabetes mellitus (T2DM). Whether thyroid-related hormones within the euthyroid range can help identify sarcopenia risk remains unclear. This study was aimed at evaluating the association of euthyroid-range thyroid markers with sarcopenia and their utility in risk stratification among adults with T2DM. Methods: We analyzed 1823 adults with T2DM (2019–2023). Sarcopenia was defined per Asian Working Group for Sarcopenia 2019 criteria. Muscle mass was assessed via multifrequency bioelectrical impedance analysis, handgrip strength via dynamometer, and gait speed via 6-m walk test. Multivariate linear and logistic models were adjusted for demographics, diabetes duration, body mass index, nephropathy, glycated hemoglobin, and antidiabetic medications. Discriminatory ability was evaluated using receiver operating characteristic curves, change in the area under the receiver operating characteristic curve (ΔAUROC), and net reclassification indices (net reclassification improvement [NRI]; bootstrapped). Results: Higher free triiodothyronine (FT3) levels correlated with greater muscle mass, handgrip strength, and gait speed. Among thyroid markers, FT3 showed the strongest discrimination for sarcopenia (AUROC = 0.633). The optimal cutoff was 3.62 pmol/L (sensitivity, 85.6% and specificity, 35.7%), although overall performance was modest. Low FT3 independently predicted sarcopenia (odds ratio [OR], 2.26; p = 0.002). The association remained significant in females (OR, 3.29) but not in males (OR, 1.83); no sex interaction was detected. Adding FT3 modestly improved the adjusted model (ΔAUROC, 0.007; NRI significant at 25th percentile risk). Conclusion: FT3 provides modest but superior discrimination than thyroxine or thyroid-stimulating hormone and may support early sarcopenia risk detection in T2DM, particularly at low levels, with a possible sex-specific pattern but no significant interaction.