Predictive Effect of FT3 Within the Euthyroid Range on LDL-C in Patients With Type 2 Diabetes: A Cross-Sectional Analysis of Inpatients in China

Evidence regarding the relationship between free triiodothyronine (FT3) and low-density lipoprotein cholesterol (LDL-C) remains limited. This study aimed to evaluate the association between FT3 and LDL-C levels in patients with type 2 diabetes mellitus (T2DM) who exhibit normal thyroid function. Between June 2022 and October 2023, a total of 3011 inpatients with T2DM and euthyroid status were continuously and non-selectively recruited from a Chinese hospital. The average age of the included individuals was 56.92 ± 12.56 years, with 1430 (47.49%) males. The mean FT3 concentration was 4.35 ± 0.56 pmol/L. A logistic regression model was applied to analyse the relationship between the FT3 and LDL-C levels, while smooth curve fitting was employed to investigate potential nonlinear associations between these variables. This study demonstrated a positive correlation (0.05 [95% CI: 0.02–0.07; p = 0.0018]) and nonlinear relationship (p = 0.0014) between FT3 and LDL-C levels in Chinese patients with diabetes. Specifically, when FT3 was below 4.28 pmol/L, LDL-C levels increased alongside rising FT3 concentration. However, when FT3 reached or exceeded 4.28 pmol/L, LDL-C levels plateaued and tended to stabilise. These findings suggest that maintaining FT3 within the range of 2.76 to 4.28 pmol/L may be most beneficial for mitigating the progression of cardiovascular disease in patients with T2DM. Our research is important for identifying the optimal FT3 range to delay the progression of cardiovascular disease in patients with T2DM. These findings provide valuable insights to guide clinicians in preventing and managing cardiovascular disease in this population.