Object To explore the relationship between free triiodothyronine (FT₃) level and left ventricular diastolic dysfunction(LVDD) in elderly male patients with type 2 diabetes mellitus. Methods A total of 208 elderly male (≥60 years) hospitalized patients with T2DM in Ruijin Hospital Shanghai Jiao Tong University School of Medicine were enrolled from January 2020 to November 2021. According to the Echocardiographic Assessment of Heart Disease for Systolic and Diastolic Function 2020, patients were divided into LVDD group (112 cases) and normal left ventricular fuction (control group, 96 cases). Baseline data, echocardiographic indicators, thyroid hormone indicators and thyroid hormone levels between 2 groups were compared. Meanwhile, all the patients were also divided into 4 groups according to the quartile level of FT₃: F1 group (2.43 pmol/L<FT₃≤3.30 pmol/L, n=52); F2 group (3.31 pmol/L<FT₃≤3.78 pmol/L, n=52); F3 group(3.78 pmol/L<FT₃≤4.65 pmol/L, n=52) and F4 group (4.66 pmol/L<FT₃≤6.01 pmol/L, n=52). Baseline data and echocardiographic indicators were compared between the four groups. Pearson correlation was used to analyze the correlation between FT₃ and echocardiographic indicators. Multivariate logistic regression analysis was used to explore the relationship between FT₃ level and LVDD. Receiver operating characteristic (ROC) curve was used to evaluate the efficacy of FT₃ and multivariate logistic regression model for diagnosing LVDD. Results Compared with the control group, the level of FT₃ were significantly lower in LVDD group [（3.51±0.24）pmol/L vs (4.72±0.33) pmol/L, P<0.01]. Echocardiographic indicators showed that the percentage of LVDD, ratio of peak flow velocity E peak in early diastole in mitral orifice and peak flow velocity E peak in early diastole in mitral annulus (E/e’) were significantly different (P<0.01) in the four groups by the quartile level of FT₃. Pearson correlation analysis showed that FT₃ was negatively correlated with E/e’ (P<0.001) and left atrial volume index (P<0.001). Multivariate logistic regression analysis showed that as the quartile level of FT₃ decreased, the risk of LVDD gradually increased (F3: OR=2.29; F2: OR=3.36; F1: OR=4.95; all P<0.05). Age, glycosylated hemoglobin (HbA1c), duration of diabetes mellitus and FT₃ were entered into the logistic regression analysis model. ROC results showed the AUC of FT₃ and multivariate logistic regression model for diagnosing LVDD were 0.742 and 0.858,respectively.When cutoff of FT₃ was taken as <3.37 pmol/L,the sensitivity and specificity for diagnosing LVDD were 66.2% and 75.0%,respectively. Conclusions The risk of LVDD increases as FT₃ level decreases in elderly male patients with T2DM, and FT₃ may serve as a biomarker for early diagnosing LVDD.