Objective: To investigate the association between triiodothyronine (T3) and inflammatory factors, and its potential effect on long-term outcomes in hospitalized patients with heart failure (HF). Methods: A total of 2 475 patients with HF admitted in Heart Failure Care Unit were consecutively enrolled in this retrospective cohort study from December 2006 to June 2018. Patients were divided into low T3 syndrome group (n=610, 24.6%) and normal thyroid function group (n=1 865, 75.4%). The median follow-up time was 2.9 (1.0, 5.0) years. A total of 1 048 all-cause deaths were recorded at the final follow-up. The effects of free T3 (FT3) and high-sensitivity C-reactive protein (hsCRP) on the risk of all-cause death were evaluated by Cox regression analysis and Kaplan-Meier analysis. Results: The age of the total population was 19-95 (57±16) years, 1 823 cases (73.7%) were male. Compared to those with normal thyroid function, albumin [(36.5±5.4) vs. (40.7±4.7) g/L], hemoglobin [(129.4±25.1) vs. (140.6±20.6) g/L], total cholesterol [3.6 (3.0, 4.4) vs. 4.2 (3.5, 4.9) mmol/L] (all P<0.001) were lower, Whereas age [(60.5±16.0) vs. (55.2±15.4) years], creatinine [105.0 (83.6, 137.0) vs. 87.8 (75.6, 106.3) mmol/L], log N-terminal B-type natriuretic peptide [(8.2±1.3) vs. (7.2±1.4) ng/L] were higher in LT3S patients (all P<0.001). In Kaplan-Meier survival analysis, patients with lower FT3 and higher hsCRP had significantly lower cumulative survival (P<0.001), lower FT3 combined with higher hsCRP subgroup had the highest risk of all-cause death (Ptrend<0.001). In multivariate Cox regression analysis, LT3S was an independent predictor of all-cause mortality (HR=1.40, 95%CI 1.16-1.69, P<0.001). Conclusion: LT3S is an independent predictor of poor prognosis in patients with heart failure. FT3 combined with hsCRP improve the predictive value of all-cause death in hospitalized patients with heart failure.