BACKGROUND: The association between free triiodothyronine (FT3) and long-term prognosis in dilated cardiomyopathy (DCM) patients has not been evaluated. The purpose of this study was to determine whether the level of FT3 could provide prognostic value in patients with DCM. METHODS: Data of consecutive patients diagnosed with DCM were collected from October 2009 to December 2014. FT3 was measured by fluoroimmunoassay. Other biochemical markers, such as free thyroxin (FT4), thyroid-stimulating hormone, red blood cell, hemoglobin, blood urea nitrogen, and serum creatinine, were tested at the same time. Follow-up was performed every 3 months. The primary endpoint was all-cause mortality. Pearson analysis was used to evaluate the correlation of FT3 and other lab metrics with DCM patients' prognosis. The association of long-term mortality in DCM and FT3 was compared using Cox hazards model. RESULTS: Data of 176 patients diagnosed with DCM were collected. Of them, 24 patients missed FT3 values and six patients were lost to follow-up. Altogether, data of 146 patients were analyzed. During the median follow-up time of 79.9 (53.5-159.6) months, nine patients lost, 61 patients died (non-survival group), and 85 patients survived (survival group). FT3 was significantly lower in non-survival group than that in survival group (3.65 ± 0.83 pmol/L vs. 4.36 ± 1.91 pmol/L; P = 0.003). FT3 also showed a significantly positive correlation with red blood cell and hemoglobin, negatively correlated with age, blood urea nitrogen and serum creatinine (P < 0.05), respectively. Patients in the group of lower FT3 levels (FT3 ≤3.49 pmol/L) suffered from a higher risk of all-cause mortality (P for log-rank = 0.001). In multivariate Cox regression analysis, FT3 level was significantly associated with all-cause mortality (hazard ratio: 0.70, 95% confidence interval 0.52-0.95, P for trend = 0.021). CONCLUSION Low levels of FT3 were associated with increased all-cause mortality in patients with DCM.