Abstract: AIM:To determine the morbidity of low triiodothyronine syndrome in patients with acute heart failure (AHF) and to evaluate the effect of low triiodothyronine syndrome on disease severity and prognosis. METHODS: Thyroid hormone levels, APACHE II scores and BNP levels were assessed in 240 consecutive patients hospitalized for AHF. Mortality during hospitalization was calculated and survival 6 months after discharge was followed-up by telephone inquiry. RESULTS: Seventy (29.2%) of the 240 patients with AHF had low thyroid hormone syndrome and the mortality rate during hospitalization and after discharge in this group was higher than in the normal T3 group (17.1% vs. 7.6%, P<0.05; 11.3% vs. 3.4%, P<0.05). APACHE II scores and BNP levels were higher in low thyroid hormone syndrome group compared with those in normal T3 group ［24±6 vs. 20±6; (784±188) mg/L vs.(697±200) mg/L］. In a multivariate Cox proportional hazards model, FT3 levels, APACHE II scores and BNP levels were independent predictors of early mortality in patients in AHF (HR=0.683, 95%CI: 0.536-0.847, P<0.05; HR=1.257, 95%CI: 1.317-1.864, P<0.05; HR=0.997, 95%CI: 0.922-1, P<0.05). Area under the curve for APACHE II scores was higher than for FT3 and BNP levels. However, there was no significant difference in area under the curve between FT3 and BNP levels (0.893, 95%CI: 0.831-0.965; 0.720, 95%CI: 0.640-0.806; 0.698, 95%CI: 0.586-0.751). CONCLUSION: Diagnosis of AHF complicated with low T3 syndrome is important in evaluating disease severity and predicting the prognosis.