Low triiodothyronine (T3) syndrome has recently been evaluated as a prognostic marker of acute heart failure (AHF). However, in which cases low T3 syndrome typically leads to adverse outcomes remain unclear. Of 1,432 AHF patients screened, 1,190 were enrolled. Euthyroidism was present in 956 patients (80.3%), who were divided into 2 groups: the normal group (n = 445, FT3 ≥1.88 µIU/L) and low-FT3 group (n = 511, FT3 <1.88 µIU/L). The survival rates and event-free rates within 365 days were significantly lower in the low-FT3 group than in the normal group. A multivariate Cox regression model showed that the low-FT3 group was an independent predictor of 365-day mortality (hazard ratio [HR] 1.429, 95% confidence interval [CI] 1.013 to 2.015) and HF events (HR 1.349, 95% CI 1.047 to 1.739). The multivariate logistic regression analysis revealed that age (per 10-year old increase, odds ratio [OR]: 1.186, 95% CI: 1.046 to 1.345) and prognostic nutritional index (PNI; per 1-point increase, OR: 1.067, 95% CI: 1.046 to 1.089) were independently associated with the low-FT3 group. The prognosis in patients with a low PNI and over 75 years old, including all-cause death within 365 days, was significantly poorer in the low-FT3 group than in the normal group. In conclusion, adverse outcomes were predicted by the presence of low T3. AHF patients with low T3 syndrome are strongly associated with aging and malnutrition. Low T3 syndrome complicated with older age and malnutrition is likely to lead to adverse outcomes in patients with AHF.
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