Background: Low plasma triiodothyronine (T 3 ) levels are considered a prognostic predictor of death in heart failure (HF) patients. Aim: To study an association between plasma T 3 levels and several cardiac, neurohormonal, and metabolic markers of HF. Methods: A total of 133 ambulatory HF patients (114 males; mean age 63.2 years) with left ventricular ejection fraction !40% were enrolled. TSH, total tetraiodothyronine (T 4 ) and T 3 , N-terminal pro-brain natriuretic peptide (NT-proBNP), and other cardiac and metabolic parameters were measured. The lowest tertile of T 3 (group 1) was compared against the two upper ones (group 2). Results: In simple logistic regression, the lowest T 3 tertile was associated with more advanced HF disease status: older (age: odds ratio (OR)Z1.05; confidence interval (CI) 95% 1.01-1.09, PZ0.004), lower functional capacity (walking test: ORZ0.996; CI 95% 0.993-0.999, PZ0.008), higher NT-proBNP (ORZ1.64; CI 95% 1.19-2.27, PZ0.003) and adiponectin levels (ORZ1.07; CI 95% 1.02-1.11, PZ0.004), lower DHEAS log-transformed (ORZ0.50; CI 95% 0.31-0.80, PZ0.004), and the presence of lower phase angle values as measured by body bioelectrical impedance analysis (ORZ3.18; CI 95% 1.50-6.71, PZ0.04) and worse renal function (ORZ0.96; CI 95% 0.94-0.98, PZ0.003). T 3 levels in the lowest tertile were independently associated with low phase angle values (ORZ2.95, CI 95% 1.16-7.50, PZ0.02) and the log transformation of DHEAS (ORZ0.56; CI 95% 0.32-0.97, PZ0.04). Conclusion: We have demonstrated an association between plasma T 3 levels in the lower range and other deranged hormonal and metabolic parameters in HF patients.