BackgroundLow T3 which is defined as decreased triiodothyronine (T3) and normal thyroid-stimulating hormone (TSH) and thyroxin (T4) levels is present in many acute diseases and is related to increased mortality. We studied low T3 level's relation to long-term mortality in non-ST-elevation acute coronary syndrome (NSTE-ACS) patients. MethodsT3, T4, and TSH levels of consecutive NSTE-ACS patients were measured. Patients with normal T4 and TSH levels, but low T3 level were defined as low T3 group. Patients with normal T3, T4, and TSH levels were defined as normal group. Clinical and laboratory findings in these two groups were compared. In addition, we examined low T3 level's relation to early and long-term mortality. ResultsMean patient age was 61 13 (67% male) and 31 (11%) patients had low T3 level. Free T3 values were negatively correlated with age, serum creatinine, and brain type natriuretic peptide values at the time of admission (r = -0.452, P < 0.0001; r = -0.255, P < 0.0001; r = -0.544, P < 0.0001, respectively). Mortality at 1 month and 1 year was higher in low T3 group (3% vs. 16%, P = 0.002; 6.4% vs. 23%, P = 0.003, respectively). In multivariate analysis, low T3 was found to be related to mortality at 1 year (OR: 2.6, 95% CI: 1.1-6.5, P = 0.02). In ROC analysis, free T3 had a good area under the curve (AUC) value for mortality at 1 year [AUC: 0.709 (95% CI: 0.619-0.799, SE: 0.0459)]. ConclusionLow T3 is related to increased early and late mortality in NSTE-ACS patients. Free T3 levels may be used to identify NSTE-ACS patients with high mortality risk.