OBJECTIVE: To investigate the association between low free triiodothyronine 
(fT3) levels and the severity and prognosis of patients with acute myocardial 
infarction.
METHODS: A total of 501 patients with acute myocardial infarctions were enrolled 
in our study. The circulating levels of thyroid hormones and clinical parameters 
were assayed. The patients were categorized into either the low fT3 group or the 
normal fT3 group according to the fT3 level on admission. All patients underwent 
a follow-up for 10±2 months for mortality from any cause and the occurrence of 
any adverse major cardiac events (MACE).
RESULTS: There were 171 patients in the low fT3 group (fT3<3.5 pmol/L) and 330 
patients in the normal fT3 group (≥3.5 pmol/L). During the follow-up period, 33 
patients died (6.6%) and the overall survival rates were 86.0% and 97.3% in 
patients with a low fT3 level and a normal fT3 level, respectively. The rates of 
MACE were 66.7% and 45.5% in the patients with and those without low fT3 levels, 
respectively. Using a multivariable Cox proportional hazards model, the fT3 
level was found to be the most important predictor of cumulative death and MACE 
(hazard ratio [HR] for death: 0.142, p<0.001 and HR for major adverse cardiac 
events: 0.748, p=0.007). A Kaplan-Meier analysis revealed that those patients 
with low fT3 levels had higher rates of MACE and death.
CONCLUSION: A low fT3 level, a common phenomenon in patients with acute 
myocardial infarctions, is a strong predictor of short-term and long-term poor 
prognoses in patients with acute myocardial infarctions.