Background Previous studies have suggested that hypothyroidism correlated with coronary heart diseases (CHD) mortality in long-term cohort,but whether the thyroid function status is associated with myocardial injury in acute ST-elevation myocardial infarction (STEMI) has not been investigated sufficiently.Methods Five hundred and eighty-two hospitalized patients from January 2010 to December 2011,with the diagnosis of STEMI,were enrolled in this study.All patients underwent testing for thyroid function status,cardiac troponin I (cTnl),cardiac enzymes,C-reactive protein (CRP).We investigated the association between thyroid hormone levels and cardiac markers (creatine kinase-MB and cTnl),and thus evaluated the potential role of thyroid function status in predicting the myocardial injury.Results There were 76 patients (13.06％)who had hypothyroidism including low-T3-syndrome (34 patients,5.84％),subclinical hypothyroidism (28 patients,4.81％) and clinical hypothyroidism (14 patients,2.41％).After adjusting for conventional risk factors (age,gender,smoking,diabetes mellitus,dyslipidemia,hypertension),free triiodothyronine (FT3) was significantly and negatively correlated with log-CKMB (r=-0.244,P ＜0.001) and log-cTnl (r=0.290,P ＜0.001),indicating that the lower thyroid hormone level correlates with the severer cardiac injury in STEMI patients.FT3 also had a moderate negative correlation with CRP (r=-0.475,P ＜0.001),which might indicate that hypothyroidism may activate the inflammation response.No significant correlation was found between other thyroid parameters (TSH,FT4) and cardiac markers.Conclusions As the lower FT3 level correlates with higher level of cardiac markers and lower left ventricular ejection fraction (LVEF),the hypothyroidism may be a predictor for myocardial injury in STEMI.And these results may warrant further study to investigate whether reversing the hypothyroidism could benefit the STEMI patients.