Subclinical hypothyroidism (SCH) is defined as a serum thyroid-stimulating hormone (TSH) level above the upper limit of normal despite normal levels of serum free thyroxine. There is growing evidence that SCH is associated with lipid abnormalities, increasing cardiovascular risk. The cross-sectional Rotterdam study showed an association of SCH with myocardial infarction.Objectives: The aim of this study was to assess prevalence of subclinical hypothyroidism among patients with acute coronary syndrome and its correlation with both in-hospital morbidity and mortality. We evaluated free T3, free T4 and TSH levels of 100 patients (mean age 55.7 years) who were admitted to the coronary intensive care unit between April 2012 to March 2013 with the diagnosis of acute coronary syndrome (ACS) either ST elevation (STEMI) or non-ST elevation acute myocardial infarction (NSTEMI) or unstable angina (UA) together with 50 apparently healthy individuals (mean age 53.2 years) matched for age and sex (control group). Our study showed statistically important difference in prevalence of SCH between both control and ACS group (it was 16% in control group Vs. 5% in the ACS group with p value 0.02). We have compared both morbidity and mortality in both patients who have normal thyroid profile (euthyroid) vs. who have subclinical hypothyroidism and we did not find a statistically important difference between both (p value for both morbidity and mortality was more than 0.05).Morbidity was 34.6% in ACS patients with normal thyroid profile (euthyroid) vs. 20% in those SCH (p value 0.7). Mortality was 2.5% in ACS patients with normal thyroid profile (euthyroid) vs. 0% in those with SCH (p value more than 0.05). Mean free T3 was significantly lower in the morbidity group of ACS (mean free T3 value was 3.2 pg/ml in the morbidity group vs. 3.68 pg/ml in the non morbidity group with p value 0.03). Mean free T3 was significantly lower in the morbidity group of ACS (mean free T3 value was 3.2 pg/ml in the morbidity group vs. 3.68 pg/ml in the non morbidity group with P value 0.03). In the ROC analysis, a level of FT3 ≤ 3.26 pmol/L was found to predict morbidity in ACS with 64% sensitivity and 53% specificity (AUC: 0.664, 95% CI: 0.553-0.776, P value 0.007).Mean free T3 value was significantly lower in patients presented by (ST elevation ACS) vs. those presented by (non ST elevation ACS) (either UA or NSTEMI). (Mean free T3 value was 3.1 pg/ml and 3.7 pg/ml, respectively, P value 0.03).Mean free T3 value was significantly lower in patients presented by myocardial infarction (MI) vs. those presented by UA. (Mean free T3 value was 3.2 pg/ml and 4 pg/ml respectively, P value 0.001).Mean TSH was significantly lower in the mortality group of ACS (mean TSH value was 0.6 UI/ml in the mortality group vs.2.9 UI/ml in the non-mortality group with p value 0.005). Mean free T4 was significantly higher in the mortality group of ACS. (Mean free T4 value was1.98 ng/dl in the mortality group vs.1.4 ng/dl in the non- mortality group with p value 0.03). This study suggest that no association between SCH and in-hospital morbidity and mortality in ACS patients. Abnormally low TSH was associated with in-hospital mortality in ACSHigh FT4 value is associated with increased mortality in patients with ACS. Low free T3 conferred an adverse prognosis in patients with CAD as evidenced by significantly lower mean free T3 in the morbidity group of ACS when compared to the non morbidity group, Patients presented by ST elevation ACS when compared to those presented by non ST elevation ACS (either UA or NSTEMI), Patients presented by myocardial infarction when compared to those presented by unstable angina.