Introduction: Subclinical hypothyroidism (SCH) is defined biochemically as a normal serum free thyroxine (T4) level in the presence of an increased serum thyroid stimulating hormone (TSH) concentration.(1) Its prevalence ranges from 4 to 15 percent and is higher in females and increasing age.(2) Overt hypothyroidism was associated with accelerated atherosclerosis and an increased risk of cardiovascular abnormalities. (3) Some studies have reported a higher atherosclerotic cardiovascular disease risk in patients with SCH. (5-8) Elevated TSH levels were observed to be associated with higher cholesterol levels.(9) Higher mortality was also reported in some studies (6,10) especially with TSH ≥ 10.0 mIU/L, in contrast to other studies.(11,12) Heart failure events and myocardial infarction have been reported to be higher.(13,14) These findings in SCH patients could be explained by mitochondrial oxidative stress due to elevated inflammatory markers, hypercoagulability, endothelial dysfunction, insulin resistance, increased vascular resistance and left ventricular diastolic and systolic dysfunction.(3,15,16) As is the case with overt hypothyroidism, SCH was observed to be associated with elevated peripheral vascular resistance and diastolic dysfunction.(17) There are a few studies evaluating the effects of subclinical hypothyroidism on the outcomes of acute coronary syndrome patients.