The goal of this clinical trial is to find out whether levothyroxine reduces complications and improves the prognosis of patients with subclinical hypothyroidism (SCH) undergoing coronary artery bypass grafting (CABG). The main questions this study aims to answer are: • Does levothyroxine reduce major adverse cardiovascular events (MACE), including cardiovascular death, nonfatal myocardial infarction, nonfatal stroke, and new arrhythmias, in SCH patients undergoing CABG? Researchers will compare patients who take levothyroxine with those who do not. Participants will: * Be randomly assigned to either receive levothyroxine or not * Start the medication one day before surgery if assigned to the treatment group. * Have regular follow-up visits to check thyroid function and cardiovascular events for up to five years after surgery.