Based on accumulating evidences showing that hypothyroid status is associated with poor prognosis among acute myocardial infarction (AMI) patients, the study is designed to evaluate whether replacement treatment with levothyroxine could have beneficial effects on patients with AMI and hypothyroidism. This is a multicenter prospective computerized-randomized trial stratified by ejection fraction with a 1:1 ratio to levothyroxine group or standard therapy group.

The primary hypothesis is that reversing the hypothyroid status with levothyroxine on top of standard therapy for AMI is safe and has beneficial effects on cardiac function, myocardial perfusion/metabolism and myocardial fibrosis.

Inclusion Criteria:

1. Age 18 - 75, male or non-pregnant female;
2. Is diagnosed acute myocardial infarction and received primary percutaneous coronary intervention (PCI);
3. With presence of hypothyroid status (i.e., TSH >7.0 mIU/L and (or) fT3 <1.79 pg/mL);
4. Is able to understand the objective of the trial, takes part voluntarily and signs the written informed consent form.

Exclusion Criteria:

1. Those who have participated in other drug or therapeutic equipment clinical trials but do not reach the main study endpoint time limit;
2. Symptoms of severe heart failure (Killip Class III and above);
3. Severely impaired renal function before surgery: serum creatinine > 2.0mg/dl;
4. Impaired liver function before surgery: Serum GPT > 120U/L;
5. Those who have prior thyroid diseases and already on levothyroxine or anti-thyroid medicines;
6. Those taking medicine which can affect the test of thyroid function;
7. Patients who plan to undergo coronary artery bypass grafting or other surgery within 3 months;
8. Those having prior myocardial infarction;
9. Patients who are deemed by the researchers to have low compliance and unable to abide by the requirements and complete the study.