Developing Oral LT3 Therapy for Heart Failure - HFrEF

Inclusion Criteria: 1. Men and women aged ≥18 years 2. NYHA Class I, II or III heart failure 3. EF≤40 percent within the past year 4. An implantable cardioverter-defibrillator (ICD) 5. Stable doses of neurohormonal blockade for 30 days 6. TSH and free T4 level within the laboratory reference range and total T3 level \<94 ng/dL Exclusion Criteria: 1. Hypertrophic or restrictive cardiomyopathy 2. Uncorrected severe primary valvular disease 3. Arrhythmia that results in irregular heart rate 4. Inability to perform VO2max exercise testing 5. Severe lung disease, including treatment with oral steroids within past 6 months for exacerbation of obstructive lung disease or use of daytime supplemental oxygen 6. Serum creatinine \> 3.0 mg/dL 7. History of cirrhosis 8. LVAD use 9. Heart failure hospitalization within past month 10. Acute coronary syndrome, coronary intervention or ablation therapy within past 2 months or cardiac surgery, percutaneous repair of a valve or septal defect repair within the past 6 months 11. Taking thyroid extract, LT4, LT3, amiodarone, or medication that affects the absorption or metabolism of thyroid hormone; gastrointestinal conditions that affect the absorption of thyroid hormone 12. If female, current or planned pregnancy within the timeframe of study participation 13. Any medical condition that in the opinion of the investigator, will interfere with the safe completion of the study.