Heart failure (HF) is a major cardiovascular disease with increasing prevalence. Thiamine deficiency is common in HF patients. Previous small studies have shown that thiamine supplementation can improve left ventricular systolic function in HF, but larger clinical studies are lacking. Given the ease of supplementation and the potential benefits in HF, we aim to conduct a pilot randomized controlled trial (RCT) using high dose thiamine supplementation in HF patients. The main goal of this pilot study is to determine the feasibility of recruitment for an RCT of thiamine supplementation.

Inclusion Criteria:

• Age ≥60
• NYHA class II-IV symptoms
• Recent HF-related admission in past 12 months OR NT-proBNP >600ng/L within 60 days of screening
• LVEF ≤45% on 2D/3D echocardiography or radionuclide angiography (RNA) in the past 12 months (on optimal therapy)
• Medically optimized prior to enrolment with angiotensin converting enzyme inhibitor or angiotensin receptor blocker (± neprilysin inhibitor), β-blocker, and/or aldosterone antagonist at target or maximally tolerated doses.
• Patients must be stable on medications without hospitalization in the past month.

Exclusion Criteria:

• Taking >2.5mg/d of thiamine supplement. Allows standard multivitamin. B complex vitamin not allowed due to high thiamine content.
• Unable to swallow study medication. A placebo swallowing test will take place at screening.
• Clinical indication for thiamine supplementation including symptomatic thiamine deficiency (Wernicke's encephalopathy, severe malnutrition, refeeding syndrome) and heavy alcohol use, >15 standard drinks per week in men and >10 standard drinks per week in women.
• End-stage renal disease on dialysis
• Severe mitral valve disease because this impacts the accuracy of speckle tracking analysis on echocardiography.
• Non-English speaking (unable to complete questionnaires).
• Unable to provide written consent.
• Cognitive impairment without a caregiver administering medications.
• Expected survival <1 year due to non-cardiac disease.
• Expected heart transplantation in <6 months (± left ventricular assistive device).
• Allergies to the ingredients of the study medication or placebo