How does hyperthyroidism cause high output cardiac failure?

From the Guidelines

Hyperthyroidism causes high output cardiac failure through several interconnected mechanisms related to excess thyroid hormone, leading to increased cardiac output, cardiac remodeling, and eventually heart failure. The condition is characterized by decreased systemic vascular resistance, increased resting heart rate and left ventricular (LV) contractility, and enhanced isovolumic ventricular relaxation, as noted in the study by Bozkurt et al 1. This results in a decreased afterload and an increased preload, with cardiac output increasing by up to 300% from a euthyroid state. The increased release of erythropoietin caused by hyperthyroidism also contributes to the circulating blood volume, which can increase by up to 25% 1.

Key mechanisms involved in the development of high output cardiac failure in hyperthyroidism include:

• Increased metabolic rate and oxygen demand, leading to peripheral vasodilation and decreased systemic vascular resistance
• Direct effects of thyroid hormone on the heart, including increased heart rate, contractility, and blood volume
• Enhanced renin-angiotensin-aldosterone system activity, leading to increased blood volume and cardiac output
• Increased sensitivity of the heart to catecholamines, resulting in tachycardia and palpitations
• Potential development of atrial fibrillation, compromising cardiac filling and reducing cardiac efficiency

According to the American Heart Association scientific statement 1, patients with hyperthyroidism may occasionally have exertional dyspnea or other symptoms and signs of heart failure, despite having high cardiac contractile function. The development of heart failure in these patients is often unexpected and may be related to hyperthyroid cardiomyopathy. Treatment of hyperthyroidism with antithyroid medications and beta-blockers can help manage cardiac symptoms and prevent long-term cardiac damage 1. Controlling the underlying hyperthyroidism is crucial to preventing the progression of high output cardiac failure and improving patient outcomes.

From the Research

Hyperthyroidism and High Output Cardiac Failure

• Hyperthyroidism is characterized by an increase in resting heart rate, blood volume, stroke volume, myocardial contractility, and ejection fraction 2
• The development of "high-output heart failure" in hyperthyroidism may be due to "tachycardia-mediated cardiomyopathy" 2
• Excess thyroid hormone affects cardiovascular hemodynamics, leading to high-output heart failure and, in late stages, dilated cardiomyopathy 3
• Hyperthyroidism can cause cardiac arrhythmias, heart failure, osteoporosis, and adverse pregnancy outcomes if left untreated 4

Mechanisms and Effects

• Thyroid hormones upregulate the expression of the sarcoplasmic reticulum calcium-activated ATPase and downregulate the expression of phospholamban, leading to increased cardiac contractility 2
• Hyperthyroidism increases the risk of cardiovascular heart disease (CHD) mortality and CHD events, particularly in patients with very low levels of thyrotropin 2
• Treatment options for hyperthyroidism, such as antithyroid drugs, radioactive iodine ablation, and surgery, can help alleviate symptoms and prevent complications 4, 5

Treatment and Management

• The combination of methimazole and propranolol is considered an effective treatment regimen for hyperthyroidism, improving heart rate, bone metabolism, and thyroid hormone levels 6
• Treatment choices should be individualized and patient-centered, taking into account the underlying diagnosis, presence of contraindications, severity of hyperthyroidism, and patient preference 4, 5