Introduction
Hyperthyroidism is a common condition that affects females and a small percentage of males. Thyroid hormones play an important role in cardiac function. Hyperthyroidism affects the cardiovascular system resulting in cardiac failure. Hyperthyroidism can cause cardiovascular disease or may complicate pre-existing cardiovascular disease and also increases cardiovascular risk. This article gives insight into the relationship between hyperthyroidism and cardiac diseases.
What Does Hyperthyroidism Mean?
Hyperthyroidism is when the thyroid gland produces and releases excess thyroid hormones. Hyperthyroidism, also called the overactive thyroid, releases high levels of triiodothyronine (T3) and thyroxine (T4). The thyroid hormones are essential for regulating body temperature, heart rate, metabolism, and maintaining homeostasis (a state of balance). Excess or reduced production of thyroid hormones can impact the entire body. Excess production (hyperthyroidism) affects cardiovascular hemodynamics (a study of cardiovascular functions such as cardiac output and arterial pressure), resulting in high-output cardiac failure and dilated cardiomyopathy (a chronic disease of the heart muscle) in later stages. Thyroid dysfunction affects blood pressure, cardiac output, contractility, and vascular resistance and causes abnormal heart rhythms.
What Is the Role of Thyroid Hormones in Cardiac Function?
Thyroid hormones produced in the thyroid gland exert various effects on the cardiovascular and peripheral vascular systems. Thyroid hormones improve the systolic and diastolic functions of the heart, and cardiac contractility, increase the heart rate and decrease systemic vascular resistance. Excess thyroid hormone production increases the risk of atrial fibrillation and heart failure.
What Are the Cardiac Manifestations of Hyperthyroidism?
Hyperthyroidism presents with cardiac and hemodynamic symptoms, including palpitations, breathlessness on exertion, widened pulse pressure, tachycardia (increased heart rate), atrial fibrillation, and exercise intolerance. Excess thyroid hormones also increase the resting heart rate and cardiac contractility. The cardiac output is also increased by 50 to 300 percent. The rise in cardiac output is due to the increase in heart rate, cardiac contractility, and dilation of peripheral blood vessels. The cardiac manifestations of hyperthyroidism can cause severe complications in individuals with underlying cardiac disease. The common cardiac manifestations of hyperthyroidism include the following:
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Tachycardia and Palpitations - Unrecognized hyperthyroidism is the common cause of increased resting heart rate and increased heart rate during exertion.
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Cardiac Arrhythmias - Hyperthyroidism causes abnormal cardiac rhythms such as atrial fibrillation, ventricular tachycardia, and premature ventricular complexes in individuals with underlying cardiac diseases.
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High Blood Pressure - An increase in cardiac contractility increases the systolic blood pressure, or the pressure exerted within the blood vessels during cardiac contraction.
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Angina - Individuals with coronary artery disease experience worsening cardiac symptoms in hyperthyroidism. This can be an increase in angina (chest pain) or a heart attack.
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Shortness of Breath - Breathlessness on exertion can be due to the weakness in skeletal muscles associated with hyperthyroidism.
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Cardiac Failure - Hyperthyroidism can result in heart failure in rare cases. However, the risk is increased in individuals with pre-existing cardiac disease.
Hyperthyroidism and the Heart Rate:
Heart rate is an essential mechanism that regulates cardiac output. Heart rate determines the rate of cardiac ejection and affects systolic and diastolic function. Sinus tachycardia is a common cardiovascular sign of hyperthyroidism. The rapid use of oxygen, increased release of metabolic end products, and the relaxation of smooth muscle fibers in the arteries by the thyroid hormones cause peripheral vasodilation in the peripheral vascular system. This decrease in peripheral vascular resistance plays a significant role in the hemodynamic changes caused by the thyroid hormones. The decrease in peripheral vascular resistance increases the heart rate and causes a decrease in diastolic pressure due to the consequent widening of pulse pressure. An increased heart rate increases cardiac output, thereby increasing the body's metabolic rate.
Hyperthyroidism and Cardiac Arrhythmias:
Cardiac arrhythmias (abnormal heart rhythms), such as sinus tachycardia, atrial fibrillation, and electrocardiogram (ECG) abnormalities, such as shortened PR (ECG waves) and QT (ECG waves), are observed in individuals with hyperthyroidism. Sinus tachycardia is the common rhythm disturbance observed in hyperthyroidism. An atrioventricular blockage may be present in some cases. Atrial fibrillation is the common supraventricular arrhythmia in individuals with thyrotoxicosis.
Hyperthyroidism and Cardiac Contractility:
Cardiac contractility refers to the intrinsic ability of the cardiac muscle to do work. Hyperthyroidism increases cardiac contractility. The term left ventricular (LV) systolic function refers to the aggregate effect of the mechanisms that control the cardiac function, such as heart rate, cardiac contractility, preload, and afterload. The left ventricular systolic function consistently improves at rest in hyperthyroid patients.
Hyperthyroidism and Preload:
Preload refers to the force exerted on the ventricular wall during ventricular filling. It corresponds to the end-diastolic wall stress. Hyperthyroidism impacts the cardiac preload and increases left ventricular performance. Hyperthyroidism activates the renin-angiotensin-aldosterone system and increases the total blood volume and cardiac preload.
Hyperthyroidism and Afterload:
Afterload refers to the force exerted on the ventricular wall during ejection, and it corresponds to the end-systolic wall stress. In hyperthyroid patients, ventricular afterload is increased, but the underlying mechanism is not known. It can be due to an increase in heart rate, which reduces the compliance of arterial trees.
Hyperthyroidism and Heart Failure:
Hyperthyroidism, if left untreated, can result in heart failure. Cardiac arrhythmias increase blood volumes, and cardiac hypertrophy in hyperthyroidism results in cardiac failure. Severe hyperthyroidism can cause high-output cardiac failure. High-output cardiac failure, referred to as congestive heart failure, results in tachycardia-induced cardiomyopathy. In hyperthyroid patients with pre-existing cardiac disorders such as ischemic heart disease, atrial fibrillation, and valvular diseases, the risk of heart failure is increased.
How Are Cardiac Diseases in Hyperthyroidism Managed?
Hyperthyroidism should be treated to prevent the progression of cardiac diseases. The treatment for hyperthyroidism depends on the cause and severity of the disease. The treatment includes antithyroid drugs like Methimazole or Propylthiouracil, radioactive iodine treatment, or surgical removal. The cardiac manifestations are managed with medications like beta-blockers. Beta-blockers help in managing the symptoms like anxiety, tachycardia, tremulousness, and palpitations. The drugs like Furosemide may help in reversing the volume overload.
Conclusion
Hyperthyroidism is a condition in which the levels of thyroid hormones are high. Excess thyroid hormone production affects the cardiovascular system and causes various symptoms, including palpitations, anxiety, increased heart rate, increased cardiac contractility, breathlessness on exertion, angina, and atrial fibrillation. Hyperthyroidism can result in heart failure if left untreated. Also, thyroid dysfunction impacts the entire body. Hence, early diagnosis and treatment of hyperthyroidism and the associated cardiac abnormalities are essential to prevent fatal complications.
