Introduction

What is hyperthyroidism?

Hyperthyroidism is a condition in which the thyroid gland becomes too active and releases too much thyroid hormone into the bloodstream. The thyroid gland is found in the neck and is shaped like a small butterfly. Thyroid hormones have an important role in controlling many bodily functions, such as:

• Metabolism: They increase the body's basal metabolic rate, affecting how quickly the body burns calories
• Body temperature regulation: They help maintain normal body temperature
• Heart rate and cardiac output: They influence how quickly and strongly the heart beats
• Nervous system function: They affect nerve signal transmission and cognitive function
• Muscle strength and control: They impact muscle tone (the amount of tension in muscles) and reflexes
• Digestion: They influence gut motility and nutrient absorption
• Skin and hair health: They affect the growth and maintenance of these tissues
• Bone metabolism: They play a role in bone density regulation

Whilst the symptoms may be challenging to manage, there are many treatments available, such as medication, radioactive therapy and surgery.

Overview of cardiomyopathy and heart enlargement

Cardiomyopathy describes diseases of the heart muscle, which ultimately affect the efficiency of blood being pumped around the body.¹ There are two types of cardiomyopathy: dilated and hypertrophic cardiomyopathy. Dilated cardiomyopathy is where the heart chambers become stretched and thin, weakening the heart’s ability to pump blood. Hypertrophic cardiomyopathy is where the heart muscles enlarge, causing a thickening of the heart chambers. This results in the chambers becoming smaller, meaning less blood can be held. There is no cure; rather, symptoms are managed and aimed at preventing further complications. 

How are they linked?

Hyperthyroidism has a major impact on the cardiovascular system and can lead to cardiomyopathy and heart enlargement.² The increased levels of thyroid hormones result in the heart working harder by increasing the heart rate and blood volume. To deal with this constant extra work, the heart responds by enlarging. This change is referred to as hyperthyroid cardiomyopathy. The larger the heart becomes, the less efficient it is at pumping blood. If both are not managed, it can lead to permanent heart damage. Hence, early diagnoses are crucial so that it can be managed to prevent any further damage. 

Symptoms and diagnosis

Symptoms of cardiomyopathy caused by hyperthyroidism include the following:³

• Abnormal and faster heartbeats
• Shortness of breath, which is worsened by physical activity 
• Increased fatigue
• Pain in the chest
• Swelling in ankles, feet and legs, referred to as oedema 
• Increased feelings of dizziness and lightheadedness
• Episodes of fainting

Diagnostic criteria 

Several tests can be done to help reach a diagnosis, for example:

• Thyroid tests are done to measure the levels of thyroid hormones. Higher levels of thyroid hormones and lower levels of thyroid-stimulating hormone suggest hyperthyroidism
• An echocardiogram is a special ultrasound that looks specifically at the heart’s structure and how well it is functioning. It can identify weaker muscles or an enlarged heart, which is indicative of cardiomyopathy caused by hyperthyroidism 
• Electrocardiograms look at the electrical activity of the heart. A higher heart rate is a sign of hyperthyroidism so this test can be used for a diagnosis
• Chest X-rays can also be done to look at whether the heart is bigger than normal, which may mean cardiomyopathy 
• Stress tests are conducted to measure how the heart responds to stress, such as during exercise, which can rule out any other possible heart issues 

All these tests combined show whether the thyroid is overactive and if this is having an effect on the activity of the heart. They create a more complete picture of the affected person’s health and a better understanding of how to diagnose and manage the illness. 

Cardiovascular effects of hyperthyroidism

Hyperthyroidism has many significant effects on the cardiovascular system due to the hormones' impact on blood vessels and overall heart function.⁴ The increased thyroid hormones heighten the body's metabolic rate, causing many cardiovascular changes. The main cardiovascular changes of hyperthyroidism include:

• Faster heart rate (tachycardia): Thyroid hormones directly stimulate the sinoatrial node, which regulates heartbeat, resulting in a faster resting heart rate.
• Increased cardiac contractility: The heart muscle contracts with more pressure and force, increasing cardiac output.
• Decreased systemic vascular resistance: Blood vessels dilate, lessening the afterload on the heart.
• Higher blood volume: The kidneys retain more sodium and water, increasing the circulatory volume.
• Change in cardiac output (how much blood is pumped per minute): A mixture of increased heart rate, stronger contractions, and decreased vascular resistance, causing increased blood flow.
• Widened pulse pressure: The difference between systolic and diastolic blood pressure increases due to increased cardiac output and decreased vascular resistance
• Left ventricular hypertrophy: Long durations of hyperthyroidism can cause thickening of the heart's left ventricular wall as a compromise to increased workload

These cardiovascular effects of hyperthyroidism can greatly impact a patient's quality of life and, if it remains untreated, can cause more severe complications, including heart failure or thromboembolism. This means that an early diagnosis and management of hyperthyroidism are very important to reduce these cardiovascular risks and restore normal thyroid function.

Management and treatment

Currently, there is no cure for hyperthyroid cardiomyopathy; rather, symptoms are managed to prevent the condition from worsening. There are several strategies to manage cardiomyopathy induced by hyperthyroidism and heart enlargement,⁵ explained below.

Treatment for cardiomyopathy and heart enlargement

• Medication that decreases the amount of work the heart does, manages heart rhythms, blood pressure and many more symptoms, e.g. beta blockers, diuretics and cardiac myosin inhibitors
• An implantable cardioverter defibrillator may be given if there is a dangerous abnormal heart rhythm. It’s an electronic device that monitors and controls abnormal heart rhythms 
• To help the heart pump more effectively, a left ventricular assist device is given only when cardiomyopathy and heart enlargement when the heart failure is severe. 
• Heart surgery to remove thickened muscle walls

Treatment for hyperthyroidism 

There are many treatment options available and tailored to each person. 

• Medications such as antithyroid drugs stop the thyroid gland from producing hormones. This is a quick way of reducing the excess thyroid levels, causing hyperthyroidism 
• Radioactive iodine can be taken orally. Once taken, it damages your thyroid cells and results in the thyroid gland shrinking in size, lowering hormone levels over a few weeks. This can be used in the long term to create a balanced hormone level
• Surgery may be an option, whereby the thyroid gland is removed. However, it may cause hypothyroidism - an underactive thyroid.
• Beta blockers can be given to manage symptoms such as a fast heartbeat 

Summary

Hyperthyroidism is a medical condition caused by an overactive thyroid gland producing too many hormones. This can greatly impact the cardiovascular system and, as a result, cause cardiomyopathy and heart enlargement. The excess thyroid hormones increase the body's metabolic rate, causing many changes in the cardiovascular system. These include a faster heart rate, increased cardiac contractility, decreased vascular resistance, and higher blood volume. Over time, these changes can cause cardiomyopathy induced by hyperthyroidism, where the heart increases in size to manage the increased workload. The enlarged heart can result in decreased efficiency in pumping blood and possibly irreversible, permanent heart damage if left untreated.

Symptoms of cardiomyopathy caused by hyperthyroidism include abnormal and faster heartbeats, shortness of breath, fatigue, chest pain, swelling in limbs, dizziness, and episodes of fainting. To make a diagnosis, there needs to be a combination of thyroid tests, echocardiograms, electrocardiograms, chest X-rays, and stress tests to assess the state of the heart, as well as how the thyroid is functioning. These tests help create a detailed picture of the patient's health and are used to formulate a treatment plan.

Managing hyperthyroid cardiomyopathy focuses on treating both the underlying hyperthyroidism and the resulting heart issues. Hyperthyroidism treatments include antithyroid medications, radioactive iodine therapy, and, in some cases, thyroid surgery. For cardiomyopathy and heart enlargement, treatments may involve taking medications to reduce heart workload and manage symptoms, implantable cardioverter defibrillators for dangerous arrhythmias, left ventricular assist devices for severe cases, and heart surgery to remove thickened muscle walls. While there is no cure for cardiomyopathy, early diagnosis and management are vital to prevent further complications and restore normal thyroid function. The goal is to relieve symptoms, prevent the condition from deteriorating, and improve the patient's quality of life.