Introduction:
The human body's delicate balance depends on the proper functioning of various systems, and the endocrine system plays a vital role in maintaining equilibrium. The thyroid gland, located in the neck, is a chief component of the endocrine system and regulates metabolism by producing thyroid hormones, namely thyroxine (T4) and triiodothyronine (T3). However, when this gland becomes overactive, it releases excessive thyroid hormones resulting in hyperthyroidism. Apart from affecting the metabolism, hyperthyroidism can have far-reaching effects on the cardiovascular system, leading to irregular heartbeat or arrhythmias.
What Is Hyperthyroidism?
Hyperthyroidism (high levels of thyroid hormones) occurs when the thyroid gland releases an abundance of T4 and T3 hormones, interfering with the body's normal processes. This condition can occur due to various factors like Grave's disease (an autoimmune disorder where the immune system attacks the thyroid gland), causing it to produce thyroid hormones at higher levels. Additionally, toxic multinodular goiter and certain medications containing excess iodine can trigger hyperthyroidism.
What Is the Role of Thyroid Hormones in Maintaining the Homeostasis?
The thyroid hormones significantly impact maintaining energy balance within the heart, and an excess of these hormones can lead to increased metabolism, known as hypermetabolism. The thyroid gland produces two main hormones: thyroxine (T4) and triiodothyronine (T3). T4 is the major form of thyroid hormone and mainly acts as a precursor to T3, the biologically active hormone.
The production and release of thyroid hormones are regulated by the hypothalamic thyrotropin-releasing hormone (TRH), which stimulates the production of thyroid-stimulating hormone (TSH). TSH, in turn, controls the concentrations of thyroid hormones. Most of the T4 is converted to active T3 through deiodinases, with Type 1 deiodinase playing a vital role in this process. Type 1 deiodinase converts T4 to active T3 and can also deactivate thyroid hormones by converting T4 to inactive reverse T3 (rT3) or T2.
It is necessary to consider that cardiac cells lack significant intracellular deiodinase activity, so the heart relies mainly on T3, as it is the hormone transported into the cardiac cells. Both T4 and T3 circulate in the bloodstream, mostly bound to proteins, particularly thyroxine-binding globulin, along with other hormone-binding proteins. The remaining unbound T3 is transported into the cells through various membrane transport proteins and then moves to the cell nucleus to regulate the expression of specific genes.
What Is the Mechanism of Thyroid Hormones?
Thyroid hormone exerts its effects through two mechanisms: genomic and nongenomic actions. The genomic actions involve T3 binding to nuclear receptors, which then attach to specific regions called thyroid-responsive elements (TREs) in the promoter of target genes. This binding can activate or repress gene expression, leading to various tissue-specific responses. Prolonged exposure to high T3 levels can cause cardiac hypertrophy and dysfunction by increasing the synthesis of certain cardiac proteins, including structural and regulatory ones.
On the other hand, the nongenomic activities of thyroid hormones bring about rapid changes in the cardiac myocyte's plasma membrane and cytoplasmic organelles. These changes include alterations in ion channels (sodium, potassium, and calcium), actin cytoskeleton polymerization, and intracellular signaling pathways in both heart and smooth muscle cells.
Both genomic and nongenomic mechanisms work together to regulate cardiac function and cardiovascular hemodynamics. They enhance myocardial relaxation by upregulating the expression of sarcoplasmic reticulum calcium-activated ATPase and downregulating phospholamban expression. Additionally, they increase the expression of faster contractile isoforms of the myosin, contributing to improved systolic function. Thyroid hormones also accelerate the sinoatrial node's depolarization and repolarization rates, leading to an increased heart rate.
Consequently, in hyperthyroidism, thyroid hormones positively affect the heart's contractility and rate, which, combined with heightened adrenergic sensitivity, contribute to the elevated heart rate and enhanced contractility observed in this condition.
How Are Higher Thyroid Hormone Levels Linked to Irregular Heartbeat?
The link between irregular heartbeat and higher thyroid hormone levels, specifically in the context of hyperthyroidism, occurs due to the effects of excess thyroid hormones on the cardiovascular system. The impact of hyperthyroidism on the heart can be diverse and may include:
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Increased Heart Rate (Tachycardia): One of the hallmark effects of hyperthyroidism is an elevated resting heart rate. Thyroid hormones directly stimulate the heart, leading to an increase in heart rate. This can result in a persistent fast heartbeat, even at rest. Resting heart rates above 100 beats per minute (bpm) are common in people with hyperthyroidism.
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Palpitations: Many individuals with hyperthyroidism experience palpitations, which are sensations of a fast, irregular, or pounding heartbeat. Palpitations can be worrisome and concerning for those affected.
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Atrial Fibrillation (AF) and Atrial Flutter: Hyperthyroidism can hinder the normal electrical signaling of the heart, resulting in abnormal heart rhythms. Atrial fibrillation (AF) and atrial flutter are two common arrhythmias associated with hyperthyroidism. AF is identified by chaotic electrical signals in the heart's upper chambers (atria), leading to irregular and rapid heartbeats.
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Increased Cardiac Output: Hyperthyroidism causes an increase in the volume of blood pumped by the heart per minute, known as cardiac output. This is primarily due to the elevated heart rate and increased force of heart contractions. Increased cardiac output can lead to symptoms such as increased blood pressure, heart murmurs, and a bounding pulse.
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Exacerbation of Pre-existing Heart Conditions: Hyperthyroidism can put additional stress on the heart, especially in individuals with pre-existing heart conditions. It can worsen symptoms and complications of heart failure, angina (chest pain), and other cardiovascular issues.
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Cardiomyopathy: In severe and untreated cases of hyperthyroidism, a condition known as thyrotoxic cardiomyopathy may develop. This condition involves direct damage to the heart muscle, leading to impaired heart function and potential heart failure.
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Risk of Blood Clots: People with hyperthyroidism, especially those with atrial fibrillation, may have an increased risk of forming blood clots within the heart. These clots move and travel to other body parts, leading to severe complications like strokes.
How to Manage the Effect of Thyroid Hormone Levels and Irregular Heartbeat?
Management includes:
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Antithyroid medications, such as Methimazole or Propylthiouracil (PTU), work by blocking the production of thyroid hormones to manage mild to moderate hyperthyroidism.
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Beta-blocker medications, like Propranolol or Atenolol, are used to manage the symptoms of hyperthyroidism, remarkably rapid heart rate, palpitations, and tremors.
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Radioactive iodine is taken orally and selectively destroys the overactive thyroid cells producing excess thyroid hormones.
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In some cases, if antithyroid medications and radioactive iodine are not suitable or effective, surgical removal of the part or whole of the thyroid gland (thyroidectomy) can be suggested.
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Adequate rest, stress management, and a balanced diet can help support the body during hyperthyroidism treatment. Reducing caffeine intake may also be beneficial, as caffeine can exacerbate symptoms such as palpitations.
Conclusion:
Hyperthyroidism is a complex endocrine disorder that can predominantly affect the cardiovascular system, leading to irregular heartbeat and other cardiac symptoms. The close association between the thyroid and heart underscores the importance of promptly managing hyperthyroidism. With effective treatment, including antithyroid medications, beta-blockers, radioactive iodine therapy, and, in some cases, surgery, individuals can restore thyroid balance and alleviate irregular heartbeat and associated symptoms. Timely diagnosis and collaborative efforts are essential to successfully managing hyperthyroidism and promoting overall heart health.
