Hypothyroidism, a condition characterized by an underactive thyroid gland, is one of the most prevalent endocrine disorders worldwide. The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), play a crucial role in regulating the body's metabolic processes, including the cardiovascular system (1). A deficiency in these hormones can lead to a variety of cardiac manifestations, some of which may be life-threatening if not promptly addressed (2).

This review aims to explore the spectrum of cardiac manifestations associated with hypothyroidism, elucidating the pathophysiological mechanisms underlying these effects and discussing their clinical implications and management strategies.

Pathophysiology of Cardiac Manifestations in Hypothyroidism

The cardiovascular system is highly sensitive to thyroid hormone levels. Thyroid hormones exert direct effects on the heart and peripheral vasculature, influencing heart rate, myocardial contractility, and systemic vascular resistance (3). In hypothyroidism, the reduced levels of T3 and T4 lead to the following pathophysiological changes:

1. Bradycardia: Thyroid hormones increase the heart rate by enhancing the pacemaker activity of the sinoatrial node. In hypothyroidism, reduced hormone levels result in bradycardia, which is often one of the earliest cardiac manifestations (4).

2. Increased Systemic Vascular Resistance: Hypothyroidism is associated with increased systemic vascular resistance, which leads to diastolic hypertension and places a greater workload on the heart (5).

3. Reduced Cardiac Output: The decreased myocardial contractility and heart rate contribute to a reduced cardiac output, which can result in symptoms of heart failure in severe cases (6).

4. Pericardial Effusion: Hypothyroidism can cause an accumulation of fluid in the pericardial sac, leading to pericardial effusion. This condition is often asymptomatic but can progress to cardiac tamponade if the effusion is large or rapidly accumulates (7).

Clinical Manifestations

The cardiac manifestations of hypothyroidism can range from mild to severe and may include:

1. Bradycardia: Patients with hypothyroidism often present with a slow heart rate, which may be asymptomatic or associated with symptoms such as fatigue, dizziness, or syncope (8).

2. Hypertension: Diastolic hypertension is commonly observed in hypothyroid patients due to increased systemic vascular resistance (9). This type of hypertension is often resistant to conventional antihypertensive therapies and improves with thyroid hormone replacement.

3. Heart Failure: Hypothyroidism can exacerbate heart failure in patients with preexisting cardiac conditions or lead to heart failure de novo in severe cases of myxedema (10). The symptoms include dyspnea, orthopnea, and peripheral edema.

4. Pericardial Effusion: Mild pericardial effusion is common in hypothyroidism and is usually detected incidentally on echocardiography. Large effusions, although rare, can cause symptoms such as chest pain, shortness of breath, and a sense of fullness in the chest (11).

5. Coronary Artery Disease (CAD): Hypothyroidism can worsen the clinical course of CAD by increasing cholesterol levels, promoting atherosclerosis, and reducing coronary blood flow (12).

Management

The management of cardiac manifestations in hypothyroidism primarily involves thyroid hormone replacement therapy. Levothyroxine, a synthetic form of T4, is the treatment of choice and is typically administered orally(13). The dosage is adjusted based on the patient's thyroid function tests and clinical response.

• Bradycardia: Bradycardia usually resolves with appropriate thyroid hormone replacement. In rare cases, temporary pacing may be required if the bradycardia is severe and symptomatic (14).

• Hypertension: Diastolic hypertension associated with hypothyroidism often improves with thyroid hormone replacement. In some cases, adjunctive antihypertensive therapy may be necessary until thyroid function normalizes (15).

• Heart Failure: Patients with hypothyroidism-induced heart failure require careful management of both thyroid function and heart failure. Diuretics, ACE inhibitors, and beta-blockers may be used alongside levothyroxine therapy (16).

• Pericardial Effusion: Small, asymptomatic pericardial effusions typically resolve with thyroid hormone replacement. Large effusions causing symptoms may require pericardiocentesis (17).

Hypothyroidism has significant effects on the cardiovascular system, ranging from mild bradycardia to severe heart failure and pericardial effusion. Early recognition and treatment of hypothyroidism are crucial to prevent the progression of cardiac complications. Regular monitoring of thyroid function in patients with unexplained cardiac symptoms is essential for timely diagnosis and management.

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