From the Guidelines
Hypothyroidism is a known cause of bradycardia, characterized by an abnormally slow heart rate typically below 60 beats per minute. This occurs because thyroid hormones play a crucial role in regulating heart function, and when thyroid hormone levels are low (hypothyroidism), the heart rate slows down 1. Thyroid hormones normally increase the force and speed of heart contractions by affecting calcium release in cardiac muscle cells and influencing beta-adrenergic receptor sensitivity. In hypothyroidism, the reduced metabolic rate leads to decreased sympathetic nervous system activity and diminished cardiac output.
Some key points to consider in the relationship between hypothyroidism and bradycardia include:
- The hemodynamic changes typical of hypothyroidism are opposite to those of hyperthyroidism, but they are accompanied by fewer symptoms and signs 1.
- The most common signs of hypothyroidism are bradycardia, mild hypertension (often diastolic), and a narrowed pulse pressure 1.
- Pericardial effusions and nonpitting edema (myxedema) can occur in patients with severe, long-standing hypothyroidism 1.
- The low cardiac output in hypothyroidism is caused by bradycardia, a decrease in ventricular filling, and a decrease in cardiac contractility 1.
Treatment of hypothyroidism-induced bradycardia involves thyroid hormone replacement therapy, typically with levothyroxine (T4) at doses individualized to the patient 1. As thyroid levels normalize with treatment, heart rate typically returns to the patient's normal range. It is essential to monitor patients closely and adjust treatment as necessary to prevent complications and improve quality of life.
From the Research
Hypothyroidism and Bradycardia
- Hypothyroidism is a condition characterized by low thyroid hormone levels that can affect multiple organ systems, including the cardiovascular system 2.
- Common cardiac manifestations of hypothyroidism include bradycardia and decreased cardiac output 2.
- Bradycardia is a condition where the heart beats too slowly, and it is a known symptom of hypothyroidism 2, 3.
- The relationship between hypothyroidism and bradycardia is well-established, with hypothyroidism affecting cardiac function and leading to changes in heart rhythm 3.
Mechanisms and Associations
- Thyroid hormones play a crucial role in regulating cardiac function, and hypothyroidism can lead to changes in cardiac contractility, vascular resistance, blood pressure, and heart rhythm 3.
- Subclinical hypothyroidism, a condition where thyroid-stimulating hormone levels are elevated but peripheral thyroid hormones are normal, has also been associated with cardiovascular disease risk factors such as hypertension and dyslipidemia 4.
- However, the use of levothyroxine to treat subclinical hypothyroidism and reduce cardiovascular disease risk is not clearly beneficial and may only provide benefit in certain subgroups 4.
Clinical Implications
- Patients with hypothyroidism may present with atypical symptoms, including bradycardia, and close monitoring is essential to permit early diagnosis and treatment 2.
- Diagnosis of hypothyroidism relies on blood levels of thyroid-stimulating hormone and free thyroxine, and treatment usually involves monotherapy with levothyroxine 5, 6.
- Regular assessment of thyroid-stimulating hormone levels and symptom relief is necessary to manage hypothyroidism effectively 6.