Causes of Sinus Bradycardia in Thyroid Disorders

Primary Mechanism: Hypothyroidism

• The American College of Cardiology recommends that hypothyroidism, a reversible cause of sinus bradycardia, should be treated with thyroxine (T4) replacement therapy rather than permanent pacing, as it represents a primary mechanism of sinus bradycardia in thyroid disorders 1, 2, 3
• Bradycardia is one of the most common cardiovascular manifestations of hypothyroidism, involving decreased metabolic rate leading to reduced sinus node automaticity 4, 2
• Increased systemic vascular resistance, slowed diastolic relaxation, and ventricular filling contribute to hemodynamic changes in hypothyroidism 4
• Low cardiac output results from the combination of bradycardia, decreased ventricular filling, and decreased cardiac contractility in hypothyroidism 4

Clinical Management Approach

• The ACC/AHA/HRS guidelines provide a Class I recommendation that patients with symptomatic sinus bradycardia secondary to hypothyroidism should first be managed by treating the underlying thyroid condition, not with permanent pacing 1, 2, 3
• Cardiovascular abnormalities from hypothyroidism respond well to replacement therapy with thyroxine (T4) 1, 2, 3
• Thyroid function tests should be obtained in all patients presenting with bradycardia when hypothyroidism is suspected 5
• Checking TSH and free T4 levels is useful in patients presenting with unexplained bradycardia 5