Will hypothyroidism cause bradycardia?

Hypothyroidism and Bradycardia

Yes, hypothyroidism commonly causes bradycardia as one of its primary cardiovascular manifestations. 1, 2, 3

Cardiovascular Effects of Hypothyroidism

Hypothyroidism affects the cardiovascular system in several distinct ways:

• Heart rate changes: Bradycardia (slow heart rate) is one of the most common cardiovascular manifestations of hypothyroidism 1, 3
• Blood pressure effects: Mild hypertension (often diastolic) and narrowed pulse pressure 1
• ECG changes: Common findings include:
  • Sinus bradycardia (observed in 10.3% of patients) 3
  • Low voltage complexes (25% of patients) 3
  • T-wave inversion (23.5% of patients) 3
  • Prolonged QT interval 1, 3
  • QRS prolongation (2.9% of patients) 3

Pathophysiology

The bradycardia in hypothyroidism occurs due to:

1. Decreased sympathetic nervous system activity
2. Reduced sensitivity to catecholamines
3. Direct effects of thyroid hormone deficiency on cardiac pacemaker cells
4. Slowed diastolic depolarization in the sinoatrial node

The hemodynamic changes in hypothyroidism are opposite to those of hyperthyroidism, with bradycardia being a characteristic finding. The low cardiac output in hypothyroidism results from bradycardia, decreased ventricular filling, and decreased cardiac contractility 1.

Clinical Significance

While bradycardia is common in hypothyroidism, it's important to note that:

• Clinical heart failure due to hypothyroidism alone is rare 1
• Cardiac output is usually sufficient to meet the lowered systemic demands in hypothyroidism 1
• In severe, long-standing hypothyroidism, pericardial effusions and nonpitting edema (myxedema) can occur 1

Unusual Presentations

While bradycardia is the typical cardiac rhythm disturbance in hypothyroidism, there are rare cases where hypothyroidism may present with tachyarrhythmias:

• Case reports exist of severe hypothyroidism presenting with supraventricular tachycardia 4
• Ventricular tachycardia has been reported as an unusual manifestation of severe undiagnosed hypothyroidism 5

Diagnostic Considerations

When evaluating bradycardia:

• Thyroid function tests should be included in the workup 1
• Serial TSH measurements are essential as TSH can vary by up to 50% day-to-day 1
• A single abnormal TSH value is insufficient to establish thyroid dysfunction 1

Treatment Implications

For patients with hypothyroidism and bradycardia:

• Thyroid hormone replacement therapy typically resolves the bradycardia 6
• Careful monitoring is needed when initiating treatment in patients with cardiac disease 2
• Elderly patients may require more gradual thyroid hormone replacement 2

Conclusion

Bradycardia is a common cardiovascular manifestation of hypothyroidism, occurring due to the direct and indirect effects of thyroid hormone deficiency on the heart. While hypothyroidism is rarely the primary cause of dilated cardiomyopathy and heart failure, thyroid function tests are recommended in all patients presenting with heart failure due to the availability of definitive treatments for thyroid disorders 1.