How does thyroid dysfunction contribute to syncope?

How Thyroid Dysfunction Causes Syncope

Thyroid dysfunction can cause syncope primarily through cardiovascular mechanisms, including arrhythmias, hemodynamic changes, and autonomic dysfunction that lead to cerebral hypoperfusion. 1, 2

Mechanisms of Syncope in Hyperthyroidism

Cardiac Arrhythmias

• Hyperthyroidism significantly increases the risk of atrial arrhythmias, particularly atrial fibrillation, which can cause syncope through rapid ventricular rates or pauses 3
• TSH levels <0.1 mIU/L are associated with a 3-fold increased risk of atrial fibrillation over 10 years in adults over 60 years 4
• Supraventricular tachyarrhythmias can reduce cardiac output and cerebral perfusion, leading to syncope 5

Hemodynamic Changes

• Hyperthyroidism creates a hyperdynamic cardiovascular state with high cardiac output and low systemic vascular resistance 5
• Increased heart rate and enhanced left ventricular systolic and diastolic function can lead to inadequate filling time and reduced stroke volume during exertion 5
• These changes may cause orthostatic intolerance and syncope, especially during position changes 1

Autonomic Dysfunction

• Thyroid hormone excess can impair normal baroreceptor function and autonomic regulation 1
• This may lead to inappropriate vasodilation when standing, causing orthostatic hypotension and syncope 1
• Vasovagal mechanisms may be triggered more easily in hyperthyroid states 1

Mechanisms of Syncope in Hypothyroidism

Bradyarrhythmias and Conduction Disorders

• Hypothyroidism can cause sinus bradycardia, AV blocks, and prolonged QT intervals that may precipitate syncope 6
• Severe hypothyroidism can lead to complete heart block requiring pacemaker placement, even after thyroid hormone replacement 6
• Ventricular arrhythmias are more common in hypothyroidism than hyperthyroidism 3

Cardiac Contractility Changes

• Hypothyroidism causes decreased cardiac contractility and impaired diastolic function 5
• Reduced stroke volume and cardiac output can lead to inadequate cerebral perfusion during increased demands 1
• Pericardial effusions in severe hypothyroidism can further compromise cardiac output 1

Vascular Effects

• Hypothyroidism increases systemic vascular resistance by up to 50% 1
• Diastolic hypertension combined with bradycardia can alter cerebral autoregulation 1
• Impaired vasodilatory responses can compromise cerebral blood flow during position changes 1

Subclinical Thyroid Dysfunction and Syncope

• Even subclinical hyperthyroidism (normal T4/T3 with suppressed TSH) can cause increased heart rate, left ventricular mass, impaired ventricular relaxation, and arrhythmias 1, 4
• Subclinical hypothyroidism is associated with impaired left ventricular diastolic function and subtle systolic dysfunction that may contribute to syncope 5
• These cardiovascular abnormalities are reversible with restoration of euthyroidism 5

Clinical Approach to Thyroid-Related Syncope

• Thyroid function tests should be performed in all patients presenting with unexplained syncope, especially with accompanying tachycardia or bradycardia 1
• ECG monitoring is essential to detect arrhythmias in patients with thyroid dysfunction and syncope 6
• Beta-blockers are indicated for initial treatment of hyperthyroidism-related cardiac symptoms, including syncope 1
• Permanent pacemaker placement may be necessary in patients with AV block associated with thyroid dysfunction, as 88% of such patients ultimately require pacing even after normalization of thyroid status 6

Important Considerations

• Thyroid dysfunction is rarely the primary cause of syncope but often coexists with other cardiovascular conditions 1
• In older patients with underlying cardiac disease, thyroid dysfunction can exacerbate the risk of syncope 1
• Treatment of the underlying thyroid disorder may not immediately resolve conduction abnormalities, and some patients may still require permanent pacing 6
• Cardiovascular complications are the chief cause of death after treatment of hyperthyroidism in patients over 50 years of age 1