Does Thyroid-Stimulating Hormone (TSH) affect patients with syncope?

Thyroid-Stimulating Hormone (TSH) and Syncope: Clinical Relationship and Management

Thyroid dysfunction, particularly hyperthyroidism with suppressed TSH levels, can cause syncope through cardiovascular mechanisms including arrhythmias and should be considered in the differential diagnosis of unexplained syncope.

Relationship Between TSH and Syncope

Hyperthyroidism and Syncope

• Low TSH (subclinical or overt hyperthyroidism) can contribute to syncope through several mechanisms:
  • Atrial fibrillation: Patients with TSH <0.1 mIU/L have a 3-fold increased risk of atrial fibrillation 1, which can lead to hemodynamic compromise and syncope
  • Cardiac dysfunction: Hyperthyroidism increases heart rate, left ventricular mass, and cardiac contractility 1
  • Vasodilation: Thyrotoxicosis can cause peripheral vasodilation leading to orthostatic hypotension 2

Hypothyroidism and Syncope

• Severe hypothyroidism (elevated TSH) can potentially cause:
  • Bradyarrhythmias: Complete atrioventricular block has been documented in severe hypothyroidism, presenting with syncope 3
  • Cardiac dysfunction: Decreased cardiac output in profound hypothyroidism

Diagnostic Approach for Syncope Patients

Initial Evaluation

• The 2017 ACC/AHA/HRS guidelines recommend a thorough history, physical examination, and 12-lead ECG for all patients with syncope 1
• TSH measurement is not routinely recommended in all syncope patients but should be considered when:
  • Clinical features suggest thyroid dysfunction (tachycardia, tremor, heat intolerance in hyperthyroidism; bradycardia, cold intolerance in hypothyroidism)
  • Unexplained arrhythmias are detected
  • Syncope remains unexplained after initial evaluation

Risk Stratification

• Patients with syncope and abnormal thyroid function tests should be evaluated for:
  • Cardiac arrhythmias (particularly atrial fibrillation in hyperthyroidism)
  • Orthostatic hypotension
  • Cardiac structural abnormalities

Management Recommendations

Hyperthyroidism-Related Syncope

• Treatment of the underlying thyroid dysfunction is essential:
  • Normalization of thyroid function may resolve cardiac arrhythmias and improve hemodynamics
  • In patients with atrial fibrillation due to hyperthyroidism, rate control and anticoagulation should be considered per standard guidelines 1

Hypothyroidism-Related Syncope

• Thyroid hormone replacement therapy can resolve bradyarrhythmias and improve cardiac function 3
• Temporary cardiac pacing may be needed in severe cases with hemodynamic compromise until thyroid function normalizes

Special Considerations

Medication Effects

• Several medications can affect thyroid function and potentially contribute to syncope:
  • Antiepileptic drugs can alter thyroid hormone levels 4
  • Amiodarone can cause both hyper- and hypothyroidism

Endocrine Causes of Syncope

• The 2017 ACC/AHA/HRS guidelines list several endocrine disorders that can cause syncope, including:
  • Carcinoid syndrome
  • Pheochromocytoma
  • Mastocytosis
  • Vasoactive intestinal peptide tumor 1
• Thyroid dysfunction should be considered alongside these conditions

Common Pitfalls

• Failing to consider thyroid dysfunction in patients with unexplained arrhythmias and syncope
• Attributing syncope solely to neurally mediated mechanisms without evaluating for underlying thyroid disease in at-risk patients
• Not recognizing that treatment of thyroid dysfunction may resolve syncope episodes without need for cardiac interventions

Conclusion

While TSH testing is not recommended as part of routine syncope evaluation, thyroid dysfunction should be considered in the differential diagnosis of unexplained syncope, particularly when associated with arrhythmias or when clinical features suggest thyroid disease.