Hyperthyroidism

Electrocardiographic Abnormalities

The most common ECG changes seen with thyrotoxicosis are:

• Sinus tachycardia
• Atrial fibrillation with rapid ventricular response
• High left-ventricular voltage – i.e “voltage criteria” for LVH without evidence of LV strain.

Other ECG abnormalities include:

• Supraventricular arrhythmias (premature atrial beats, paroxysmal supraventricular tachycardia, multifocal atrial tachycardia, atrial flutter)
• Non-specific ST and T wave changes
• Ventricular extrasystoles

Around 50% of patients with thyrotoxicosis will have a resting heart rate > 100 bpm. Atrial fibrillation is seen in up to 20% of patients. Severe thyrotoxicosis (thyroid storm) may present with atrial tachycardias at rates > 200 bpm.

Mechanism

• The ECG changes in thyrotoxicosis are principally related to increased activity of the sympathetic nervous system and the stimulatory effects of thyroid hormone on the myocardium.
• Atrial tissue is very sensitive to the effects of thyroid hormone, hence the preponderance of atrial tachydysrhythmias.

Useful Tips

• Patients with unexplained sinus tachycardia or atrial fibrillation should have their TSH and T4 checked to look for evidence of thyrotoxicosis.
• Thyrotoxic atrial tachydysrhythmias are treated with intravenous beta-blockers (e.g. titrated IV boluses of propranolol or an esmolol infusion).

Example ECGs

Example 1 – Sinus tachycardia

Example 2 – Rapid atrial fibrillation with high left ventricular voltage

Related Topics

• Hypothyroidism
• Left ventricular hypertrophy

Further Reading

• ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
• ECG CLINICAL CASES — Your favourite ECG’s placed in clinical context with a challenging Q&A approach
• ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
• ECG Exam Template — a framework for the FACEM part 2 exam.
• ECG Reference Sites on the WEB — the best of the rest

Author Credits

References

• Cameron P, Jelinek G, Kelly AM, Murray L, Brown AFT. Textbook of Adult Emergency Medicine. Elsevier 2009.
• Slovis C, Jenkins R. ABC of clinical electrocardiography: Conditions not primarily affecting the heart. BMJ. 2002 Jun 1;324(7349):1320-3. Review. PubMed PMID: 12039829; PubMed Central PMCID: PMC1123277. Full text article.
• Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.