Hypothyroidism

Electrocardiographic Abnormalities

Severe hypothyroidism (myxoedema) causes a triad of:

• Bradycardia
• Low QRS voltage
• Widespread T-wave inversions (usually without ST deviation)

Other ECG changes that may be seen include:

• QT prolongation
• First degree AV block
• Interventricular conduction delay

Mechanism

ECG changes may be secondary to:

• Myxoedematous deposits of gelatinous connective tissue within the myocardium.
• Decreased activity of the sympathetic nervous system.
• Effects on the myocardium of reduced levels of thyroxine (i.e. reduced inotropy/chronotropy)

ECG Examples

Example 1 – Myxoedema coma

• This is the admission ECG of a 79-year old man who was referred to ICU with coma, hypothermia, severe bradycardia and hypotension refractory to inotropes.
• TSH was markedly elevated with an undetectable T4.
• The ECG shows marked bradycardia (30 bpm) with low QRS voltages (esp. in the limb leads) and widespread T-wave inversions, typical of severe myxoedema.

 

Example 2 – Myxoedema coma (after treatment)

• An ECG of the same patient shortly after initiation of thyroid replacement with intravenous T3 and T4.
• The heart rate has normalised and the T-wave inversion has corrected.
• Low voltage in the limb leads persists and is likely due to myxoedematous infiltration of the myocardium.

 

Example 3 – Myxoedema

• Another example of low QRS voltage due to myxoedema.

 

Related Topics

• Hyperthyroidism
• Low QRS voltage

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

• Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.