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


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

ECG myxoedema low voltage

  • Another example of low QRS voltage due to myxoedema.


Related Topics

Further Reading

Author Credits


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