Hypothyroidism causing prolonged QTc

Sheenam, VG Aarella, United Lincolnshire Hospitals NHS Trust

Introduction: Hypothyroidism can cause electrocardiographic abnormalities like- Bradycardia, ST segment depression, QT interval prolongation and flattening or inversion of T wave.

In an ECG, the QT duration represents the total time for de- and repolarization. Prolonged QT duration predisposes to life-threatening ventricular arrhythmias and therefore QT duration must always be assessed in an ECG.

Prolonged QT duration may either be congenital or acquired. One of the main causes of QT prolongation is Hypothyroidism.

Abstract: We describe the case of an 84 year lady who was admitted to the hospital after having a collapse outside the hospital. Blood tests done on presentation in A&E suggested severe Hypothyroidism (TSH > 100 and T4 <1 with TPO antibody level 174) and her ECG showed prolonged QTc.  These findings were not known in the past.

All other blood tests, CT Head and Chest X ray showed normal findings.

Investigations: July 2020: TSH > 100, T4 <1 with TPO antibody level 174

Management: Whilst in the hospital she was started on Levothyroxine 100 mcg and Cardiology review was sought in view of the new ECG findings.

After review by the Cardiology team it was established that she needed no further treatment from the Cardiology point of view for the QTc prolongation as Hypothyroidism itself can prolong the QTc and once it is corrected, the QTc will get back to normal.

Her TFTs were monitored over time and the TSH levels are 1.6 after the commencement of treatment with Levothyroxine.

She was recently admitted to the hospital again with pneumonia and her ECG showed normal QTc.

Findings before and after starting treatment with Levothyroxine:

Test Name

Before Levothyroxine

After Levothyroxine







Key learning: Normally an electrical impulse starts in the sinus node and then travels down to the ventricles, causing contraction of the ventricles' muscle cells. This contraction causes the blood to flow out of the heart and the heart muscle cells then relax. During this relaxation phase, the electrical charges of the cells need to recover. The recovery time is known as the QT interval.

The QT interval lasts just a fraction of a second but in people with prolonged QT interval, it lasts longer than it should, and this delay makes the heart more likely to develop arrhythmias.

QT duration is inversely related to heart rate; QT duration increases at low heart rate and vice versa. Therefore, one must adjust the QT duration for the heart rate, which yield corrected QT duration (QTc). Bazett’s formula is used to calculate the corrected QT duration. The formula follows (all variables in seconds):

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ECG interpretation always includes assessment of the QT (QTc) duration. Increased QT interval dispersion (QTd) is a parameter shown to be associated with ventricular arrhythmias and death, so QT dispersion corrected for heart rate (QTc) is an important predictor of cardiac death.

In some cases, a prolonged QTc doesn’t even need any extra treatment other than correction of hypothyroidism.

Hence it is very important to timely manage the patients with hypothyroidism as they may have prolonged QTc in ECG and if left untreated, these patients can have life threatening complications, like malignant ventricular arrhythmias and death.



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2.   Strachan SR, Afolabi O, Brown N, et al Chest pain, enzymes and hypothyroidism Postgraduate Medical Journal 2000;76:168-169.