ECG features: may precede clinical manifestations
Rhythm disturbances:

• Atrial fibrillation:
• Incidence increases with age [1,2]
• May spontaneous revert to sinus rhythm after treatment [1,2]
• Frequency may be greater in triiodothyronine (T3) toxicosis compared to when both T3 & thyroxine (T4) are increased [3]

• Sinus tachycardia:
• More common than atrial fibrillation [1,4]
• Incidence decreases with age [1]
• Tachycardia severity parallels disease severity

• Atrial flutter:
• Incidence: 1.5-2.0% [1,5]

• Paroxysmal supraventricular tachycardia & ventricular tachycardia:
• Possible, but uncommon

Conduction disturbances:

• Most common: incomplete RBBB (9.2%) followed by left anterior fascicular block [1]
• Intraatrial conduction disturbances: prolongation (>0.10 second) or notching of P wave (17.4%; more common in women) [1]
• Prolonged PR interval (even without heart disease or digitalis treatment) [6]
• 1st-degree AV block incidence: 3.2-8.9% [1,4,7]
• 2nd- & 3rd-degree AV block less common [6]
• May precede atrial fibrillation
• Typically returns to normal when euthyroid
• Intraventricular conduction delays in 13.1% without heart disease of other etiology [1]
• Typically disappear when euthyroid
• Wolff-Parkinson-White (WPW) pattern: higher incidence in hyperthyroidism than general population [6]

P wave & QRS amplitude: often increased [6]

• Tall P wave mimics “P-pulmonale”
• Large QRS amplitude mimics LVH

Nonspecific ST segment & T wave abnormalities in ~25% patients [6]

• Some with abnormal T waves may have STE mimicking pericarditis pattern [4]
• Typically transient with daily variation [4]
• Typically return to normal after treatment [3,4,8]

Thyrotoxic periodic paralysis:

• ECG changes mainly caused by hypokalemia [9]

REFERENCES

1. Staffurth JS, Gibberd MB, Hilton PS. Atrial fibrillation in thyrotoxicosis treated with radioiodine. Postgrad Med J. 41:663. 1965.

2. Benker VG, Preiss, H, Kreuser H, et al. EKG VaraenderungenbeiHyperthyreose: Untersuchungenan 542 Patienten. Z Kardiol. 63:799. 1974.

3. Woeber KA. Thyrotoxicosis and the heart. N Engl J Med. 392:94. 1992.

4. Hoffman L, Lowrey RD. The electrocardiogram in thyrotoxicosis. Am J Cardiol. 6:893. 1960.

5. Goss JE, Schmock CI, Pryor R, et al. Cardiac arrhythmias with thyrotoxicosis. Rocky Mountain Med J. 64:31. 1967.

6. Surawicz B, Mangiardi ML. Electrocardiogram in endocrine and metabolic disorders. Cardiovasc Clin North Am. 8:243. 1977.

7. Blizzard JJ, Rupp JJ. Prolongation of the PR interval as a manifestation of thyrotoxicosis. JAMA 173:1845. 1960.

8. Jost VF. Kammerendteilveraenderungenim EKG beiThyreotoxicosis factitial. Z Kardiol. 62:864. 1974.

9. Boccalandro C, Lopez L, Boccalandro F, et al. Electrocardiographic changes in thyrotoxic periodic paralysis. Am J Cardiol. 91:775. 2003.