ECG features: may precede clinical manifestations
- 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 
- Sinus tachycardia:
- More common than atrial fibrillation [1,4]
- Incidence decreases with age 
- Tachycardia severity parallels disease severity
- Atrial flutter:
- Incidence: 1.5-2.0% [1,5]
- Paroxysmal supraventricular tachycardia & ventricular tachycardia:
- Possible, but uncommon
- Most common: incomplete RBBB (9.2%) followed by left anterior fascicular block 
- Intraatrial conduction disturbances: prolongation (>0.10 second) or notching of P wave (17.4%; more common in women) 
- Prolonged PR interval (even without heart disease or digitalis treatment) 
- 1st-degree AV block incidence: 3.2-8.9% [1,4,7]
- 2nd- & 3rd-degree AV block less common 
- May precede atrial fibrillation
- Typically returns to normal when euthyroid
- Intraventricular conduction delays in 13.1% without heart disease of other etiology 
- Typically disappear when euthyroid
- Wolff-Parkinson-White (WPW) pattern: higher incidence in hyperthyroidism than general population 
P wave & QRS amplitude: often increased 
- Tall P wave mimics “P-pulmonale”
- Large QRS amplitude mimics LVH
Nonspecific ST segment & T wave abnormalities in ~25% patients 
- Some with abnormal T waves may have STE mimicking pericarditis pattern 
- Typically transient with daily variation 
- Typically return to normal after treatment [3,4,8]
Thyrotoxic periodic paralysis:
- ECG changes mainly caused by hypokalemia 
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.