Electrocardiographic manifestations in patients with thyrotoxic periodic paralysis

Background: Thyrotoxic periodic paralysis (TPP) commonly precedes the overt symptoms and signs of hyperthyroidism and may be misdiagnosed as other causes of paralysis (non-TPP). Because the cardiovascular system is very sensitive to elevation of thyroid hormone, we hypothesize that electrocardiographic manifestations may aid in early diagnosis of TPP. Methods: We retrospectively identified 54 patients who presented to the emergency department (ED) with hypokalemic paralysis during a 3.5-year period. Thirty-one patients had TPP and 23 patients had non-TPP, including sporadic periodic paralysis, distal renal tubular acidosis, diuretic use, licorice intoxication, primary hyperaldosteronism, and Bartter-like syndrome. Electrocardiograms during attacks were analyzed for rate, rhythm, conduction, PR interval, QRS voltage, ST segment, QT interval, U waves, and T waves. Results: There were no significant differences in age, sex distribution, and plasma K⁺ concentration between the TPP and non-TPP groups. Plasma phosphate was significantly lower in TPP than non-TPP. Heart rate, PR interval, and QRS voltage were significantly higher in TPP than non-TPP. Forty-five percent of TPP patients had first-degree atrioventricular block compared with 13% in the non-TPP group. There were no significant differences in QT shortening, ST depression, U wave appearance, or T wave flattening between the 2 groups. Conclusion: Relatively rapid heart rate, high QRS voltage, and first-degree AV block are important clues suggesting TPP in patients who present with hypokalemia and paralysis.