Hyperthyroidism is associated with lengthening of ventricular repolarization

OBJECTIVE Lengthened ventricular repolarization, as assessed by the QT interval on electrocardiogram (ECG), can predispose to an increased risk of cardiac dysrhythmias; no data are available on QT corrected for heart rate (QTc) in hyperthyroidism in vivo. DESIGN QT and RR intervals from 24 h ambulatory ECG Holter recording were measured in patients with hyperthyroidism and again following pharmacological achievement of stable euthyroidism for at least 2 months. PATIENTS We enrolled a total of 16 hyperthyroid patients with Graves' disease, six males and 10 females (mean age 47 ± 4 years, mean ± SEM); 13 healthy age‐ and sex‐matched subjects were utilized as a control group. MEASUREMENTS The QT analysis was carried out by a computerized algorithm (QTc was corrected by the heart rate by Bazett's formula). Serum total T4, total T3, free T4, free T3 and TSH concentrations were measured by a fully automated immunoenzymometric assay; plasma norepinephrine by automatized high‐pressure liquid chromatography, potassium and chloride by a potentiometric method, magnesium and calcium by a colourimetric method. RESULTS The 24‐h average QTc in the hyperthyroid patients was significantly prolonged compared to controls (458 ± 7 vs. 431 ± 6 ms, P = 0·01) and it returned to normal after treatment of thyrotoxicosis (432 ± 6 ms, P