Hyperthyroidism is associated with lengthening of ventricular repolarization

OBJECTIVE Lengthened ventricular repolarization, as assessed by the QT inte rval on electrocardiogram (ECG), can predispose to an increased risk of car diac dysrhythmias; no data are available on QT corrected for heart rate (QT c) in hyperthyroidism in vivo. DESIGN QT and RR intervals from 24 h ambulatory ECG Holter recording were m easured in patients with hyperthyroidism and again following pharmacologica l achievement of stable euthyroidism for at least 2 months. PATIENTS We enrolled a total of 16 hyperthyroid patients with Graves' disea se, six males and 10 females (mean age 47 +/- 4 years, mean +/- SEM); 13 he althy age- and sex-matched subjects were utilized as a control group. MEASUREMENTS The QT analysis was carried out by a computerized algorithm (Q Tc was corrected by the heart rate by Bazett's formula). Serum total T4, to tal T3, free T4, free T3 and TSH concentrations were measured by a fully au tomated 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 < 0.05 vs. time H, NS vs. controls). QTc positively correlated with FT3 (r = 0.63, P < 0.001) and with FT4 (r = 0.481, P < 0.02). Conversely, QTc did no t correlate with plasma basal norepinephrine levels, nor with electrolytes. CONCLUSIONS Hyperthyroidism is associated with prolonged QTc that normalize s once the patient becomes euthyroid. The strong positive correlation betwe en FT3 and QTc supports the hypothesis of an important role of thyroid horm one on modulation of QTc lengthening.