Few publications report the possibility of "torsade de pointes"" (TdP) type tachycardia and ventricular fibrillation occurring in patients with prolonged QT syndrome in the course of hypothyroidism. The authors present the case of a 51-year-old woman with Hashimoto disease and hypothyroidism, inadequately supplemented with L-thyroxine. This patient was admitted to the ward with symptoms of hypometabolic crisis. She was diagnosed with shock and had marked sinus bradycardia with a QT interval prolongation of 0.8 s and paroxysmal TdP-type ventricular tachycardia. TdP was triggered by R-on-T ventricular extrasystoles. Laboratory tests revealed profound hypothyroidism. As pharmacological treatment of the arrhythmia and bradycardia was ineffective, an endocavital electrode was introduced. As a result of ventricular stimulation with a frequency of 90/min, shortening of the QT interval and disappearance of the cardiac arrhythmia and cardiogenic shock symptoms were obtained. L-thyroxine was administered and two days later ventricular stimulation was discontinued. When corrected for heart rate, the QT interval (QTc) was 0.43 s and no QT prolongation was detected in the course of a follow-up period of several months, during which L-thyroxine supplementation was adequate. Hypothyroidism may be the cause of life-threatening arrhythmias secondary to acquired long QT syndrome. Ventricular electrostimulation proved to be a life-saving intervention in this case in which prolonged QT syndrome and ventricular cardiac arrhythmias complicated hypometabolic crisis. (Cardiol J 2007; 14: 198-201).