The article describes a clinical case of the occurrence of complex disorders of the heart rhythm and conductivity in a patient of 45 years with decompensated hypothyroidism. The literature data on specific cardiomyopathy appearing in patients with hypothyroidism (“hypothyroid heart”), metabolic, hemodynamic changes against the background of hypothyroidism and possible violations of heart rhythm and conductivity are given. In the described clinical case the patient’s medical history is given, who has been observed in an endocrinologist for hypothyroidism for the last 3 years. 6 months before this hospitalization patient stopped taking levothyroxine on her own initiative. She noticed deterioration in the last 3 months: the clinic of hypothyroidism appeared and began to grow, a frequent extrasystole appeared. At an objective survey — a symptomatology of the expressed hypothyroidism. A comprehensive examination of the patient was performed, which revealed atrioventricular blockade of the 1st degree, frequent ventricular extrasystole, prolongation of the QT interval on the ECG, late ventricular potentials against the background of low concentrations of sodium and potassium in the blood. During the first 24 hours of hospitalization, there was a clinical death. A clear positive dynamics was traced against the background of therapy aimed at the correction of electrolyte disorders, as well as hormone replacement therapy with levothyroxine.