Background. Elderly patients in the presence of iodine deficiency most often develop functional autonomy of the thyroid gland, accompanied by dysfunction — hyperthyroidism. Due to an excess of thyroid hormones, thyrotoxic cardiomyopathy is formed, in which in most cases there is a syndrome of heart rhythm disturbances — atrial fibrillation. The aim of the study: to find out the algorithm for the differential search for a disease in the presence of hyperthyroidism with severe cardiovascular disorders using the example of a specific elderly patient from real clinical practice. Clinical case. A 61-year-old patient was hospitalized in the intensive care unit for arrhythmogenic shock against the background of an attack of atrial fibrillation. The absence of myocardial infarction, pulmonary embolism, acute adrenal insufficiency was stated. The examination revealed a significant decrease in the level of TSH along with an increase in the levels of thyroid hormones without the phenomena of autoaggression, hyperglycemia, hypocholesterolemia, a tendency to hypoproteinemia, leukopenia. An increase in the thyroid gland, heterogeneity of the structure with hypovascularization, the presence of small multiple nodular formations that require clarification were determined. The patient has atrial fibrillation with variability in blood pressure, impaired repolarization processes, hyperkinetic type of hemodynamics, reflecting cardiomyopathy. In this case, there are all prerequisites for the development of functional autonomy of the thyroid gland: age, endemic region, absence of autoimmune changes, dermopathy, ophthalmopathy. Attention is drawn to the severity of manifestations of thyrotoxic heart with dyshormonal and catabolic syndromes, superimposed on comorbid coronary heart disease, hypertension. Conclusions. Hyperthyroidism in the elderly is manifested primarily by cardiovascular disorders, most often with the presence of an arrhythmia syndrome in the form of atrial fibrillation, which can provoke arrhythmogenic shock. Functional autonomy of the thyroid gland with the formation of thyrotoxic cardiomyopathy has specific clinical course, especially in comorbid conditions, which significantly impairs the quality of life.