It imposes significant socio-economic and health care burden to both patients and healthcare systems. Although the most common cause of HF is ischemic heart diseases, other less common causes such as hyperthyroidism (thyrotoxicosis), severe anemia, arrhythmia should also be considered during diagnosis to improve overall clinical management of HF. Case report. The 42-year-old man was admitted to cardiology department with mixed (inspiratory and expiratory) dyspnea at moderate effort, palpitations, fatigue, the loss in weight of about 15 kg during 9-10 months. Anamnesis: general condition worsened the last 2 months when appeared generalized edema and mixed dyspnea. During this time did not address to doctor, any treatment has not received. Physical examination revealed swelling in the legs, ankles, ascites, an irregular pulse, at a rate of 130 beats/min, BP- 110/70mmHg. On ECG atrial fibrillation with rate - 120-57 b/min, electric axis of heart is normal. Signs of left ventricular hypertrophy. The chest X-ray -pulmonary congestion, bilateral pleural effusion. The abdominal X-ray – fluid levels with air on the left. On TTE- thickening of the walls of the aorta and valve apparatus. Dilatation of all heart chambers, significant dilatation of the right atrium and right ventricle, and moderate dilatation of the left atrium and the left ventricle. Contractile function of the left ventricular myocardium is moderately reduced. Ejection fraction = 42%. The second degree mitral regurgitation and third-fourth -degree tricuspid regurgitation. Moderate pulmonary arterial hypertension (PASP= 52mmHg). Sheets of the pericardium are thickened. Fluid in the pleural cavity up to 11 millimeters in the region of the right atrium. Bilateral pleurisy - inhomogeneous fluid with floating elements on the left - about 1,000 milliliters, to the right - about 800 milliliters. Сytological analysis of fluid from pleural cavity pointed to the inflammatory etiology of the effusion. On the ultrasound examination of the thyroid gland – fourth –degree hyperplasia, multiple diffuse changes.On the ultrasound examination of abdominal cavity - ascites, bilateral pleuritic, diffuse changes in the parenchyma of the liver. The glycemic profile -7-00: 4.7 mmol/l, 13-00: 6.3 mmol/l, 17-00: 10.6 mmol/l, glycated hemoglobin - 5,6%. Analysis of thyroid hormones- free Triiodothyronine – 17,22 Pmol/l, free Thyroxine – 79,52 Pmol/l. TSH – ‹ 0, 05 uIU/ml; anti TPO- 144 IU/ml. Tumor marker CA 19-9 - <3.0 U/ml. During hospitalization was consulted by endocrinologist, surgeon. After 11 days of complex treatment with diuretics, anticoagulants, beta-adrenoblockers, antithyroid drugs, cardiac glycosides, corticosteroids, histamine-2receptor blockers - the general condition improved: dyspnea and general swelling disappeared, general weakness was reduced. Conclusions. The incidence and prevalence of thyrotoxic heart failure (THF) provide a wide variation from 12% to 68% in hyperthyroid patients. Up to 90% of patients with thyrotoxicosis may develop Atrial Fibrillation, 47% Left Ventricle systolic dysfunction and 1% dilated THF and a third of these cases are reversible. Mortality in THF patients is 1.2 higher than in patients with hypertension, valvular heart disease or coronary artery disease, and 1.4 higher than in the general population. Hyperthyroidism is a potentially reversible and curable cause of THF, so it should be excluded in every new patient with HF, especially in young patients and in the absence of coronary artery disease and other structural heart diseases.