The heart is a major target organ for thyroid hormone action. Severe overt hypothyroidism can result in diastolic hypertension, lowered cardiac output, impaired left ventricular contractility and diastolic relaxation, pericardial effusion and bradycardia. However, the function of the atrial pacemaker is usually normal and the degree by which the heart rate slows down is often modest. Here we report the case of a 20 year old male Caucasian with severe overt hypothyroidism. He presented with syncopation due to second degree atrioventricular block type Mobitz 2 and heart failure with reduced ejection fraction (38 %). Laboratory testing revealed a severe overt hypothyroidism with markedly elevated TSH (>100 mIU/L) and reduced fT3 and fT4 levels. The condition was caused by hypothyroid Graves' disease (Graves' disease with Hashimoto component). Although magnetic resonance imaging of the heart demonstrated decreased cardiac contractility and pericardial effusion, suggesting peri-myocarditis, plasma levels for BNP and troponin I were low. A possible infectious cause was unlikely, since testing for cardiotropic viruses was negative. The patient was treated with intravenous levothyroxine and after peripheral euthyroidism had been achieved, left ventricular ejection fraction returned to normal and pericardial effusion dissolved. Additionally, bradycardiac episodes abated, although intermittent second degree AV block was still occasionally present during the night. In conclusion, overt hypothyroidism may be associated by cardiac myxedema affecting both electrophysiology and contractility, observations that underscore the necessity of thyroid testing in different phenotypes of heart failure.