Authors report about a left ventricular thrombus as a complication of thyrotoxic heart disease, on a 45 years old woman. Clinical state featured signs of thyrotoxicosis, global cardiac failure and hypertension. The electrocardiogram showed a left axis deviation, the chest X-ray a cardiac enlargement (cardio-thoracic index = 0.55) on behalf of left movement of heart walls and severe left ventricule dysfunction (Ejection fraction about 18%). This exam also found a big thrombus at he left ventricule apex. The treatment disappearance of the thrombosis and improvement of cardiac failure signs and echographic parameters. Authors discuss circumstances leading to thrombosis in thyrotoxic heart disease, and the usefulness of anticoagulant drug therapy.