Enlarged heart: Enlargement of the heart. An enlarged heart is a descriptive term that is used to refer to the physical finding of an enlarged heart and is not a disease itself. Heart enlargement can be caused by a number of different conditions including diseases of the heart muscle or heart valves, high blood pressure, arrhythmias, and pulmonary hypertension. Enlarged heart can also sometimes accompany longstanding anemia and thyroid disease, among other conditions. Treatment and prognosis are dependent upon the underlying cause. Also referred to medically as cardiomegaly.