Possible causes of an enlarged heart include conditions that cause the heart to pump harder than normal, such as disease of the heart valve or muscle, and high blood pressure, according to Mayo Clinic. In some cases, the cause of an enlarged heart is unknown.
Several conditions may cause disease of a patient's heart valve, such as rheumatic fever or an infection, according to Mayo Clinic. Such conditions can cause damage to the heart valve, causing it to enlarge. Similarly, disease of the heart muscle can cause the tissue to stiffen and expand. As the muscles expand, it causes the heart to pump harder to move blood, which may cause an enlarged heart. High blood pressure can also cause a patient's heart to become enlarged by causing the heart to pump harder to circulate blood.
Pericardial effusion is another condition that can cause the heart to become enlarged, states Mayo Clinic. In patients with this condition, fluid fills the pericardium, which may cause the heart to appear enlarged when viewing it through an X-ray. Thyroid problems, including overactive and underactive thyroid, have also been known to cause an enlarged heart.
Other causes of the condition include the excessive buildup of iron in the body, called hemochromatosis, says Mayo Clinic. In people with this disease, the body is unable to metabolize iron, which causes it to build up inside different organs, including the heart.