An enlarged heart is dangerous because it can be indicative of congenital heart problems, arrhythmia, high blood pressure and diseases of the heart valve, according to Mayo Clinic. Kidney disease and HIV infections can also cause an enlarged heart, according to WebMD.
High blood pressure requires the heart to pump harder to circulate blood through the body. This increased effort can cause the left ventricle and the upper chambers of the heart to begin to grow bigger, as noted by Mayo Clinic. Additionally, even a slight enlargement of the heart can signify the weakening of the heart valves due to certain medicines, radiation treatment, rheumatic fever or infectious endocarditis.
A condition called pericardial effusion, where fluid collects in the pericardium - the sac that surrounds the heart, can make the heart appear to be enlarged on an X-ray, as stated by Mayo Clinic. Another cause for an enlarged heart is chronic anemia. This condition causes an irregular heartbeat when the heart tries to pump harder to compensate for the lack of oxygen and red blood cells in the blood. An enlarged heart can also be caused by an underactive thyroid gland, or hypothyroidism, and an overactive thyroid gland, or hyperthyroidism.