How do other illnesses affect polycythemia?

One form of polycythemia, secondary polycythemia, is related to long-term exposure to low oxygen levels, or the presence of other diseases. Certain cysts or tumors on the liver, kidney, or brain can result in secondary polycythemia. These abnormal growths release a chemical that triggers bone marrow to produce large numbers of red blood cells, leading to thick blood as seen in other forms of polycythemia. In addition, diseases of the heart and lung that result in low levels of oxygen can also be the cause of secondary polycythemia.