Thrombocytopenia (low platelet count) can occur due to any type of cancer that has metastasized to the bone marrow, or cancers that directly affect the bone marrow (e.g., blood and marrow cancers like leukemia, lymphoma). If thrombocytopenia is due to bone marrow involvement by cancer, a decrease in other blood values such as hemoglobin levels and leukocyte (white blood cell) count is usually also observed. Less frequently, the cancer itself can trigger an abnormal immune response, leading to the destruction of platelets and consequently, thrombocytopenia in isolation.