What causes Thrombocythemia and Thrombocytosis ?
Primary Thrombocythemia
                
In this condition, faulty stem cells in the bone marrow make too many platelets. What causes this to happen usually isn't known. When this process occurs without other blood cell disorders, it's called essential thrombocythemia.
                
A rare form of thrombocythemia is inherited. ("Inherited" means the condition is passed from parents to children through the genes.) In some cases, a genetic mutation may cause the condition.
                
In addition to the bone marrow making too many platelets, the platelets also are abnormal in primary thrombocythemia. They may form blood clots or, surprisingly, cause bleeding when they don't work well.
                
Bleeding also can occur because of a condition that develops called von Willebrand disease. This condition affects the blood clotting process.
                
After many years, scarring of the bone marrow can occur.
                
Secondary Thrombocytosis
                
This condition occurs if another disease, condition, or outside factor causes the platelet count to rise. For example, 35 percent of people who have high platelet counts also have cancermostly lung, gastrointestinal, breast, ovarian, and lymphoma. Sometimes a high platelet count is the first sign of cancer.
                
Other conditions or factors that can cause a high platelet count are:
                
Iron-deficiency anemia (uh-NEE-me-uh)
                
Hemolytic (HEE-moh-lit-ick) anemia
                
Absence of a spleen (after surgery to remove the organ)
                
Inflammatory or infectious diseases, such as connective tissue disorders, inflammatory bowel disease, and tuberculosis
                
Reactions to medicine
                
Some conditions can lead to a high platelet count that lasts for only a short time. Examples of such conditions include:
                
Recovery from serious blood loss
                
Recovery from a very low platelet count caused by excessive alcohol use and lack of vitamin B12 or folate
                
Acute (short-term) infection or inflammation
                
Response to physical activity
                
Although the platelet count is high in secondary thrombocytosis, the platelets are normal (unlike in primary thrombocythemia). Thus, people who have secondary thrombocytosis have a lower risk of blood clots and bleeding.