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.
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 cancer—mostly 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:
Some conditions can lead to a high platelet count that lasts for only a short time. Examples of such conditions include:
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.