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Thrombocythemia and Thrombocytosis


The term “thrombocythemia” is preferred when the cause of a high platelet count isn’t known. The condition sometimes is called primary or essential thrombocythemia. This condition occurs if faulty cells in the bone marrow make too many platelets. Bone marrow is the sponge-like tissue inside the bones. It contains stem cells that develop into red blood cells, white blood cells, and platelets. What causes the bone marrow to make too many platelets often isn’t known.1


With primary thrombocythemia, a high platelet count may occur alone or with other blood cell disorders. This condition isn’t common. When another disease or condition causes a high platelet count, the term “thrombocytosis” is preferred. This condition often is called secondary or reactive thrombocytosis. Secondary thrombocytosis is more common than primary thrombocythemia.1


Often, a high platelet count doesn’t cause signs or symptoms. Rarely, serious or life-threatening symptoms can develop, such as blood clots and bleeding. These symptoms are more likely to occur in people who have primary thrombocythemia.1



People who have thrombocythemia or thrombocytosis may not have signs or symptoms. These conditions might be discovered only after routine blood tests. However, people who have primary thrombocythemia are more likely than those who have secondary thrombocytosis to have serious signs and symptoms. The signs and symptoms of a high platelet count are linked to blood clots and bleeding. They include weakness, bleeding, headache, dizziness, chest pain, and tingling in the hands and feet.1


Causes and Risk Factors

Primary Thrombocythemia – A rare form of thrombocythemia is inherited. A genetic mutation may cause the condition.


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 cancer — mostly lung, gastrointestinal, breast, ovarian, and lymphoma. Sometimes a high platelet count is the first sign of cancer.1


Risk Factors
  • Iron-deficiency anemia 
  • Hemolytic anemia
  • Absence of a spleen 
  • Inflammatory or infectious diseases
  • Reactions to medicine


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.1

1. National Heart, Lung and Blood Institute.
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