Essential thrombocythemia is a type of chronic myeloproliferative disorder. That means the bone marrow, the spongy tissue inside your bones, makes too many of a certain type of cell. In the case of essential thrombocythemia, the bone marrow makes too many cells that create platelets.
It's not clear what causes this to happen. About 90 percent of people with the disorder have an acquired gene mutation contributing to the disease.
Platelets stick together to help form blood clots. Normally, blood clots stop bleeding when you damage a blood vessel, such as when you get a cut.
A normal platelet count ranges from 150,000 to 450,000 platelets per microliter of blood. Someone with essential thrombocythemia has more than 450,000 platelets per microliter of blood.
Plus, the excess platelets may not function normally, leading to abnormal clotting or bleeding.
If an underlying condition such as an infection or iron deficiency causes a high platelet count, it's called reactive, or secondary, thrombocythemia. Secondary thrombocythemia causes less risk of blood clots and bleeding than does essential thrombocythemia.