Bone marrow — spongy tissue inside your bones — contains stem cells that can become red blood cells, white blood cells or platelets (thrombocytes). Platelets stick together, helping blood to form a clot that stops 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.

If you have essential, or primary, thrombocythemia, your bone marrow makes too many platelet-forming cells (megakaryocytes). The excess platelets may not function normally, leading to abnormal clotting or bleeding.

The exact cause of essential thrombocythemia and similar conditions, known as myeloproliferative disorders, isn't known. About 80 percent of people with the disorder have an acquired gene mutation contributing to the disease. This may involve the Janus kinase 2 (JAK2), calreticulin (CALR) or MPL gene mutation.

A high platelet count that's caused by an underlying condition such as an infection or iron deficiency is called reactive, or secondary, thrombocythemia. People with secondary thrombocythemia have less risk of blood clots and bleeding than do people with essential thrombocythemia.

June 24, 2015