In hematology, thrombocythemia is a condition of high platelet (thrombocyte) count in the blood. Normal count is in the range of 150×10 to 450×10 platelets per liter of blood, but investigation is typically only considered if the upper limit exceeds 750×10/L.
When the cause is unknown, the term thrombocythemia is used, as either primary thrombocythemia or essential thrombocythemia. The condition arises from a fault in the bone marrow cells leading to over-production of platelets but the cause of the fault is unknown, and this type is not common.