Thrombocythemia is a condition of high platelet (thrombocyte) count in the blood. Normal count is in the range of 150x109 to 450x109 platelets per liter of blood, but investigation is typically only considered if the upper limit exceeds 750x109/L. When the cause is unknown, the term thrombocythemia is used, as either primary thrombocythemia or essential thrombocythemia. [Source: Wikipedia ]