Thrombocythemia may be a condition of high platelet (thrombocyte) count within the blood. A normal count is in the range of 150,000 to 450,000 platelets per microlitre of blood, but the investigation is only considered if the upper limit exceeds 750,000. When the cause is unknown, the term thrombocythemia is employed, as either primary thrombocythyemia or essential thrombocythemia. The condition arises from a fault in the bone marrow cells that overproduce the platelets. The cause of the fault is unknown, and this type is not common. When the cause is understood like another disorder or disease, the term thrombocytosis is preferred, as either secondary or reactive thrombocytosis. Reactive thrombocytosis is that the commonest type and though it can often haven't any symptoms it can sometimes predispose to thrombosis. In contrast, thrombocytopenia refers to abnormally low blood platelet numbers in the blood.