Thrombocytosis and thrombocythemia are conditions in which your blood has a high number of platelets, also called thrombocytes, because a clot is also called a thrombus. When another disease or condition causes a high platelet count, the term "thrombocytosis" is preferred, or secondary or reactive thrombocytosis.

"Thrombocythemia" or primary or essential thrombocythemia are used when the cause of the high platelet count is not known.

A normal platelet count ranges from 150,000 to 450,000 platelets per microliter of blood. When the platelet count is less than 1 million platelets per microliter of blood, secondary thrombocytosis is more common than primary thrombocythemia.

With primary thrombocythemia, a high platelet count may occur alone or with other blood cell disorders. The platelet count can be as low as 500,000 platelets per microliter of blood or higher than 1 million platelets per microliter of blood. This is a rare occurrence.

Most people who have a high platelet count don't have signs or symptoms and those symptoms are rarely serious or life-threatening, but can include blood clots and bleeding. These symptoms mostly occur in people who have primary thrombocythemia.

People who have primary thrombocythemia without symptoms don't need treatment, as long as the condition remains stable. Others might need medicines or procedures to treat it. Most people who have primary thrombocythemia will live a normal life span.
Treatment and outlook for secondary thrombocytosis depends on its underlying cause.