Thrombocythemia (THROM-bo-si-THE-me-ah) and thrombocytosis (THROM-bo-si-TO-sis) are conditions in which your blood has a higher than normal number of platelets (PLATE-lets).
Platelets are blood cell fragments. They're made in your bone marrow along with other kinds of blood cells.
Platelets travel through your blood vessels and stick together (clot). Clotting helps stop any bleeding that may occur if a blood vessel is damaged. Platelets also are called thrombocytes (THROM-bo-sites) because a blood clot also is called a thrombus.
A normal platelet count ranges from 150,000 to 450,000 platelets per microliter of blood.
The term "thrombocythemia" is preferred when the cause of a high platelet count isn't known. The condition sometimes is called primary or essential thrombocythemia.
This condition occurs if faulty cells in the bone marrow make too many platelets. Bone marrow is the sponge-like tissue inside the bones. It contains stem cells that develop into red blood cells, white blood cells, and platelets. What causes the bone marrow to make too many platelets often isn't known.
With primary thrombocythemia, a high platelet count may occur alone or with other blood cell disorders. This condition isn't common.
When another disease or condition causes a high platelet count, the term "thrombocytosis" is preferred. This condition often is called secondary or reactive thrombocytosis. Secondary thrombocytosis is more common than primary thrombocythemia.
Often, a high platelet count doesn't cause signs or symptoms. Rarely, serious or life-threatening symptoms can develop, such as blood clots and bleeding. These symptoms are more likely to occur in people who have primary thrombocythemia.
People who have primary thrombocythemia with no signs or symptoms don't need treatment, as long as the condition remains stable.
Other people who have this condition may need medicines or procedures to treat it. Most people who have primary thrombocythemia will live a normal lifespan.
Treatment and outlook for secondary thrombocytosis depend on its underlying cause.