Thrombocythemia isn't common. The exact number of people who have the condition isn't known. Some estimates suggest that 24 out of every 100,000 people have primary thrombocythemia.
Primary thrombocythemia is more common in people aged 50 to 70, but it can occur at any age. For unknown reasons, more women around the age of 30 have primary thrombocythemia than men of the same age.
You might be at risk for secondary thrombocytosis if you have a disease, condition, or factor that can cause it. (For more information, go to "What Causes Thrombocythemia and Thrombocytosis?")
Secondary thrombocytosis is more common than primary thrombocythemia. Studies have shown that most people who have platelet counts over 500,000 have secondary thrombocytosis.