Thrombocytosis is a condition where you have more platelets than normal in your blood. Platelets are a kind of blood cell that helps your blood clot. Your bone marrow makes your platelets along with your red and white blood cells. Most often, you will not have any signs. Some people have signs like bruises, bleeding, or blood clots. Too many platelets can lead to stroke, heart problems, or blood clots. Thrombocytosis is caused by some other health problem or certain drugs. Your platelet count is high, but the platelets themselves are normal.