According to the search results, the most common cause of high platelet count is reactive thrombocytosis or secondary thrombocytosis. This occurs when something outside the bone marrow stimulates the marrow to make more platelets. Inflammatory conditions like autoimmune diseases, cancer or trauma, as well as certain infections and iron deficiency, are common causes of a high platelet count. Other possible causes of high platelet count include essential thrombocytosis, which is a less common cause of high platelet counts and is a blood and bone marrow disease, and thrombocythemia, which is most often caused by genetic mutations in the genes that control how bone marrow forms platelets. It is important to determine the cause of high platelet count to choose the best treatment options. A referral to a hematologist can help discern the cause of high platelet numbers.