Causes

Bone marrow — spongy tissue inside your bones — contains stem cells that can become red blood cells, white blood cells or platelets. Platelets stick together, helping blood to form a clot that stops bleeding when you damage a blood vessel, such as when you cut yourself. Thrombocytosis occurs when your body produces too many platelets.

Reactive thrombocytosis

This is the more common type of thrombocytosis. It’s caused by an underlying medical problem, such as:

• Acute bleeding and blood loss
• Cancer
• Infections
• Iron deficiency
• Removal of your spleen
• Hemolytic anemia — a type of anemia in which your body destroys red blood cells faster than it produces them, often due to certain blood diseases or autoimmune disorders
• Inflammatory disorders, such as rheumatoid arthritis, sarcoidosis or inflammatory bowel disease
• Surgery or other type of trauma

Essential thrombocythemia

The cause of this disorder is unclear but it often appears to be connected to mutations in a variety of genes. Your bone marrow produces too many of the cells that form platelets and these platelets are often abnormal. This poses a much higher risk of clotting or bleeding complications than does reactive thrombocytosis.