Thrombocytosis is a disorder when the body produces too many platelets (thrombocytes), which play an important role in blood clotting. The disorder is called reactive thrombocytosis when it is caused by an underlying condition, such as an infection.

Thrombocytosis may also be caused by a blood and bone marrow disease. When caused by a bone marrow disorder, thrombocytosis is called autonomous, primary or essential thrombocytosis or essential thrombocythemia.

Thrombocytosis may be detected in routine blood test results which show a high platelet level. If the blood test indicates thrombocytosis, the doctor should determine whether  it is reactive thrombocytosis or thrombocythemia, the latter more likely to cause blood clots.


Reactive thrombocytosis does not cause many symptoms. When they occur, they may include:

  • Headache
  • Dizziness or lightheadedness
  • Chest pain
  • Weakness
  • Fainting
  • Temporary vision changes
  • Numbness or tingling of the hands and feet


The bones contain a spongy tissue, the bone marrow, which has stem cells that can be transformed into red blood cells, white blood cells or platelets. Platelets travel through  the blood vessels and stick together to form clots that stop the bleeding when the blood vessel is damaged, e.g. when you cut the finger. Normally, platelet count ranges from 150,000 to 450,000 platelets per microliter of blood.

In case of  thrombocytosis caused by a bone marrow disorder (essential thrombocythemia), the bone marrow overproduces the cells that form platelets and releases too many platelets into the blood. If the blood test results reveal a high platelet count, it is essential to determine whether it is the case of essential thrombocythemia or reactive thrombocytosis.

Reactive thrombocytosis causes are:

  • Iron deficiency anemia
  • Acute bleeding and blood loss
  • Allergic reactions
  • Cancer
  • Chronic kidney failure or another kidney disorder
  • Exercise
  • Heart attack
  • Infections
  • 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
  • Inflammation, such as from rheumatoid arthritis, celiac disease, connective tissue disorders or inflammatory bowel disease
  • Major surgery
  • Pancreatitis
  • Trauma

Some medications can cause reactive thrombocytosis such as:

  • Epinephrine (Adrenalin Chloride, EpiPen)
  • Tretinoin
  • Vincristine

Risk Factors

A person may be at risk of thrombocytosis if they have a medical condition such as iron deficiency anemia or after surgery.


If the high platelet count results from a bone marrow disease (essential thrombocythemia), and not  reactive thrombocytosis, the patient may be at risk of developing blood clots, some of which can be life-threatening.


  • Sometimes it is not possible to prevent conditions that lead to secondary thrombocytosis. But, with routine medical care, these conditions may be detected prior to the development of a high platelet count.