Thrombocytosis (throm-boe-sie-TOE-sis) is a disorder in which your body produces too many platelets (thrombocytes), which play an important role in blood clotting. The disorder is called reactive thrombocytosis when it's 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.

Your doctor may detect thrombocytosis in routine blood test results that show a high platelet level. If your blood test indicates thrombocytosis, it's important for your doctor to determine whether it's reactive thrombocytosis or if you have thrombocythemia, which is more likely to cause blood clots.

Symptoms Causes Risk factors Complications

Reactive thrombocytosis rarely causes symptoms. More often, signs and symptoms relate to the underlying condition. If symptoms of reactive thrombocytosis do occur, they may include:

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

When to see a doctor

Because thrombocytosis isn't likely to cause symptoms, you probably won't know you have the condition unless a routine blood test reveals a higher than normal number of platelets. If your blood test results show a high platelet count, your doctor will try to determine the reason.

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