Platelets are parts of the blood that help form blood clots. Thrombocytosis (throm-boe-sie-TOE-sis) is a disorder in which your body produces too many platelets.

It's called reactive thrombocytosis or secondary thrombocytosis when the cause is an underlying condition, such as an infection.

Less commonly, when the high platelet count has no apparent underlying condition as a cause, the disorder is called primary thrombocythemia or essential thrombocythemia. This is a blood and bone marrow disease.

A high platelet level may be detected in a routine blood test known as a complete blood count. It's important to determine whether it's reactive thrombocytosis or essential thrombocythemia to choose the best treatment options.


People with high platelet levels often don't have signs or symptoms. When symptoms occur, they're often related to blood clots. Examples include:

  • Headache.
  • Confusion or changes in speech.
  • Chest pain.
  • Shortness of breath and nausea.
  • Weakness.
  • Burning pain in the hands or feet.

Less commonly, very high platelet levels may cause bleeding. This can cause:

  • Nosebleeds.
  • Bruising.
  • Bleeding from your mouth or gums.
  • Bloody stool.


Bone marrow is a spongy tissue inside your bones. It 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:

  • 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 and other types of traumas.

Essential thrombocythemia

The cause of this disorder is unclear. It often appears to be connected to changes in certain genes. The bone marrow produces too many of the cells that form platelets, and these platelets often don't work properly. This poses a much higher risk of clotting or bleeding complications than does reactive thrombocytosis.


Essential thrombocythemia can lead to a variety of potentially life-threatening complications, such as:

  • Strokes. If a blood clot occurs in the arteries that supply the brain, it may cause a stroke. A mini stroke, also called a transient ischemic attack, is a temporary interruption of blood flow to part of the brain.
  • Heart attacks. Less commonly, essential thrombocythemia can cause clots in the arteries that supply blood to your heart.
  • Cancer. Rarely, essential thrombocythemia may lead to a type of leukemia that progresses quickly.

Pregnancy complications

Most women who have essential thrombocythemia have normal, healthy pregnancies. But uncontrolled thrombocythemia can lead to miscarriage and other complications. Your risk of pregnancy complications may be reduced with regular checkups and medication, so be sure to have your doctor regularly monitor your condition.