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What Are Thrombocythemia and Thrombocytosis?

Thrombocythemia (THROM-bo-si-THE-me-ah) and thrombocytosis (THROM-bo-si-TO-sis) are conditions in which your blood has a higher than normal number of platelets (PLATE-lets).

Platelets are blood cell fragments. They're made in your bone marrow along with other kinds of blood cells.

Platelets travel through your blood vessels and stick together (clot). Clotting helps stop any bleeding that may occur if a blood vessel is damaged. Platelets also are called thrombocytes (THROM-bo-sites) because a blood clot also is called a thrombus.

A normal platelet count ranges from 150,000 to 450,000 platelets per microliter of blood.


The term "thrombocythemia" is preferred when the cause of a high platelet count isn't known. The condition sometimes is called primary or essential thrombocythemia.

This condition occurs if faulty cells in the bone marrow make too many platelets. Bone marrow is the sponge-like tissue inside the bones. It contains stem cells that develop into red blood cells, white blood cells, and platelets. What causes the bone marrow to make too many platelets often isn't known.

With primary thrombocythemia, a high platelet count may occur alone or with other blood cell disorders. This condition isn't common.

When another disease or condition causes a high platelet count, the term "thrombocytosis" is preferred. This condition often is called secondary or reactive thrombocytosis. Secondary thrombocytosis is more common than primary thrombocythemia.

Often, a high platelet count doesn't cause signs or symptoms. Rarely, serious or life-threatening symptoms can develop, such as blood clots and bleeding. These symptoms are more likely to occur in people who have primary thrombocythemia.


People who have primary thrombocythemia with no signs or symptoms don't need treatment, as long as the condition remains stable.

Other people who have this condition may need medicines or procedures to treat it. Most people who have primary thrombocythemia will live a normal lifespan.

Treatment and outlook for secondary thrombocytosis depend on its underlying cause.

Other Names for Thrombocythemia and Thrombocytosis

Primary thrombocythemia also is called:

  • Essential thrombocythemia. This term is used when a high platelet count occurs alone (that is, without other blood cell disorders).
  • Idiopathic (id-ee-o-PATH-ick) thrombocythemia.
  • Primary or essential thrombocytosis (these are less favored terms).

Thrombocytosis also is known as:

  • Secondary or reactive thrombocytosis.
  • Secondary thrombocythemia (this is a less favored term).

What Causes Thrombocythemia and Thrombocytosis?

Primary Thrombocythemia

In this condition, faulty stem cells in the bone marrow make too many platelets. What causes this to happen usually isn't known. When this process occurs without other blood cell disorders, it's called essential thrombocythemia.

A rare form of thrombocythemia is inherited. ("Inherited" means the condition is passed from parents to children through the genes.) In some cases, a genetic mutation may cause the condition.

In addition to the bone marrow making too many platelets, the platelets also are abnormal in primary thrombocythemia. They may form blood clots or, surprisingly, cause bleeding when they don't work well.

Bleeding also can occur because of a condition that develops called von Willebrand disease. This condition affects the blood clotting process.

After many years, scarring of the bone marrow can occur.

Secondary Thrombocytosis

This condition occurs if another disease, condition, or outside factor causes the platelet count to rise. For example, 35 percent of people who have high platelet counts also have cancer—mostly lung, gastrointestinal, breast, ovarian, and lymphoma. Sometimes a high platelet count is the first sign of cancer.

Other conditions or factors that can cause a high platelet count are:

Some conditions can lead to a high platelet count that lasts for only a short time. Examples of such conditions include:

  • Recovery from serious blood loss
  • Recovery from a very low platelet count caused by excessive alcohol use and lack of vitamin B12 or folate
  • Acute (short-term) infection or inflammation
  • Response to physical activity

Although the platelet count is high in secondary thrombocytosis, the platelets are normal (unlike in primary thrombocythemia). Thus, people who have secondary thrombocytosis have a lower risk of blood clots and bleeding.

Who Is at Risk for Thrombocythemia or Thrombocytosis?

Primary Thrombocythemia

Thrombocythemia isn't common. The exact number of people who have the condition isn't known. Some estimates suggest that 24 out of every 100,000 people have primary thrombocythemia.

Primary thrombocythemia is more common in people aged 50 to 70, but it can occur at any age. For unknown reasons, more women around the age of 30 have primary thrombocythemia than men of the same age.

Secondary Thrombocytosis

You might be at risk for secondary thrombocytosis if you have a disease, condition, or factor that can cause it. (For more information, go to "What Causes Thrombocythemia and Thrombocytosis?")

Secondary thrombocytosis is more common than primary thrombocythemia. Studies have shown that most people who have platelet counts over 500,000 have secondary thrombocytosis.

What Are the Signs and Symptoms of Thrombocythemia and Thrombocytosis?

People who have thrombocythemia or thrombocytosis may not have signs or symptoms. These conditions might be discovered only after routine blood tests.

However, people who have primary thrombocythemia are more likely than those who have secondary thrombocytosis to have serious signs and symptoms.

The signs and symptoms of a high platelet count are linked to blood clots and bleeding. They include weakness, bleeding, headache, dizziness, chest pain, and tingling in the hands and feet.

Blood Clots

In primary thrombocythemia, blood clots most often develop in the brain, hands, and feet. But they can develop anywhere in the body, including in the heart and intestines.

Blood clots in the brain may cause symptoms such as chronic (ongoing) headache and dizziness. In extreme cases, stroke may occur.

Blood clots in the tiny blood vessels of the hands and feet leave them numb and red. This may lead to an intense burning and throbbing pain felt mainly on the palms of the hands and the soles of the feet.

Other signs and symptoms of blood clots may include:

  • Changes in speech or awareness, ranging from confusion to passing out
  • Seizures
  • Upper body discomfort in one or both arms, the back, neck, jaw, or abdomen
  • Shortness of breath and nausea (feeling sick to your stomach)

In pregnant women, blood clots in the placenta can cause miscarriage or problems with fetal growth and development.

Women who have primary thrombocythemia or secondary thrombocytosis and take birth control pills are at increased risk for blood clots.

Blood clots are related to other conditions and factors as well. Older age, prior blood clots, diabetes, high blood pressure, and smoking also increase your risk for blood clots.


If bleeding occurs, it most often affects people who have platelet counts higher than 1 million platelets per microliter of blood. Signs of bleeding include nosebleeds, bruising, bleeding from the mouth or gums, or blood in the stools.

Although bleeding usually is associated with a low platelet count, it also can occur in people who have high platelet counts. Blood clots that develop in thrombocythemia or thrombocytosis may use up your body's platelets. This means that not enough platelets are left in your bloodstream to seal off cuts or breaks on the blood vessel walls.

Another cause of bleeding in people who have very high platelets counts is a condition called von Willebrand Disease. This condition affects the blood clotting process.

In rare cases of primary thrombocythemia, the faulty bone marrow cells will cause a form of leukemia (lu-KE-me-ah). Leukemia is a cancer of the blood cells.

How Are Thrombocythemia and Thrombocytosis Diagnosed?

Your doctor will diagnose thrombocythemia or thrombocytosis based on your medical history, a physical exam, and test results. A hematologist also may be involved in your care. This is a doctor who specializes in blood diseases and conditions.

Medical History

Your doctor may ask you about factors that can affect your platelets, such as:

  • Any medical procedures or blood transfusions you've had
  • Any recent infections or vaccines you've had
  • The medicines you take, including over-the-counter medicines
  • Your general eating habits, including the amount of alcohol you normally drink
  • Any family history of high platelet counts

Physical Exam

Your doctor will do a physical exam to look for signs and symptoms of blood clots and bleeding. He or she also will check for signs of conditions that can cause secondary thrombocytosis, such as an infection.

Primary thrombocythemia is diagnosed only after all possible causes of a high platelet count are ruled out. For example, your doctor may recommend tests to check for early, undiagnosed cancer. If another disease, condition, or factor is causing a high platelet count, the diagnosis is secondary thrombocytosis.

Diagnostic Tests

Your doctor may recommend one or more of the following tests to help diagnose a high platelet count.

Complete Blood Count

A complete blood count (CBC) measures the levels of red blood cells, white blood cells, and platelets in your blood. For this test, a small amount of blood is drawn from a blood vessel, usually in your arm.

If you have thrombocythemia or thrombocytosis, the CBC results will show that your platelet count is high.

Blood Smear

A blood smear is used to check the condition of your platelets. For this test, a small amount of blood is drawn from a blood vessel, usually in your arm. Some of your blood is put on a glass slide. A microscope is then used to look at your platelets.

Bone Marrow Tests

Bone marrow tests check whether your bone marrow is healthy. Blood cells, including platelets, are made in the bone marrow. The two bone marrow tests are aspiration (as-pih-RA-shun) and biopsy.

Bone marrow aspiration might be done to find out whether your bone marrow is making too many platelets. For this test, your doctor removes a sample of fluid bone marrow through a needle. He or she examines the sample under a microscope to check for faulty cells.

A bone marrow biopsy often is done right after an aspiration. For this test, your doctor removes a small amount of bone marrow tissue through a needle. He or she examines the tissue to check the number and types of cells in the bone marrow.

With thrombocythemia and thrombocytosis, the bone marrow has a higher than normal number of the very large cells that make platelets.

Other Tests

Your doctor may recommend other blood tests to look for genetic factors that can cause a high platelet count.

How Are Thrombocythemia and Thrombocytosis Treated?

Primary Thrombocythemia

This condition is considered less harmful today than in the past, and its outlook often is good. People who have no signs or symptoms don't need treatment, as long as the condition remains stable.

Taking aspirin may help people who are at risk for blood clots (aspirin thins the blood). However, talk with your doctor about using aspirin because it can cause bleeding.

Doctors prescribe aspirin to most pregnant women who have primary thrombocythemia. This is because it doesn't have a high risk of side effects for the fetus.

Some people who have primary thrombocythemia may need medicines or medical procedures to lower their platelet counts.

Medicines To Lower Platelet Counts

You may need medicines to lower your platelet count if you:

You'll need to take these medicines throughout your life.

Hydroxyurea. This platelet-lowering medicine is used to treat cancers and other life-threatening diseases. Hydroxyurea most often is given under the care of doctors who specialize in cancer or blood diseases. Patients on hydroxyurea are closely monitored.

Currently, hydroxyurea plus aspirin is the standard treatment for people who have primary thrombocythemia and are at high risk for blood clots.

Anagrelide. This medicine also has been used to lower platelet counts in people who have thrombocythemia. However, research shows that when compared with hydroxyurea, anagrelide has worse outcomes. Anagrelide also has side effects, such as fluid retention, palpitations (pal-pih-TA-shuns), arrhythmias (ah-RITH-me-ahs), heart failure, and headaches.

Interferon alfa. This medicine lowers platelet counts, but 20 percent of patients can't handle its side effects. Side effects include a flu-like feeling, decreased appetite, nausea (feeling sick to the stomach), diarrhea, seizures, irritability, and sleepiness.

Doctors may prescribe this medicine to pregnant women who have primary thrombocythemia because it's safer for a fetus than hydroxyurea and anagrelide.


Plateletpheresis (PLATE-let-fe-REH-sis) is a procedure used to rapidly lower your platelet count. This procedure is used only for emergencies. For example, if you're having a stroke due to primary thrombocythemia, you may need plateletpheresis.

An intravenous (IV) needle that's connected to a tube is placed in one of your blood vessels to remove blood. The blood goes through a machine that removes platelets from the blood. The remaining blood is then put back into you through an IV line in one of your blood vessels.

One or two procedures might be enough to reduce your platelet count to a safe level.

Secondary Thrombocytosis

Secondary thrombocytosis is treated by addressing the condition that's causing it.

People who have secondary thrombocytosis usually don't need platelet-lowering medicines or procedures. This is because their platelets usually are normal (unlike in primary thrombocythemia).

Also, secondary thrombocytosis is less likely than primary thrombocythemia to cause serious problems related to blood clots and bleeding.

How Can Thrombocythemia and Thrombocytosis Be Prevented?

You can't prevent primary thrombocythemia. However, you can take steps to reduce your risk for complications. For example, you can control many of the risk factors for blood clots, such as high blood cholesterol, high blood pressure, diabetes, and smoking.

To reduce your risk, quit smoking, adopt healthy lifestyle habits, and work with your doctor to manage your risk factors.

It's not always possible to prevent conditions that lead to secondary thrombocytosis. But, if you have routine medical care, your doctor may detect these conditions before you develop a high platelet count.

Living With Thrombocythemia or Thrombocytosis

If you have thrombocythemia or thrombocytosis:

  • See your doctor for ongoing medical care.
  • Control risk factors for blood clots—for example, quit smoking and work to manage risk factors such as high blood cholesterol, high blood pressure, and diabetes.
  • Watch for signs and symptoms of blood clots and bleeding and report them to your doctor right away.
  • Take all medicines as prescribed.

If you're taking medicines to lower your platelet count, tell your doctor or dentist about them before any surgical or dental procedures. These medicines thin your blood and may increase bleeding during these procedures.

Medicines that thin the blood also may cause internal bleeding. Signs of internal bleeding include bruises, bloody or tarry-looking stools, pink or bloody urine, increased menstrual bleeding, bleeding gums, and nosebleeds. Contact your doctor right away if you have any of these signs.

Avoid over-the-counter pain medicines such as ibuprofen (except Tylenol®). These medicines may raise your risk of bleeding in the stomach or intestines and may limit the effect of aspirin. Be aware that cold and pain medicines and other over-the-counter products may contain ibuprofen.

Clinical Trials

The National Heart, Lung, and Blood Institute (NHLBI) is strongly committed to supporting research aimed at preventing and treating heart, lung, and blood diseases and conditions and sleep disorders.

NHLBI-supported research has led to many advances in medical knowledge and care. Often, these advances depend on the willingness of volunteers to take part in clinical trials.

Clinical trials test new ways to prevent, diagnose, or treat various diseases and conditions. For example, new treatments for a disease or condition (such as medicines, medical devices, surgeries, or procedures) are tested in volunteers who have the illness. Testing shows whether a treatment is safe and effective in humans before it is made available for widespread use.

By taking part in a clinical trial, you can gain access to new treatments before they're widely available. You also will have the support of a team of health care providers, who will likely monitor your health closely. Even if you don't directly benefit from the results of a clinical trial, the information gathered can help others and add to scientific knowledge.

If you volunteer for a clinical trial, the research will be explained to you in detail. You'll learn about treatments and tests you may receive, and the benefits and risks they may pose. You'll also be given a chance to ask questions about the research. This process is called informed consent.

If you agree to take part in the trial, you'll be asked to sign an informed consent form. This form is not a contract. You have the right to withdraw from a study at any time, for any reason. Also, you have the right to learn about new risks or findings that emerge during the trial.

For more information about clinical trials related to thrombocythemia or thrombocytosis, talk with your doctor. You also can visit the following Web sites to learn more about clinical research and to search for clinical trials:

For more information about clinical trials for children, visit the NHLBI's Children and Clinical Studies Web page.

Links to Other Information About Thrombocythemia and Thrombocytosis

NHLBI Resources

Non-NHLBI Resources

Clinical Trials

July 31, 2012 Last Updated Icon

The NHLBI updates Health Topics articles on a biennial cycle based on a thorough review of research findings and new literature. The articles also are updated as needed if important new research is published. The date on each Health Topics article reflects when the content was originally posted or last revised.