Thrombocythemia is a condition caused by high platelet count. Platelets (also known as thrombocytes) are responsible for helping your blood clot, and high levels can lead to excess clotting and other health complications. The type of thrombocythemia you have depends on what’s causing it. Knowing what high platelet count means and signs that you may have it can help you get the treatment you need. If left untreated, thrombocythemia can result in health complications from blood clots becoming stuck in veins around your body or cause bleeding.¹

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What Is Thrombocythemia?

In healthy individuals, platelet levels can range from approximately 150,000 to 400,000 platelets per microliter of blood. Thrombocythemia is diagnosed when your levels are above 450,000 platelets per microliter.² This increase means that more platelets are floating in your bloodstream, so they are more likely to stick together and create blood clots. Many people don’t notice that they have thrombocythemia until they’re diagnosed with a blood clot and receive treatment for it.

Thrombocythemia vs. Thrombocytosis

There are two main types of conditions caused by high platelet levels — essential thrombocythemia and secondary thrombocythemia (thrombocytosis). Essential thrombocythemia is a rare disorder that doctors and researchers believe is caused by certain gene mutations.³ It’s classified as a myeloproliferative neoplasm (MPN), or a group of blood diseases and cancers caused by changes in the stem cells in the bone marrow.⁴

On the other hand, secondary thrombocytosis is caused by an underlying condition such as iron deficiency or infection. Thrombocytosis also tends to be less severe than essential thrombocythemia, with less risk of bleeding and blood clots.⁵

What Causes a High Platelet Count?

Essential thrombocytopenia is thought to be primarily caused by gene mutations that control blood cell growth and division. Other, less common mutations can also occur in genes involved in protein folding and blood clotting pathways. While some gene mutations are passed down from parents to their children, the mutations that cause essential thrombocythemia develop spontaneously in a person’s somatic (body) cells.⁶

When cell growth genes are turned on in essential thrombocythemia, they cause the bone marrow to make too many megakaryocytes. These large cells are responsible for making platelets.⁷

On the other hand, thrombocytosis is a reaction to an underlying health condition that causes your body to overproduce platelets. It can also develop if your body isn’t breaking down platelets normally, which can lead to a buildup over time.⁵

Risk Factors for Thrombocythemia

Essential thrombocythemia is a rare condition that affects only 2 in every 100,000 people in the United States. It’s more common in older individuals aged 60 to 80 years, though nearly 20 percent of cases occur in people aged 40 or younger. Women are twice as likely to be diagnosed as men. If essential thrombocythemia is diagnosed in a child, it’s likely due to the rare case where the condition was passed down from their parents.³

Thrombocytosis is also more common in older individuals and women. Your risk of thrombocytosis increases if you:⁸

Have cancer
Are iron deficient
Have hemolytic anemia, which destroys red blood cells (RBCs) too quickly
Have another blood disorder
Are in kidney failure
Take certain medications
Have had your spleen removed, which normally stores platelets and releases them as your body needs them
Have recently had surgery, particularly abdominal surgery
Have an infection or are injured
Have an inflammatory or autoimmune disease, including sarcoidosis, inflammatory bowel diseases, or rheumatoid arthritis

Thrombocythemia Symptoms

The symptoms of essential thrombocythemia often differ from person to person. Many people don’t notice they have thrombocythemia until they’re diagnosed with a blood clot and receive treatment for it or if their doctor sees high platelet levels during routine blood work. This is because it takes time for platelet levels to rise, so symptoms may show up gradually.

Essential thrombocythemia symptoms can affect the whole body or produce symptoms in an area where a blood clot is. Common locations for blood clot formation include the hands, feet, and brain. If you notice any of the following signs, seek medical attention immediately, as blood clots can be life-threatening:³

Redness or numbness in your hands or feet, caused by tiny blood clots in blood vessels (erythromelalgia)
Throbbing or burning sensation in the palms of your hands or soles of your feet
Nausea
Enlarged spleen
Frequent nosebleeds
Blood in your urine or stool
Seizures
Migraines
Bruising easily with small injuries
Bleeding gums
Heavy menstrual periods

Symptoms of thrombocytosis are often accompanied by symptoms of the underlying cause driving your high platelet count. These tend to improve once the problem is addressed. Thrombocytosis symptoms related to blood clots or abnormal bleeding in your body include:⁵

Bruising easily with small injuries
Headache
Bleeding gums
Frequent nosebleeds
Abdominal bleeding (stomach or intestines)
Erythromelalgia (a condition that includes burning pain and redness in the feet, hands, and other body parts)
Blood clots in your abdomen, typically in severe cases (known as Budd-Chari syndrome)

How Is Thrombocythemia Diagnosed?

Essential thrombocythemia and thrombocytosis are diagnosed with a series of blood and bone marrow tests. Once your doctor confirms your platelet levels are above 450,000 platelets per microliter, they’ll investigate whether it’s being caused by a gene mutation or an underlying health condition.

Blood tests for thrombocythemia include:¹

Genetic tests for changes in known genes involved in essential thrombocythemia, including CALR, JAK2, THPO, MPL, and TET2
Peripheral blood smear to look at your platelets under a microscope for any abnormalities
Bone marrow aspiration (liquid sample) or biopsy (tissue sample) to look for an abnormally high amount of megakaryocytes
Iron levels, to determine if you have thrombocytosis caused by iron deficiency anemia
Inflammatory markers, pointing to inflammation which may be causing thrombocytosis

Complications from Thrombocythemia

Thrombocythemia increases your risk of heart attack or stroke, which occur when a blood clot travels to your heart or brain, respectively.^3,5 The clot cuts off blood flow into the organ, depriving it of oxygen and nutrients. These are both life-threatening complications that require immediate treatment. It’s important to know the signs of a heart attack or stroke in case of an emergency. If you notice any of the following signs, dial 9-1-1 immediately.

Signs of a heart attack include:⁹

Chest pain or discomfort that feels like pressure or squeezing in the middle or left side of your chest; this typically lasts for a few minutes and may continue to come and go over time
Shortness of breath that occurs with chest pain
Pain or discomfort in your back, neck, or jaw
Pain or discomfort in one or both of your shoulders or arms

Signs of a stroke include:¹⁰

Sudden trouble walking or balancing
Sudden slurred speech or trouble understanding speech from others
Sudden vision loss in one or both eyes
Sudden weakness or numbness on one side of your body in your arms, legs, or face
Sudden unexplainable headache

Rarely, people with thrombocythemia may also develop bone marrow problems and even cancer. Myelofibrosis is a progressive bone marrow disorder that causes scarring, eventually leading to spleen and liver enlargement and severe anemia. There’s also a risk of developing acute myelogenous leukemia (AML), which is a type of bone marrow cancer that affects your white blood cells.¹¹

Pregnant women with thrombocythemia also need to be careful, as the condition can cause complications. Women are already at an increased risk of the condition, and pregnant women are also more likely to develop blood clots. If you’re pregnant or planning on becoming pregnant, be sure to talk with your doctor about the best way to manage your thrombocythemia.³

Resources for Living with Thrombocythemia

If you or a loved one are living with thrombocythemia, there are patient support groups and resources available to help you connect with others who understand what you’re going through. These organizations offer online community platforms, questions to ask your doctor, support hotlines, and peer-to-peer support.