What is essential thrombocythaemia?

Essential thrombocythaemia (ET) is a rare blood disorder that causes a high number of blood cells called platelets to form. These are blood cells involved in blood clotting. Thrombo means clotting and cythaemia relates to blood cells. It is also known as primary thrombocythaemia or essential thrombocytosis.

ET is a type blood disorder called a myeloproliferative neoplasm. These are conditions that cause an increase in the number of blood cells.

The World Health Organisation (WHO) classes all myeloproliferative neoplasms as blood cancers. This is because the bone marrow is producing blood cells in an uncontrolled way. But many people with myeloproliferative neoplasms feel well and only need gentle treatment. The disorders often develop slowly and progress slowly. Or they can remain stable for a while.

ET usually develops very slowly. It can affect people of any age. But it usually affects middle aged and older people. The average age at diagnosis is 50 to 70 years, although it can affect children and young adults.

The bone marrow

The bone marrow is the soft inner part of our bones that makes blood cells. All blood cells start from the same type of cell called a stem cell. The stem cell makes immature blood cells. The immature cells go through various stages of development before they become fully developed blood cells and are released into the blood. As well as platelets, the bone marrow makes:

  • red blood cells to carry oxygen around our bodies
  • white blood cells to fight infection

This diagram shows how different types of cells develop from a single blood stem cell:

A simplified diagram showing how blood cells are made

What happens in essential thrombocythameia (ET)?

In essential thrombocythaemia, the stem cells make too many platelets. The extra platelets might form blood clots. The platelets can also collect in the spleen, which makes it bigger.

Treatment controls ET for most people for many years. But for some, ET can lead to other problems, such as scarring of their bone marrow. This is called myelofibrosis. The risk of this happening increases over time.

More rarely ET can develop into acute myeloid leukaemia (AML). Fewer than 5 in 100 people (5%) with ET develop AML.

Risks and causes

We don’t know exactly what causes ET. Researchers have found that around 60 in 100 people (60%) with ET have a change in the JAK2 gene Open a glossary item. The JAK2 gene makes a protein that controls how many blood cells are made by the stem cells. This gene change causes too much of this protein to be made and that causes the stem cells to make more platelets. Or, people with ET might have a genetic change in either the CALR or MPL gene. Genetic faults might happen because you’ve been exposed to hazardous chemicals, but more often it's because of a copying mistake when a cell was dividing.

In most cases, these genetic faults happen during a person’s lifetime. You are not born with them, so you can’t pass them onto your children.

Family history

In rarer cases, you might have a history of myeloproliferative neoplasms (MPNs) in your family. This might mean there is a faulty gene in your family that increases your risk of developing MPNs.

Your specialist can talk with you about any genetic changes and family risk of MPNs. Ask if you have any questions or if this is worrying you.


Many people find out they have ET when they are having a blood test for something else. This is because ET usually develops slowly and doesn’t cause symptoms at first. As it progresses it causes symptoms.

Most of the symptoms happen as a result of blood clots forming or because of bleeding problems. Bleeding problems are less common than clots. They can develop because the platelets are not fully mature and don’t work normally.

Symptoms include:

  • headaches
  • dizziness, feeling lightheaded and blurred vision
  • tiredness (fatigue)
  • burning and tingling in hands and feet - this often feels worse with heat and better with cold
  • pain or discomfort in the tummy (abdomen) from an enlarged spleen
  • purplish mottled skin changes, usually on the legs
  • nosebleeds or bleeding gums
  • black or tarry poo, if there is any bleeding in the bowel
  • vaginal bleeding when you are not due to have a period, or abnormally heavy periods

Blood clots

The symptoms of a blood clot depend on where it is in the body. They can form anywhere but most commonly develop in the deep veins in the:

  • leg
  • lung
  • brain
  • heart

Not everyone with ET will have these problems. People who are over 60 years old, have diabetes or heart disease, and are overweight are at higher risk.

Contact your doctor straight away if you have:

  • pain, redness and a feeling of hotness in your leg, this could be a blood clot in the leg
  • breathlessness and chest pain, this could be a clot in the lung (pulmonary embolism)
  • headache, dizziness and blurred vision, this could be a clot in the brain

Chest pain could also be a clot in the heart, which in serious cases can cause a heart attack.

Essential thrombocythameia is rare, so if you have any of these symptoms it is more likely to be something else. But it is still important to contact your doctor to find out what is causing them.


The first test to diagnose essential thrombocythaemia is a blood test.

Last reviewed: 
21 May 2020
Next review due: 
21 May 2023
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