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About essential thrombocythaemia (ET)

Essential thrombocythaemia is one of a group of conditions called myeloproliferative disorders, or neoplasms (MPN).

Essential thrombocythaemia is called ET for short.

Myeloproliferative disorders cause an increase in the number of blood cells.

Essential thrombocythaemia

Essential thrombocythaemia 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.

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:

Diagram showing how blood cells are made

So, in essential thrombocythaemia, the stem cells make too many platelets. The excess platelets may form blood clots and can collect in the spleen, making it become enlarged.

Risks and causes

We don’t know exactly what causes ET. Researchers have found that around 60% of people with ET have a change in the JAK2 gene. 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.

People with ET may also have a mutation with the CALR or MPL gene. Genetic faults may happen because you’ve been exposed to hazardous chemicals, but more often it's because of a copying mistake when a cell was dividing.

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

Outlook for essential thrombocythaemia

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.


ET usually develops slowly and doesn’t cause symptoms at first. So some people find out they have it when they are having a blood test for something else. As the condition 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, but can develop because the platelets are not fully mature and don’t work normally.

Symptoms include:

  • headaches
  • blurred vision
  • hearing problems
  • burning and tingling in hands and feet
  • swelling on your left side caused by an enlarged spleen
  • nosebleeds
  • bruising
  • bleeding gums
  • 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 - could be a blood clot in the leg
  • breathlessness and chest pain - could be a lung clot (pulmonary embolism)
  • headache, dizziness and blurred vision - could be a clot in the brain
  • chest pain could be a clot in the heart, and in more serious cases can cause a heart attack


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

Last reviewed: 
13 Sep 2017
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