Essential thrombocythemia (ET)

Platelets are tiny blood cells that help blood clot. In essential thrombocythemia, the bone marrow makes far too many of them. This can lead to unwanted blood clots and, in some people, to bleeding.

ET is one of the myeloproliferative neoplasms (MPNs), the same family of blood cancers as polycythemia vera and myelofibrosis. Its course is often slow, and many people live with it for many years.

A high platelet count has many causes, such as low iron or another illness, so doctors rule those out first. A bone marrow biopsy often helps too, because an early (prefibrotic) form of primary myelofibrosis can look like ET but tends to behave differently.

ET is caused by gene changes that happen in a blood-forming stem cell during a person’s life. About 6 in 10 people have a change in the JAK2 gene, and about 5 to 10 in 100 have a change in the MPL gene. Most of the rest have a change in the CALR gene, and a smaller group has none of the three. These changes switch on growth signals that drive the marrow to make extra platelets.

The gene change found can matter. A CALR change is linked to a milder course, and a JAK2 change to a higher chance of clots. MPL changes and one type of CALR change have been linked to a higher chance of turning into myelofibrosis.

These changes are not usually inherited, and ET cannot be caught from anyone. In rare families, several relatives have ET or another MPN.

Many people have no symptoms and learn they have ET from a routine blood test. Others have headaches, lightheadedness, vision or hearing problems, or burning, tingling, redness or warmth in the hands and feet.

The main risks are blood clots, which can cause a stroke, a heart attack, or a clot in a deep vein or the lungs, and bleeding. Clots are more likely in people over 60, in those who have already had a clot and in those with a JAK2 change.

Over many years, a small number of people see ET change into myelofibrosis, in which scar-like fibers build up in the marrow. Change into acute myeloid leukemia is less common. Reported rates vary, partly because some people told they had ET may have had early myelofibrosis instead.

Treatment aims to prevent clots and bleeding, and it depends on risk. Doctors look at age, any past clot and whether a JAK2 change is present. Some younger people with no symptoms or past clots are simply watched, and many people take low-dose aspirin. Expert sources differ on exactly who needs aspirin, so the team also weighs the chance of bleeding.

People at higher risk usually also take a medicine that lowers platelets. Hydroxyurea, a pill, is one of the usual first choices. Anagrelide is another pill that lowers platelets. In a large UK trial, both lowered platelets equally when taken with aspirin, but people on anagrelide had more clots, bleeding and myelofibrosis.

Interferon injections are another choice. In August 2026 the FDA approved ropeginterferon alfa-2b (Besremi), given every two weeks, for adults with ET; in the European Union it is approved only for polycythemia vera. Interferons can cause or worsen serious mental health, autoimmune, blood-flow and infection problems, so people taking them are checked closely.

ET is usually a long-term condition. Care means regular blood tests and check-ups, and for many people, medicine taken for years. The care team weighs the side effects of each medicine against its benefits.

Surgery and pregnancy need special planning with the care team. Some medicines used for ET need a pregnancy test before starting.

The team also watches for signs that ET may be changing, such as anemia, blood counts that fall after years of being high, a growing spleen, or new symptoms common in myelofibrosis, like fever, drenching night sweats or weight loss. If that happens, a bone marrow biopsy can show whether it has become myelofibrosis.

ET itself is not treated with a stem cell transplant. European transplant experts (EBMT) state that ET is not a reason for a donor transplant unless it has turned into myelofibrosis, or into MDS or leukemia. They also do not recommend a transplant using a person’s own cells for MPNs.

If ET does progress, a donor (allogeneic) transplant may be considered for people with higher-risk myelofibrosis who are fit enough, because it is the only treatment that may cure it. In a study by EBMT, the European transplant society, of 2,809 people who had a first donor transplant for myelofibrosis in 2015–2021, about 6 in 10 received cells from an unrelated volunteer, either fully or partly matched.

Most people with ET will never need a donor. For the few whose disease becomes higher-risk myelofibrosis or leukemia, a registry volunteer may make a transplant possible. No one can promise a match for a particular person.