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What is Essential Thrombocythaemia?

Essential thrombocythaemia (ET) occurs when too many platelets are made.

Essential thrombocythaemia (previously known as primary thrombocythaemia) is a rare blood disorder that occurs when the bone marrow makes more platelets than the body needs. Platelets are needed to help the blood clot, but in people with ET, their overproduction means they don’t work properly. This results in serious complications like thrombosis – where excess platelets cause a blood clot that blocks a vein or an artery and stops blood flowing, or excess bleeding (due to the platelets not functioning properly). Although it’s platelets that are mostly affected, red blood cells and white blood cells may also be involved. It’s a condition that slowly gets worse over time.


Learn more about the role of bone marrow and stem cells.




A cross section of bone (marrow)

Bone marrow is the soft, spongy tissue that fills the inner cavities of the bone. It’s where red blood cells, white blood cells and platelets are formed. It also contains fluid, blood vessels and fat.


The bone marrow is either red or yellow. When we are born, we only have red marrow, which is responsible for creating new blood cells. As we get older, some of the red marrow is replaced by yellow marrow, which is largely fat. A healthy adult has both red and yellow marrow.

ET is most common among those over the age of 60, though it does sometimes affect younger people, more often women under the age of 40. The average age at diagnosis is between 65 and 70 years. It’s more common among women, with two women diagnosed for every man.


How rare is essential thrombocythaemia?

The annual incidence of ET is between 1.5 and 3 cases per 100,000 people. ET can occur at any age, (although rarely in children); however, most patients are diagnosed between 50-70 years. essential thrombocythaemia (ET) affects men and women in relatively equal numbers.