In hematology, essential thrombocythemia (ET) is a rare chronic blood cancer (myeloproliferative neoplasm) characterised by the overproduction of platelets (thrombocytes) by megakaryocytes in the bone marrow.

[3] It may, albeit rarely, develop into acute myeloid leukemia or myelofibrosis.

[3] Most people with essential thrombocythemia are without symptoms at the time of diagnosis, which is usually made after noting an elevated platelet level on a routine complete blood count (CBC).

[4] The most common symptoms are bleeding (due to dysfunctional platelets), blood clots (e.g., deep vein thrombosis or pulmonary embolism), fatigue, headache, nausea, vomiting, abdominal pain, visual disturbances, dizziness, fainting, and numbness in the extremities; the most common signs are increased white blood cell count, reduced red blood cell count, and an enlarged spleen.

All mutations (insertions or deletions) affected the last exon, generating a reading frame shift of the resulting protein, that creates a novel terminal peptide and causes a loss of endoplasmic reticulum KDEL retention signal.

This mutation results in the JAK2 protein constantly being turned on, which leads to the overproduction of abnormal blood cells, in ET it is platelets or megakaryocytes.

There is also a small number of people who have a different mutation called CALR, which is abbreviated from calreticulin.

The MPL gene is responsible for making thrombopoeitin receptor proteins which promote the growth and division of cells.

[3][15][16][17] Currently unapproved but in late-stage clinical trials (NCT04254978) are agents that lower platelets such as bomedemstat.

[18] Essential thrombocythemia is sometimes described as a slowly progressive disorder with long asymptomatic periods punctuated by thrombotic or hemorrhagic events.

[20] Essential thrombocythemia can be linked with a three-fold increase in risk of miscarriage.

[20] Jill Kaplan, the female protagonist of The Pajama Diaries comic strip was diagnosed with essential thrombocythemia.