Essential thrombocythemia (ET) is a chronic myeloproliferative disorder characterized by abnormal proliferation of megakaryocytes and platelets that originate from clonal proliferation in the bone marrow. Patients have significant and sustained elevated platelet counts (≥ 450 x 10⁹/L).

Most cases occur in patients aged 50-60 years, although there is another peak incidence in female patients of approximately 30 years of age. ET accounts for approximately one-third of BCR-ABL-negative myeloproliferative neoplasms.

Almost half of all patients with ET present incidentally when thrombocytosis is noted on routine blood work. Others will present with complications (vascular occlusion / thrombosis, hemorrhage, bleeding, or first trimester pregnancy loss) or disease-related symptoms (headache, vision changes, dizziness, splenomegaly). Between 13% and 40% of patients will have "vasomotor" symptoms due to microvascular disturbance, which can include headache, syncope or presyncope, chest pain, transient visual disturbances, acral paresthesias, and skin findings such as livedo reticularis or erythromelalgia. Molecular testing in these patients usually shows presence of either JAK2, CALR, or MPL mutations (90% of cases) or some other clonal marker.

ET is an indolent disorder; however, in advanced cases, ET patients will eventually develop bone marrow fibrosis (leading to cytopenias), or the disease will transform into acute leukemia.