Essential Thrombocythemia (Essential Thrombocytosis; Primary Thrombocythemia) By Jane Liesveld, MD, James P Wilmot Cancer Institute, University of Rochester Medical Center Reviewed / Revised Jul 2022 | Modified Sep 2022 View Patient Education Etiology Pathophysiology Symptoms and Signs Diagnosis Prognosis Treatment Key Points Essential thrombocythemia (ET) is a myeloproliferative neoplasm characterized by an increased platelet count, megakaryocytic hyperplasia, and a hemorrhagic or microvascular vasospastic tendency.