Thrombocytosis describes a platelet count elevated above 450 × 109/litre, which can be (1) primary—including essential thrombocythaemia, chronic myeloid leukaemia, polycythaemia vera and myelodysplastic syndromes; or (2) secondary—including iron deficiency, infection, blood loss, malignancy.
Platelets are released from megakaryocytes, whose development is principally regulated by thrombopoietin. This is chiefly produced in the liver and binds to its receptor (c-Mpl) to cause activation via the JAK-STAT signalling pathway at different levels of the platelet production pathway, ranging from the proliferation and survival of haematopoietic stem cell/progenitor cells to megakaryocyte maturation. Thrombopoietin production is increased by a wide variety of stimuli, which explains the many causes of secondary thrombocytosis....