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The three major pathophysiologic causes of thrombocytosis are (1) clonal, including essential (or primary) thrombocythemia and other myeloproliferative neoplasms; (2) familial, including rare cases of nonclonal myeloproliferation resulting from thrombopoietin and thrombopoietin receptor mutations; and (3) reactive, in which thrombocytosis occurs secondary to a variety of acute and chronic clinical conditions. This chapter deals with the latter two causes of thrombocytosis.

Acronyms and Abbreviations

EPO, erythropoietin; ESA, erythropoiesis-stimulating agent; FGF, fibroblast growth factor; GM-CSF, granulocyte-macrophage colony-stimulating factor; IFN, interferon; IL, interleukin; JAK, Janus kinase; LIF, leukemia inhibitory factor; MHC, major histocompatibility complex; NF, nuclear factor; ORF, open reading frame; SDF, stromal cell-derived factor; STAT, signal transducer and activator of transcription; TPO, thrombopoietin.

The upper limit of the normal platelet count in most clinical laboratories is between 350 × 109/L and 450 × 109/L. In a sample of 10,000 healthy individuals 18 to 65 years of age, 1% had platelet counts greater than 400 × 109/L. Only in eight of these 99 individuals was thrombocytosis confirmed 6 months to 1 year later.1 Nevertheless, it is clear that thrombocytosis is a feature of several important disorders, including cancer and numerous, serious inflammatory disorders, and that even a high normal platelet count is associated with morbidity and mortality. In a longitudinal study of healthy Norwegian men, a platelet count in the top quartile of the normal range (from 275 × 109/L to 350 × 109/L) was associated with a twofold increase in cardiovascular mortality over a 12-year follow-up.2 Whether the platelet count per se or an underlying inflammatory condition resulting in both thrombocytosis and accelerated atherogenesis is responsible for these observations is not certain.

The causes of thrombocytosis in which the platelet count exceeds the upper limit can be broadly categorized as (1) clonal, including essential thrombocythemia and other myeloproliferative neoplasms; (2) familial; and (3) reactive, or secondary. This chapter focuses on the causes and molecular mechanisms that underlie reactive and hereditary thrombocytosis. A full discussion of neoplastic thrombocytosis, termed essential thrombocythemia, is found in Chap. 84.


The regulation of platelet production is discussed extensively in Chap. 110, but a brief discussion here provides the appropriate background for discussion of reactive and hereditary thrombocytosis. Thrombopoietin (TPO), the ligand for the megakaryocytic growth factor receptor c-MPL, is the major humoral regulator of megakaryocyte survival, growth, and development, although it does not stimulate the final step in thrombopoiesis: platelet release from megakaryocyte proplatelet processes. Although TPO supports the entire continuum of megakaryocyte development from stem cell to mature megakaryocyte,3 other cytokines including interleukin (IL)-6,4 IL-3,5,6 IL-11,7 leukemia inhibitory factor,8,9 fibroblast growth factor–4,10 stromal cell–derived factor–1,10,11 interferon (IFN)-γ,12 and granulocyte-macrophage colony-stimulating factor13 also affect thrombopoiesis, both in ...

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