Lineage-specific cytokine affecting the proliferation and maturation of megakaryocytes from their committed progenitor cells. It acts at a late stage of megakaryocyte development. It may be the major physiological regulator of circulating platelets. Found mainly in the liver, kidney and skeletal muscle. Expression in the liver and bone marrow displays diurnal rhythmicity (a circadian rhythm that is synchronized with the day/night cycle). Two-domain structure with an erythropoietin-like N-terminal and a Ser/Pro/Thr-rich C-terminal. Belongs to the EPO/TPO family.