Thrombopoietin （THPO） also known as megakaryocyte growth and development factor （MGDF） is a protein that in humans is encoded by the THPO gene. Thrombopoietin is a glycoprotein hormone produced by the liver and kidney which regulates the production of platelets. It stimulates the production and differentiation of megakaryocytes, the bone marrow cells that bud off large numbers of platelets. Megakaryocytopoiesis is the cellular development process that leads to platelet production. Thrombopoietin is a humoral growth factor necessary for megakaryocyte proliferation and maturation, as well as for thrombopoiesis.