Thrombopoietin (TPO) also known as megakaryocyte growth and development factor (MGDF) is a glycoprotein hormone produced in the liver by both parenchymal cells and sinusoidal endothelial cells, in the kidney by proximal convoluted tubule cells. Small amounts are also made by striated muscle and bone marrow stromal cells. It stimulates the production and differentiation of megakaryocytes, the bone marrow cells that bud off large numbers of platelets. Circulating TPO has been measured in both normal and pathophysiological conditions, including idiopathic thrombocytopenia purpura (ITP), essential thrombocythemia (ET), liver cirrhosis, amegakaryocytic thrombocytopenia (AMT), and aplastic anemia (AA). While TPO levels may be regulated by megakaryocyte counts, and platelets may play a role in regulating plasma TPO levels, there does not seem to be a direct correlation between platelet count and blood TPO levels.