ENDOTOXIN LEVEL <0.01 EU per 1 μg of the protein by the LAL method. Human Recombinant GMP Grade TPO (Thrombopoietin). Thrombopoietin (TPO) is a glycoprotein hormone produced mainly by the liver and the kidney that regulates the production of platelets by the bone marrow. The protein functions in the iodination of tyrosine residues in thyroglobulin and phenoxy-ester formation between pairs of iodinated tyrosines to generate the thyroid hormones, thyroxine and triiodothyronine. It stimulates the production and differentiation of megakaryocytes, the bone marrow cells that fragment into large numbers of platelets.