Time course of Hb level, sEpo, and sTfR in a patient with megaloblastic anemia due to vitamin B12 deficiency treated with vitamin B12 (IM as cyanocobalamin, 500 μg per day). A marked decrease in serum Epo was seen after the first injection and before any increase in Hb level. There was a parallel increase in serum transferrin receptor, indicating a rapid expansion of the erythroid marrow during the first days of treatment.