First change of improvement, noted after iron therapy is initiated

12-24 hr Replacement of intracellular iron enzymes; subjective improvement; decreased irritability; increased Appetite 36-48 hr Initial bone marrow response; erythroid hyperplasia 48-72 hr Reticulocytosis, peaking at 5-7 days 4-30 days Increase in hemoglobin level 1-3 mo Repletion of stores