Specifically, anemia-related factors have been shown to increase FGF23 production, including iron deficiency [16–19], which is common in adult [20] and pediatric [21] CKD, and increased erythropoietin [22–27], levels of which increase in response to decreasing hemoglobin concentrations in mild–moderate CKD [28]. This evidence concerns the gene FGF23 and anemia (phenotype).