Inclusion Criteria:

• Age 18 years or greater
• Moderate to severe CKD not requiring dialysis (eGFR 15 - 45 ml/min/1.73 m2 by CKD-EPI)
• Absolute iron deficiency (serum ferritin <300ng/ml and Transferrin Saturation < 30%)

Exclusion Criteria:

• Hemoglobin concentrations > 13 g/dL
• Known disorder of iron homeostasis (e.g., hemochromatosis)
• Known gastrointestinal disorder (irritable bowel disease, inflammatory bowel disease)
• Known liver disease (ALT/AST or bilirubin > 3x normal)
• Serum phosphorus concentrations < 3.0 mg/dL
• Any known cause of anemia other than iron deficiency or CKD (e.g., sickle cell anemia)
• Symptomatic gastrointestinal bleeding within 12 weeks prior to the screening visit.
• Subjects receiving any form of renal replacement therapy including hemodialysis, peritoneal dialysis, or renal transplant.
• Pregnancy or lactation in female participants
• Severe anemia defined as a hemoglobin < 8.0 g/dL for males or a hemoglobin <7.0 g/dL for females.
• Receipt of erythropoiesis stimulating agents within 4 weeks of screening.
• Receipt of intravenous iron therapy within 8 weeks of screening.
• Blood transfusion within 4 weeks of screening
• Known allergies or severe adverse reactions to previous oral iron therapy
• Current use of oral phosphorus binders.
• Current use of an active vitamin D analog