Inclusion Criteria:  Informed Consent as documented by signature (Appendix 5: Informed Consent Form) Male or female ASA 1 to 3 patients, at least 18 years old Scheduled for an elective major abdominal or thoracic surgery. Patients with isolated iron deficiency (defined as hemoglobin concentration (Hb) > 130 g/l, and plasma Ferritin < 100 ng/ml or TSAT < 20%), or iron deficiency anemia (defined as Hb 100-130 g/l, and plasma Ferritin < 100 ng/ml or TSAT < 20%). Patients scheduled to be hospitalized ≥ 3 days.  Exclusion Criteria:  Patients with known anaphylactic reactions to parenteral iron products. Patients with allergy/hypersensitivity to any contents of Ferinject® or Phoscap®. Patients with iron overload or disturbances in utilization of iron (e.g. haemochromatosis, hemosiderosis). Patients with ≥3 times increase in aspartate aminotransferase or alanine aminotransferase as per reference range. Patients with excessive blood loss requiring massive transfusion (≥ 10 more red blood cell units). Patients with known myelodysplastic syndromes. Patients with chronic kidney disease with an estimated GFR < 30 ml/min or with end-stage renal disease requiring scheduled dialysis. Patients with known urinary tract infections with urea-splitting bacteria. Patients with known diseases influencing phosphate, calcium or vitamin D homeostasis (e.g. Hyperparathyroidism, X-linked hypophosphatemia, renal tubular acidosis). Patients taking medication significantly influencing phosphate, calcium or vitamin D homeostasis (e.g. vitamin D substitution > 800 I.E. per day in the last 4 months, pre-existing phosphate substitution, phosphate binders). Patients with preexisting hypophosphatemia Any patient judged to lack the ability to give informed consent or perform the trial assessments (e.g. due to dementia, insufficient knowledge of the German language). Women who are pregnant or breast feeding, Intention to become pregnant during the course of the study, Lack of safe contraception, defined as: Female participants of childbearing potential, not using and not willing to continue using a medically reliable method of contraception for the entire study duration, such as oral, injectable, or implantable contraceptives, or intrauterine contraceptive devices, or who are not using any other method considered sufficiently reliable by the investigator in individual cases (Female participants who are surgically sterilized / hysterectomized or post-menopausal for longer than 2 years are not considered as being of child bearing potential), Known or suspected non-compliance, drug or alcohol abuse. Participation in another study with investigational drug within the 30 days preceding and during the present study. Any patient judged by the Principal Investigator or Sub-Investigator to be inappropriate for the trial for any other reason.