Ferric Carboxymaltose-Induced Hypophosphatemia

Definition and Prevalence

Ferric carboxymaltose (FCM) causes moderate to severe hypophosphatemia in 47-75% of patients due to its specific physicochemical properties triggering increased FGF23, which can lead to serious complications including osteomalacia and fractures with repeated use 1, 2
Hypophosphatemia is classified by severity based on serum phosphate levels, with mild being less than the lower limit of normal, moderate being less than 2.5 mg/dL, severe being less than 2.0 mg/dL, and potentially life-threatening being less than 1.0 mg/dL 2

FCM causes hypophosphatemia in 47-75% of patients, with 51% developing levels less than 2 mg/dL within 35 days of administration, due to the specific physicochemical properties of FCM triggering a sharp increase in intact fibroblast growth factor 23 (iFGF23) 1, 2
Elevated iFGF23 leads to hyperphosphaturic hypophosphatemia through increased urinary phosphate excretion, creating a cascade of biochemical changes known as "6H-syndrome": high FGF23, hyperphosphaturia, hypophosphatemia, hypovitaminosis D, hypocalcemia, and secondary hyperparathyroidism 1, 3, 4

Patients at highest risk for FCM-induced hypophosphatemia include those with recurrent or ongoing blood loss, malabsorptive disorders, normal renal function, severe iron deficiency, lower body weight, low baseline serum phosphate, and higher serum PTH 1, 2, 5
The American Society of Hematology recommends cessation of FCM in patients who develop hypophosphatemia, and treatment should focus on mitigating secondary hyperparathyroidism with vitamin D supplementation 1, 6
Phosphate repletion should be avoided as it raises parathyroid hormone and worsens phosphaturia, and alternative IV iron formulations should be considered in high-risk patients, as other preparations have much lower rates of hypophosphatemia (<10%) 1, 2, 6, 7

The FDA mandates monitoring serum phosphate levels in patients at risk for chronic low serum phosphate, and patients should be educated to report symptoms like worsening fatigue, muscle pain, or bone pain following infusion 1
Vitamin D supplementation before FCM infusion does not reduce hypophosphatemia risk, and repeated FCM infusions in high-risk patients can lead to osteomalacia and fractures 1