Treatment-resistant hypophosphataemia after ferric carboxymaltose: expanding the spectrum of 6H syndrome (hypophosphataemia, hyperphosphaturia, hypovitaminosis D, hypocalcaemia, secondary hyperparathyroidism, high fibroblast growth factor-23)

Ferric carboxymaltose can cause severe, prolonged hypophosphatemia that is treatment-resistant and may necessitate weeks of continuous intravenous phosphate and activated vitamin D therapy.The full constellation of 6H syndrome (hypophosphatemia, hypophosphaturia, hypovitaminosis D, hypocalcaemia, secondary hyperparathyroidism, high fibroblast growth factor-23) offers a practical diagnostic framework when evaluating post-iron infusion phosphate disturbances.Coexistent malabsorptive disorders can exacerbate renal phosphate wasting and complicate management, therefore at-risk patients need proactive phosphate monitoring and early specialist input, and consideration of fibroblast growth factor-23 directed management.