[Muscle weakness after intravenous iron replacement].

After a young female patient received an intravenous iron infusion for severe symptomatic iron-deficiency-anemia, a severe hypophosphatemia was diagnosed with associated mild muscle weakness. With renal phosphate loss and elevated serum FGF23 (Fibroblast Growth Factor 23), the diagnosis of a hypophosphatemia caused by ferric carboxymaltose was made. Upregulation of FGF23 inhibits renal phosphate reabsorption and also the activation of vitamin D, which additionally reduces intestinal phosphate absorption. The symptoms of hypophosphatemia may be masked after iron replacement.