FGF23 and hypophosphatemia: Laboratory workup to elucidate the cause of hypophosphatemia includes confirmation of reduced renal tubular reabsorption of phosphate, serum phosphate and calcium, alkaline phosphatase, 25-hydroxy vitamin D, 1–25-hydroxy vitamin D, genetic testing for hypophosphatemic osteomalacia, and, if available, FGF-23 concentrations [3, 4].