[Chronic bone pain due to raised FGF23 production? The importance of determining phosphate levels]

Hypophosphatemia is an important finding in the evaluation of patients with chronic bone pain. Fibroblast-growth factor 23 (FGF23) plays a role in the differential diagnosis of hypophosphatemia. A 34-year-old man had progressive pain in both shoulders and hips due to hypophosphatemic osteomalacia. He had elevated FGF23 levels, induced by a FGF23-producing tumour in the right acetabulum. Thus, he had tumour-induced hypophosphatemic osteomalacia. A 50-year-old man had had bowed legs and joint pains since his youth due to osteomalacia. Several family members also had osteomalacia. His phosphate concentration was low. Genetic testing revealed a mutation on the PHEX gene which results in high FGF23 levels. Thus, he had X-linked hereditary hypophosphatemic osteomalacia. In patients with bone pain, the measurement of a phosphate concentration is important. In renal phosphate loss, the measurement of FGF23 is an important next step if parathormone concentrations are low or normal.