FGF23 and mesenchymal cell neoplasm: Tumor‐induced osteomalacia (TIO) is a rare paraneoplastic syndrome, caused by overproduction of fibroblast growth factor 23 (FGF23)—or other phosphatonins—secreted by a mesenchymal tumor.(1, 2) The largest cohort from a single center included 230 patients.(3) FGF23 excess leads to hypophosphatemia due to renal phosphate wasting and inappropriately normal or low 1.25 dihydroxyvitamin D. As a result, patients often present with a long history of musculoskeletal pain, fatigue, proximal muscle weakness, gait disturbance, and multiple fractures.