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ABSTRACT: Objective: To report the effectiveness of cinacalcet in treating tertiary hyperparathyroidism (THPT) associated with chronic treatment of hypophosphatemia in a patient with fibroblast growth factor-23 (FGF-23)-mediated tumor-induced osteomalacia (TIO).Methods: We review the clinical course, imaging studies, pathology, and results of laboratory testing of our patient and review relevant literature.Results: A 52-year-old woman presented with hypercalcemia. At age 26, she developed hypophosphatemia, inappropriately low levels of calcitriol, calcium, and intact parathyroid hormone (iPTH) and was diagnosed with vitamin D–resistant osteomalacia. She was initiated on high-dose phosphate but remained hypophosphatemic and developed progressive elevation in iPTH levels along with hypercalcemia. Twenty years after the initial presentation, a mass was found on her right forearm, and surgical resection revealed a phosphaturic mesenchymal tumor diagnostic of TIO. Her hypophosphatemia resolved and she was titrated off phosphate supplements; however, the THPT persisted. Treatment with cinacalcet resulted in lowering of calcium levels and normalization of iPTH levels.Conclusion: This may be the first reported case of managing THPT associated with chronic phosphate replacement therapy without parathyroidectomy in an adult with a FGF-23–producing tumor. Medical therapy with cinacalcet should be considered in patients with THPT associated with reversible hypophosphatemia.Abbreviations: 1,25(OH)2D 1,25-dihydroxyvitamin D 18F-FDG PET/CT 18-fluorodeoxyglucose positron emission tomography/computed tomography ADHR autosomal dominant hypophosphatemic rickets ARHR autosomal recessive hypophosphatemic rickets FGF-23 fibroblast growth factor-23 iPTH intact parathyroid hormone TIO tumor-induced osteomalacia THPT tertiary hyperparathyroidism XLH X-linked hypophosphatemic rickets