A Challenging Case of Phosphaturic Mesenchymal Tumor With Primary Hyperparathyroidism

Background: Localization of tumor-induced osteomalacia (TIO) is often challenging. Primary hyperparathyroidism (HPT) following curative surgery for TIO is rarely reported. Clinical Case: A 49-year-old man presented with fragility rib fractures, generalized bone pain, and muscle weakness worsening over the past 3 years. Rheumatologic workup was negative. Initial tests showed elevated levels of parathyroid hormone (PTH) 114.1 pg/mL (14–72 pg/mL) and alkaline phosphatase (ALP) 283 IU/L (44–174 IU/L), reduced levels of 25(OH)D 16 ng/mL (30–100 ng/mL), 1,25(OH)(2)D 9 pg/mL (18–72 pg/mL), and phosphorus 1.6 mg/dL (2.5–4.9 mg/dL), calcium levels of 9.2 mg/dL (8.5–10.1 mg/dL), and eGFR 58 mL/min/1.73 m(2). A sestamibi scan showed normal parathyroid uptake. The diagnosis was secondary HPT due to chronic kidney disease and vitamin D deficiency. The patient was treated with D(3) and phosphate. During a 2-year follow-up, the patient reported improvement of pain and weakness with no additional fractures. Further investigations showed persistent hypophosphatemia with elevated urinary fractional phosphate excretion (44%, ref. <20%), indicating renal phosphate wasting. Fibroblast Growth Factor 23 (FGF23) was high, 291 RU/mL (0–180 RU/mL). DXA results were consistent with osteopenia. TIO was suspected. At a 3-year follow-up, investigations included three whole-body (18)F-FDG PET-CT scans revealing several areas suspicious for tumor presence. However, multiple MRIs were inconclusive. Laboratory tests showed persistent hypophosphatemia (despite D(3) and phosphate treatment), elevated FGF23 (1330 RU/mL) and PTH (274.4 pg/mL), and normal calcium, 25(OH)D, and 1,25(OH)(2)D. The patient subsequently underwent (68)Ga DOTATATE PET-CT, which revealed a somatostatin receptor-positive lesion involving the left upper lobe of the lung. The mass was resected without complications. Histopathology was compatible with a phosphaturic mesenchymal tumor. At a 6-month postoperative follow-up, the patient reported dramatically improved symptoms with decreased weakness and pain, normal phosphate, calcium, ALP, and FGF23 (160 RU/mL) levels, while DXA results were significantly improved. Phosphorus supplementation was discontinued. At follow-up 3 years post-surgery, the patient had slowly rising PTH (126.3 pg/mL) and calcium (10.1–10.6 mg/dL) levels with normal phosphate, 25(OH)D, and FGF23 (174 RU/mL) levels. A diagnosis of primary HPT was made. Further evaluation was deemed unnecessary since the patient did not meet the criteria for surgical treatment. The development of primary HPT was considered mechanistically related to long-standing hypophosphatemia and hypovitaminosis D stimulating PTH production. Conclusion: This case report highlights the pitfalls contributing to delayed diagnosis of TIO and alerts clinicians to the potential development of primary HPT after curative surgery for TIO.