PSAT223 A Delayed Diagnosis of Oncogenic Osteomalacia Presenting with Bone Pain, Muscle Weakness and Elevated Alkaline Phosphatase

BACKGROUND: Tumor-induced osteomalacia (TIO) is a rare bone disease caused by excess production of FGF23 by a phosphaturic mesenchymal tumor, resulting in severe hypophosphatemia, bone demineralization, insufficiency fractures, bone pain and muscle weakness. The Fibroblast Growth Factor-23 (FGF23) is a phosphaturic hormone produced by osteocytes, whose primary effect is to decrease serum phosphate by suppressing proximal tubular phosphate reabsorption and intestinal phosphate absorption (via diminished 1,25D production). CLINICAL CASE: A 65-year-old male was referred to endocrinology clinic for a second opinion due to severe rib pain and elevated alkaline phosphatase levels. His first endocrinologist diagnosed Paget's disease of the bone based on symptoms of rib pain, an elevated alkaline phosphatase level, and abnormal Tc99 bone scintigraphy demonstrating uptake in multiple areas, including bilateral humeri, ribs, knees, ankles, and L4 vertebral body. He received 5 mg IV zoledronate for treatment of Paget's disease, and immediately post-infusion reported worsening of severe rib pain. Upon presentation to our institution, he reported ongoing severe rib pain and bilateral knee pain. On exam, he had extreme difficulty rising from a seated position. Laboratories revealed calcium 9.0 mg/dL, albumin 5.0 g/dL, alkaline phosphatase 116 U/L, 25D 43 ng/dL, PTH 66 pg/mL and serum phosphorous 1.5 mg/dL. Tubular Reabsorption of Phosphate (TRP) was 59%, based on a spot second morning void with corresponding fasting morning blood draw. 1,25D was 13 pg/mL (19-79 pg/mL). Intact FGF23 was 346 pg/mL (<59 pg/nL). CT revealed bilateral tibial plateau insufficiency fractures. When asked if he noticed any "lumps or bumps", the patient reported a lump under his left second toe. MRI showed a 1.8 cm soft tissue mass. A GA68-DOTA-TATE PET CT revealed a 2.3 cm soft tissue mass plantar to the 2nd left proximal phalanx with intense activity mass (SUVmax 39.8). This mass was surgically excised. Post-operative FGF23 levels were as follows: 38 pg/mL → 15 pg/mL → <14 pg/mL. Within 14 days, the patient's ability to rise from a seated position was markedly improved. His severe rib pain persisted for three weeks but was significantly improved by six weeks post-op. Despite treatment with 1 mcg calcitriol daily and 5000 IU vitamin D daily, he developed evidence of post-operative hungry bone syndrome with calcium 8.3 mg/dL, PTH 64 pg/mL, and alk phos 157 U/L. CONCLUSION: Tumor-induced osteomalacia is a rare but treatable condition, presenting with severe hypophosphatemia, bone pain, bone demineralization, insufficiency fractures, and muscle weakness. Due to the nonspecific nature of the presenting symptoms, delays in diagnosis are common. Serum phosphorous should be obtained in all patients presenting with unexplained elevations in alkaline phosphatase. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.