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Abstract: SA-PO204

Frustratingly Frail: Following the Phosphorous

Session Information

Category: Bone and Mineral Metabolism

  • 402 Bone and Mineral Metabolism: Clinical

Authors

  • Giehl, Nolan M., University of California Los Angeles David Geffen School of Medicine, Los Angeles, California, United States
  • Nguyen, Minhtri K., University of California Los Angeles David Geffen School of Medicine, Los Angeles, California, United States
Introduction

Determining the etiology of persistent hypophosphatemia can be humbling and elusive. We present an extraordinary case of tumor-induced osteomalacia (TIO) after steadfast investigation.

Case Description

A 71-year-old man presented after 4 years of progressive muscle weakness, bony pain and debilitating osteoporosis with multiple pathologic fractures. He had no history of kidney disease, steroid use, growth deficiencies, hypogonadism, thyroid, or prostatic disease. Past bone marrow biopsy was normal. Initial laboratory data revealed severe hypophosphatemia (1.8 mg/dl), low vitamin-D-1,25 (10 pg/ml), but high-normal parathyroid hormone (57 pg/ml). Subsequent urine testing showed inappropriate phosphate wasting (740 mg/24hr) and high fractional excretion of phosphate (20.4%). Urinalysis was negative for glucosuria or proteinuria. Further serum testing for FGF23 was remarkably high (321 RU/ml). Follow up functional radiographic testing via 68Ga-DOTATATE-based PET/CT showed a hyperdense lesion in C7-T1 concerning for mesenchymal tumor (Figure 1). This was confirmed on cervical spine MRI. Upon surgical consultation for tumor removal, he was deemed not a candidate based on tumor location. Started on anti-FGF23 therapy with IV burosumab monthly while awaiting reassessment for surgical candidacy, he has had normalization of serum phosphate and near resolution of musculoskeletal symptoms.

Discussion

TIO is a rare paraneoplastic syndrome of abnormal phosphorus metabolism caused by small mesenchymal tumors that secrete FGF23 leading to impaired bone metabolism and disability. Delayed diagnosis is common due to nonspecific symptoms, and years may elapse before patients receive a correct diagnosis and proper treatment. Increased awareness of the appropriate recognition and management of TIO is vital among providers who may encounter patients with suspected TIO to reduce morbidity. Furthermore, our case demonstrates that while curative surgical intervention is preferred, the newly FDA-approved anti-FGF23 therapy (burosumab) is an effective second line therapy at achieving phosphorus hemostasis and clinical improvement.