Endocrine Abstracts

Background: Oncogenic osteomalacia is a rare, acquired disorder caused by phosphaturic mesenchymal tumours that secrete excess fibroblast growth factor 23 (FGF23), leading to renal phosphate wasting, hypophosphatemia, and defective bone mineralisation. Diagnosis is often delayed due to non-specific symptoms and overlap with osteoporosis.

Case Presentation: A 60-year-old man with asthma and hiatus hernia initially presented in 2019 with progressive right hip and chest wall pain. Blood tests showed persistent hypophosphatemia, low calcium, normal vitamin D, mildly raised alkaline phosphatase, and elevated PTH. He was diagnosed with secondary hyperparathyroidism and treated with calcium and phosphate supplements. Over the next two years, he developed persistent pain requiring hospital admissions and sustained multiple atraumatic fractures. DEXA scanning confirmed osteoporosis, and alendronate was started. Despite treatment, hypophosphatemia persisted, prompting further endocrine review. Followed by rheumatology team for possible seronegative inflammatory spondylarthritis, but analgesics and Trail of disease-modifying therapy failed to improve symptoms. A decline in renal function was noted, suspected to be analgesia-related interstitial nephritis, and Fanconi syndrome was excluded. In March 2022, endocrine review revealed elevated 24-hour urinary phosphate excretion, high serum FGF23, hypocalciuric, and elevated P1 NP, raising suspicion of oncogenic osteomalacia.He underwent a bone scan which revealed which showed multiple areas of increase uptake. This, along with high FGF23, prompt a referral to Sarcoma MDT to look for oncogenic osteomalacia from mesenchymal tumour with the outcome being that ‘no evidence of tumour’ and they advised a review by metabolic bone clinic. In September 2023, he was reviewed by metabolic bone clinic, PET/CT was arranged and identified intense uptake in the proximal right humerus. MRI confirmed a 34 x21 x18 mm marrow-replacing lesion, and subsequent CT-guided biopsy confirmed aphosphaturic mesenchymal tumour. Genetic testing for PHEX mutations was conducted to exclude X-linked hypophosphatemia. Outcome and follow-up In October 2024, the tumour was surgically resected. Post-operatively, the patient reported significant improvement in mobility and pain reduction. Planned for repeat DEXA scanning to monitor bone recovery and tapering of Phosphate and calcitriol therapy under endocrine supervision.

Conclusion: Oncogenic osteomalacia, although rare, should be considered in adults with persistent hypophosphatemia, renal phosphate wasting, and multiple atraumatic fractures, particularly when elevated FGF23 is present. Tumour localisation often requires dedicated imaging and a multidisciplinary approach. Early identification and resection of the underlying tumour are crucial to reversing metabolic bone disease, improving bone health, and preventing further fractures.