SUN-745 - When Hypophosphatemia Leads to Fractures: A Case of Tumor-Induced Osteomalacia

Tumor-induced osteomalacia (TIO), is an ultrarare paraneoplastic syndrome caused by excess fibroblast growth factor 23 (FGF23), resulting in renal phosphate wasting and impaired bone mineralization. The true epidemiology of TIO remains unknown, as no large population-based studies exist. Limited registry data from Denmark and Germany highlight its extreme rarity, with a reported incidence of ≤0.13 per 100,000 person-years and prevalence of ≤0.70 per 100,000 persons. Diagnosis is frequently delayed due to nonspecific symptoms and difficulty localizing the typically small, slow-growing tumors.
A 72-year-old man was referred to endocrinology for evaluation of multiple non-traumatic fractures and electrolyte abnormalities. Initial laboratory testing demonstrated severe hypophosphatemia with a serum phosphorus level of 1 mg/dL with normal calcium levels. Renal phosphate wasting was evident, with a urine phosphorus-to-creatinine ratio of 1580 mg/g (reference range 60–962 mg/g). Alkaline phosphatase was markedly elevated at 642 U/L. Serum 25-hydroxyvitamin D was normal at 40 ng/mL, and intact parathyroid hormone was low-normal at 23.1 pg/mL. Subsequent testing revealed a significantly elevated FGF23 level of 756 RU/mL, exceeding the reference value of < 180 RU/mL. Physical examination was notable for wheelchair dependence and bilateral lower extremity weakness.
Dual-energy X-ray absorptiometry demonstrated osteopenia to osteoporosis, with an AP spine T-score of −2.0 and a right femur T-score of −3.0. Radiographs and bone scintigraphy revealed diffuse osteopenia, multiple rib fractures, and femoral neck fractures consistent with osteomalacia. 68Ga-DOTATATE PET/CT localized a somatostatin receptor–avid right-sided scalp lesion superficial to the temporal bone, measuring 1.2 cm with a maximum standardized uptake value of 71.5, without evidence of additional lesions.
This case highlights the severe skeletal and functional consequences of delayed recognition of TIO and underscores the diagnostic value of FGF23 measurement and 68Ga-DOTATATE PET/CT for tumor localization. TIO should be considered in patients with unexplained hypophosphatemia, renal phosphate wasting, and fragility fractures to enable timely diagnosis and prevent irreversible morbidity.

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