Endocrine Abstracts

JOINT2378

Tumor-induced osteomalacia (TIO) is a rare cause of bone demineralization, primarily affecting adults. It results from the hypersecretion of fibroblast growth factor 23 (FGF23) by typically benign, small, and elusive mesenchymal tumors, which can arise anywhere in the body, but are most commonly found in the lower limbs. Excess FGF23 reduces renal phosphate reabsorption in the proximal tubules, and inhibits the activation of 25-OH-vitamin D, thereby impairing intestinal phosphate absorption. Consequently, patients develop non-specific yet debilitating symptoms, including fatigue, chronic pain, muscle weakness, fragility fractures, and mobility impairment. We report the case of a 59-year-old female referred to our endocrinology department for progressive bone pain, muscle weakness, and multiple fragility fractures (ribs, femur, tibia, talus, and calcaneus). Extensive hematologic, rheumatologic and orthopedic evaluations revealed persistent hypophosphatemia. Renal phosphate wasting was confirmed by a low tubular maximum reabsorption of phosphate to the glomerular filtration rate (TmP/GFR) for her age. Elevated FGF23 levels, along with low 25-OH-vitamin D and 1,25-(OH)₂-vitamin D, raised suspicion of an FGF23-secreting tumor. Magnetic resonance imaging (MRI) identified a 50/24/64 mm mass in the retrocrural space, initially suggesting a paraganglioma. However, biochemical studies ruled out a secretory paraganglioma. Surgical excision confirmed a mesenchymal neoplasm, establishing the diagnosis of TIO. Postoperatively the patient experienced significant symptomatic improvement, including relief from pain and depression, along with restored mobility. Biochemical follow-up demonstrated normalization of FGF23, high 25-OH-vitamin D, 1,25-(OH)₂-vitamin D, normal serum phosphate, and phosphaturia. Diagnosing TIO remains challenging due to the difficulty in locating the causative tumor. However, it should be considered in patients with unexplained progressive muscle weakness, bone pain, and multiple fragility fractures. Early recognition and appropriate biochemical and imaging workups are crucial for timely diagnosis and treatment. While wide surgical resection remains the gold standard, newer approaches such as minimally invasive radiofrequency ablation, radiotherapy, or cryoablation are under investigation. This case is particularly notable due to the tumor’s unusual size and retrocrural location, initially raising the suspicion of an alternative diagnosis.