Endocrine Abstracts

Background: Moderate hypophosphatemia is not an uncommon finding. As usually requiring no specific intervention, it might be neglected and serum phosphate measurements are sometimes skipped. Only 1% of phosphorus is in extracellular fluid so it is not a reliable marker of body phosphorus reserve and refractory chronic hypophosphatemia might be a sign of severe phosphorus depletion.

Case presentation: A fifty-two-year-old male patient presented with a 12-month history of lumbar, thoracal, and inguinal pain with weight loss, progressive severe muscle weakness, impaired gait and he became wheel-chair dependent. After thorough oncological, hematological, and neurological investigation imaging revealed rib fractures, compressive lumbar vertebral fractures, and bilateral femoral neck insufficiency fractures with no confirmation of metastatic tumor disease. He was finally sent for metabolic bone disease consultation and it was immediately noticed that not a single serum phosphate measurement was performed. Moderate hypophosphatemia, low ratio of tubular maximum phosphate reabsorption to glomerular filtration rate, elevated alkaline phosphatase, and elevated fibroblast growth factor-23 (FGF-23) were found. Considering the late age of onset and no family history, the abnormal secretion of FGF-23 impairing proximal tubule phosphate reabsorption was recognized. The rare diagnosis of tumor induced osteomalacia was presumed. No evident tumor on ⁶⁸Ga-DOTATE-PET/CT scan was revealed. High phosphorus diet and calcitriol were prescribed. Joulie’s solution as the only available phosphate supplement was administered. Patient’s clinical condition progressively improved during several months. His mobility recovered and bone pain almost disappeared, but secondary hyperparathyroidism as a limiting factor of conservative therapy developed.

Conclusions: ⁶⁸Ga-DOTATE-PET/CT is planned to be repeated after a year of follow-up. If unsuccessful again, selective FGF23 venous sampling might be tried to localize the responsible tumor. Newly developed treatments aimed to block FGF-23 might also be only temporary measures and every effort should be made toward tumor detection and final cure.