SUN-715 FGF23 Mutation Causing Hypophosphatemia

Abstract Disclosure: A. Safdar: None. A. Altaf: None. A. Arooj: None. N.L. Shah: None. Background: Fibroblast growth factor (FGF) 23 is a potent regulator of vitamin D and phosphate homeostasis. We describe a case of refractory hypophosphatemia in a patient with elevated levels of FGF23 levels. Clinical Case: A 62 years-old male with prior history of hypertension, gastro esophageal reflux disease (GERD) was referred to endocrine clinic for initial evaluation of hypophosphatemia. His metabolic panel showed serum phosphorous level of 1.2 mg/dL (reference 2.5-5 mg/dL), calcium levels of 10.6 mg/dL (reference 8.6-10.3 mg/dL), albumin 4.7 g/dL (reference 4.2-5.5 g/dL) and vitamin D 25-hydroxy levels 24 ng/mL (reference 30-100 ng/mL), PTH 37.5 pg/mL (reference 87.7-77.1 pg/mL). The patient was not taking any calcium or phosphorous supplements at the time. When corrected for albumin, his corrected calcium level was 10.0 mg/dL and within the reference range for normal. This prompted further work up for underlying etiology of his low vitamin D levels, and low phosphorous levels and FGF23 was checked. The serum FGF23 levels returned high at 344 RU/mL (Reference rage 44-215 RU/mL). Upon next clinic visit, his phosphorous level when checked again was at lower limits of normal at 2.5 mg/dL and hence it was decided to monitor without any supplemental phosphorous replacement. He was referred for genetic testing to rule out X-linked hypophosphatemia and sent for whole body PET scan to rule out tumor induced osteomalacia. Discussion: X-linked hурοрhοѕрhаtemiа (XLH), tumor-induced οѕtеοmalаϲia (ΤΙО) and other inherited forms of fibroblast growth factor 23 (FGF23)-mediated hypophosphatemic ricketѕ are all characterized by hурοрhοѕрhatemiа, normal serum levels of ϲalсium, and either normal or modestly elevated levels of parathyroid hormone (РTH). These disorders have high or inappropriately normal circulating levels of FGF23, a circulating hormone that causes renal рhοѕphаtе wasting and is a common final pathway. This case demonstrates the importance of FGF23 in regulating vitamin D and phosphorus metabolism and should prompt physicians to check it when dealing with low phosphorus without any clear underlying etiology. Reference: (1). Quarles LD. Role of FGF23 in vitamin D and phosphate metabolism: implications in chronic kidney disease. Exp Cell Res. 2012 May 15;318(9):1040-8. doi: 10.1016/j.yexcr.2012.02.027. Epub 2012 Mar 7. PMID: 22421513; PMCID: PMC3336874.(2) Razzaque MS, Lanske B. The emerging role of the fibroblast growth factor-23-klotho axis in renal regulation of phosphate homeostasis. J Endocrinol 2007; 194:1. Presentation: Sunday, July 13, 2025