Severe Hypophosphatemia and Elevated FGF23 Level Following Zoledronic Acid Infusion

Background: Severe hypophosphatemia may be seen following zoledronic acid infusion, however FGF23 elevation has not been previously reported. Clinical Case: A 67-year-old man with Crohn’s disease status post remote ileocolonic resection, malnutrition (BMI 16.7), anemia, history of hypovitaminosis D, secondary hyperparathyroidism, and severe osteoporosis, was advised to present to the emergency room for hypocalcemia and hypophosphatemia after routine lab draw. His initial ionized calcium was 0.89 mmol/L (1.09–1.29 mmol/L) and phosphorus was 0.9 mg/dL (2.3–4.4 mg/dL), with a 25-hydroxyvitamin D level of 62 ng/mL (20–50 ng/mL), PTH of 132 pg/mL (11–51 pg/mL), and normal renal function. Hemoglobin was stable between 8–9 g/dL (13.5–17.1 g/dL). He endorsed ongoing fatigue, oral ulcers, and perioral numbness, which had been attributed to Humira infusion 2 months prior. He denied other paresthesias, carpopedal spasms, seizures, bone pain, or confusion. He reported receiving his second annual infusion of zoledronic acid 10 days prior in Turkey. He was started on high-dose oral calcium, calcitriol, and phosphate, which were continued on discharge 5 days later. Several days into his hospitalization, the patient spoke with his wife in Turkey who confirmed that his calcium and phosphorus were both within normal limits immediately prior to his infusion. A 1,25-hydroxyvitamin D level obtained during his workup was normal at 42.5 pg/mL (19.9–79.3 pg/mL), however an FGF23 level, which returned 2 weeks later, was elevated at 287 RU/mL (<=180 RU/mL). Dotatate PET to rule out oncogenic osteomalacia was negative. One month later, after normalization of calcium and phosphorus levels, repeat PTH and FGF23 levels were both within normal limits. Conclusion: This case demonstrates that a transient increase in FGF23 levels may accompany and exacerbate hypophosphatemia following zoledronic acid infusion. The mechanism for this elevation is unclear, though we speculate that in the setting of acute hypocalcemia and hypophosphatemia due to zoledronic acid infusion, secondary hyperparathyroidism may upregulate FGF23 production, which further decreases phosphorus levels. If this scenario is accurate, the transient FGF23 elevation seen is not pathologic, but physiologic. Indeed, in the patient above, calcium and phosphorus repletion lead to normalization of both PTH and FGF23.