Objective: To determine whether fibroblast growth factor 23 (FGF 23) concentrations are increased in the serum of patients with humoral hypercalcemia of malignancy and in those with hyperparathyroidism. Patients and Methods: Serum samples from patients with hypercalcemia and documented elevations of parathyroid hormone-related protein or parathyroid hormone levels were examined for FGF 23 concentrations. Specimens from healthy age-matched controls were evaluated to establish a reference range for FGF 23. Results: Mean ± SEM concentrations of FGF 23 were elevated in the 7 patients with humoral hypercalcemia of malignancy (385±134 relative unit [RU]/mL) compared with 11 healthy controls (43.9±5.8 RU/mL; P=.005). In the 11 patients with hyperparathyroidism, FGF 23 concentrations were increased (mean ± SEM 86.6±22.8 RU/mL), but the increases were not statistically significant. The increases in FGF 23 in these disorders did not correlate with changes in serum phosphate or 1α,25-dihydroxyvitamin D levels. Conclusion: Levels of FGF 23 are elevated in patients with tumors associated with humoral hypercalcemia of malignancy. The precise cause of such elevations requires further investigation.
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