The Identification and characterization of FGF-23 has provided an opportunity to gain new insight into phosphorus metabolism. Circulating FGF-23 promotes renal excretion of phosphorus, and FGF-23 is measurable in the serum of normal subjects. Serum levels of FGF-23 are elevated in patients with renal phosphate wasting disorders such as tumor induced osteomalacia, X-linked hypophosphatemia and fibrous dysplasia. However, the factors that alter its serum concentration are not known. The study of serum FGF-23 is confounded by the fact that high serum calcium, PTH, and any other putative phosphotonins, have similar effects on serum and urine phosphorus. To circumvent the confounding effect of serum PTH and calcium, we studied serum FGF-23 and phosphate levels in patients with chronic hypoparathyroidism and hyperphosphatemia. Serum was collected in the morning after an overnight fast from three groups: 1) 9 patients with chronic hypoparathyroidism on stable treatment with calcium and calcitriol, 2) 9 patients with primary hyperparathyroidism, and 3) 77 normal controls. Patients with hypoparathyroidism had predictably higher levels of serum phosphorus than patients with hyperparathyroidism or normal controls (5.6 +/- 1.1, 3.1 +/- 0.6, and 3.1 +/- 0.5 mg/dL, mean +/- 1 SD, respectively (p < 0.01 for hypoparathyroid vs. either group)). They also had higher levels of FGF-23 (150 +/- 120 vs. 70 +/- 60, or 55 +/- 20 RIU/ml, respectively (p < 0.05 vs. either group)). In conclusion, serum FGF-23 levels are elevated in patients with hyperphosphatemia and chronic hypoparathyroidism, suggesting a feedback system in which serum FGF-23 responds to serum phosphorus and regulates it. However, in the setting of chronic hypoparathyroidism, the degree of elevation of FGF-23 is insufficient to normalize serum phosphorus.