Effect of Paricalcitol on Circulating Parathyroid Hormone in X-Linked Hypophosphatemia: A Randomized, Double-Blind, Placebo-Controlled Study

Context: Hyperparathyroidism occurs frequently in X-linked hypophosphatemia (XLH) and may exacerbate phosphaturia, potentially affecting skeletal abnormalities. Objective: The objective of the study was to suppress elevated PTH levels in XLH patients. Design: This was a prospective, randomized, placebo-controlled, double-blind, 1-year trial of paricalcitol, with outcomes measured at entry and 1 year later. Setting: Patients were recruited from the investigators' clinics or referred from throughout the United States. Data were collected in an in-patient hospital research unit. Patients: Subjects with a clinical diagnosis of XLH and hyperparathyroidism were offered participation and were eligible if they were 9 years old or older and not pregnant, and their serum calcium level was less than 10.7 mg/dL, their 25-hydroxyvitamin D level was 20 ng/mL or greater, and their creatinine level was 1.5 mg/dL or less. Intervention: The intervention for this study was the use of paricalcitol or placebo for 1 year. Main Outcome Measures: Determined prior to trial onset was the change in PTH area under the curve. Secondary outcomes included renal phosphate threshold per glomerular filtration rate, serum phosphorus, serum alkaline phosphatase activity, and Tc-99m-methylenediphosphonate bone scans. Results: PTH area under the curve decreased 17% with paricalcitol, differing (P = .007) from the 20% increase with placebo. The renal phosphate threshold per glomerular filtration rate increased 17% with paricalcitol and decreased 21% with placebo (P = .05). Serum phosphorus increased 12% with paricalcitol but did not differ from placebo. Paricalcitol decreased alkaline phosphatase activity in adults by 21%(no change with placebo, P = .04). Bone scans improved in 6 of 17 paricalcitol subjects, whereas no placebo-treated subject improved. Hypercalciuria developed in six paricalcitol subjects and persisted from baseline in one placebo subject. Conclusions: Suppression of PTH may be a useful strategy for skeletal improvement in XLH patients with hyperparathyroidism, and paricalcitol appears to be an effective adjunct to standard therapy in this setting. Although paricalcitol was well tolerated, urinary calcium and serum calcium and creatinine should be monitored closely with its use.