Calcitonin for Treating X-linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is the most common form of inherited rickets in the United States. It also causes bone disease in adults. XLH is caused by overproduction of a hormone call FGF23, which makes the body waste phosphate. This study is designed to determine if nasal calcitonin, an already approved drug in the US, can lower blood levels of FGF23 and reduce phosphate wasting in patients with XLH. In this study the investigators will: 1. Determine whether nasal calcitonin significantly lowers integrated 24-hour blood levels of FGF23 in patients with XLH. 2. Evaluate whether nasal calcitonin improves serum phosphate levels in XLH. 3. Assess whether nasal calcitonin improves blood levels of the active form of vitamin D and calcium absorption from the intestine. 4. Make sure that nasal calcitonin is safe and well tolerated.