Hypophosphatemic rickets (HR) is a rare inherited disorder characterized by a classic rickets phenotype with low plasma phosphate levels and resistance to treatment with vitamin D. Development of secondary hyperparathyroidism (SHPT) as a direct consequence of treatment is a frequent complication and a major clinical challenge, as this may increase risk of further comorbidity. Cinacalcet, a calcimimetic agent that reduces the secretion of PTH from the parathyroid glands, has been suggested as adjuvant treatment to SHPT in patients with HR. However, only two papers have previously been published and no data are available on effects of treatment for more than six months. We now report a case of 3-year treatment with cinacalcet in a patient with HR complicated by SHPT. A 53-year-old woman with genetically confirmed X-linked dominant hypophosphatemic rickets developed SHPT after 25 years of conventional treatment with alfacalcidol and phosphate supplements. Cinacalcet was added to her treatment, causing a sustained normalization of PTH. Ionized calcium decreased, requiring reduction of cinacalcet, though asymptomatical. Level of phosphate was unchanged, but alkaline phosphatase increased in response to treatment. Cinacalcet appeared to be efficient, safe, and well tolerated. We recommend close control of plasma calcium to avoid hypocalcemia. 1. Introduction Familial hypophosphatemic rickets (HR) is a rare inherited disorder characterized by renal phosphate wasting and inappropriately low levels of calcitriol causing hypophosphatemia and abnormal bone mineralization, first described by Albright . The typical phenotype is characterized by severe bone deformities and growth retardation presenting in childhood with bowing of legs and shortness of length. In adulthood, the disease is often complicated by osteomalacia with bone pain and arthralgias due to joint deformities and enthesopathies. Occurrence of dental problems like tooth abscesses is also common [2, 3]. In most instances, HR is caused by mutations in genes affecting the metabolism of fibroblast growth factor 23 (FGF-23) . The prevailing form of HR is X-linked dominant hypophosphatemia (XLH) caused by an inactivating mutation of the PHEX gene . Medical treatment of HR includes activated vitamin D analogues and supplements with high amounts of oral inorganic phosphate salts which heal osteomalacia, but neither corrects the biochemical abnormalities nor the bone deformities . The renal phosphate leak is not improved; in contrary, treatment has been shown to increase levels of FGF23 . Also,
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