Figure 126

Several inherited disorders have been described that result in isolated renal phosphate wasting. These disorders include X-linked hypo-phosphatemic rickets (HYP), hereditary hypophosphatemic rickets with hypercalciuria (HHRH), hypophosphatemic bone disease (HBD), autosomal dominant hypophosphatemic rickets (ADHR), autosomal recessive hypophosphatemic rickets (ARHR), and X-linked recessive hypophosphatemic rickets (XLRH). These inherited disorders share two common features: persistent hypophosphatemia caused by decreased renal tubular phosphate (Pi) reabsorption (expressed as decreased ratio of plasma concentration at which maximal phosphate reabsorption occurs [TmP] to glomerular filtration rate [GFR], [TmP/GFR], a normogram derivative of the fractional excretion of

Pi); and associated metabolic bone disease, eg, rickets in children or osteomalacia in adults [5]. These disorders can be distinguished on the basis of the renal hormonal response to hypophosphatemia, the biochemical profile, and responsiveness to therapy. In addition, the rare disorder XLRH is associated with nephrolithiasis. The clinical features of the two most common disorders HYP and HHRH are contrasted here. Whereas both disorders have defects in renal Pi reabsorption, the renal hormonal response to hypophosphatemia is impaired in HYP but not in HHRH. Indeed, in children with HHRH, phosphate supplementation alone can improve growth rates, resolve the radiologic evidence of rickets, and correct all biochemical abnormalities except the reduced TmP GFR [5].

PEX (endopeptidase)

PEX (endopeptidase)

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