PTH and hypophosphatemia: Hypophosphatemia stimulates the renal 1a-hydrolase, increasing in turn the synthesis and serum levels of 1,25 dihydroxyvitamin D, with the resultant suppression of parathyroid hormone (PTH) levels, increased intestinal reabsorption of calcium and hypercalciuria, which differentiates HHRH from autosomal dominant hypophosphatemic rickets (ADHR) and X-linked hypophosphatemia (XLH).