FGF23 and hypophosphatemia: In patients with mutations in the genes encoding Npt2a or Npt2c, the decreased tubular phosphate reabsorption induces both a “downregulation” of FGF23 (which in turn decreases the inhibition of 1,25(OH)2D by FGF23) and a direct stimulation of 1,25(OH)2D to increase intestinal phosphate absorption to maintain circulating phosphate levels, even in the absence of overt hypophosphatemia.