FGF23 and rickets: At low concentrations, increased local secretion of FGF23 by osteocytes may promote osteoblast differentiation and bone formation by acting diffusely on neighbouring cells, whereas at a high concentration, systemic overproduction of FGF23 enters the circulation and acts on target organs such as the kidney, inhibiting renal phosphorus reabsorption and leading to hypophosphatasia rickets.