FIGURE 1.

Calcium and phosphorus homeostasis. Regulation of calcium and phosphorus homeostasis tightly relies on a variety of concerted actions of three target organs: intestine, kidney and bone, whose functions are principally regulated by a panel of hormones, including PTH, vitamin D, FGF23 and various factors. Tiny changes in calcium could be sensed by CaSR and Calcitonin located in the thyroid gland, thereby modulating the flow of PTH accordingly. Under the influence of effectors of TRPVs, NaPi-II cotransporters, PTH1R, etc., PTH mediates the absorption and reabsorption of calcium and phosphorus in the bones and kidneys. 1,25(OH)₂D, the chief mediator for calcium and phosphorus absorption at intestine, could also regulate the bone calcium absorption via RANKL system. FGF23, another critical hormone that promote phosphorus urinary excretion, has been shown to decrease NaPi-II cotransporters expression in the proximal tubule.