FGF23 and chronic kidney disease: Several studies have shown that FGF23 levels increase as the glomerular filtration rate decreases in both children and adults with CKD as a result of the decrease in functional nephrons, the excreted fraction of phosphate decreases, and this is detected by specific extracellular sensors, such as the PiT1/PiT2 cotransporters in osteocytes, FGFR1c, and the calcium-sensitive receptor (CaSR).