Pathways for phosphate absorption in the small intestine. In addition to passive paracellular transport, two transporters, sodium-dependent phosphate transporter 2b (Npt2b) and Pit1, account for 50% or more of phosphate absorption. Npt2b is down-regulated by fibroblast growth factor 23 (FGF-23), and up-regulated by 1,25-dihydroxyvitamin D (1,25(OH)2D) and a low phosphate (P) diet. Little is known about the regulation of the Pit1 transporter at the present time. Abbreviation: Pi, inorganic phosphate.