Phosphate (inorganic phosphate) is an essential mineral and the primary intracellular anion, playing crucial roles in bone and teeth mineralisation, energy production (ATP), DNA and cell membrane synthesis, acid-base buffering, and intracellular signalling. About 85% of body phosphate is in bone and teeth; the remainder is in soft tissues and extracellular fluid.

Phosphate levels are regulated by PTH (which promotes renal phosphate excretion), FGF-23 (fibroblast growth factor 23, produced by bone), and vitamin D (which promotes intestinal absorption). Abnormal phosphate is most clinically significant in chronic kidney disease, where reduced renal phosphate excretion causes hyperphosphataemia, contributing to cardiovascular calcification, bone disease, and accelerated CKD progression.