Phosphate Total body phosphate is found mostly in bone (80-85%), with smaller amounts in muscle and the ECF (<1%). Phosphate is the major intracellular anion. Serum phosphate consists of both organic and inorganic phosphate. Organic phosphate is in phospholipids, phosphate esters, phosphoproteins, nucleic acids, etc. Inorganic phosphate is measured in assays, most of which is present as mono- and dihydrophosphate. 10-25% of phosphate is protein bound, and the rest is free or complexed to cations such as calcium, sodium and magnesium. Results for Pi are expressed as milligrams per deciliter (mg/dl). Phosphate is absorbed in the small intestine (especially the jejunum) by both paracellular and active transport. Absorption is decreased by low vitamin D, high calcium and low phosphate levels in the diet and other compounds such as iron and aluminium. Absorption is enhanced by low dietary calcium, increased dietary acidity, growth hormone and vitamin D. Phosphate is excreted in saliva, feces (30-40%) and urine (60-90%). In the kidney, 80-90% of filtered phosphate is resorbed in the PCT with sodium. The remainder is absorbed in the distal nephron. Renal excretion is increased by PTH, saline distension, steroids, calcitonin, bicarbonate; whereas excretion is reduced by thyroid hormone, growth hormone and insulin-like growth factor. [Hypophosphatemia] [Hyperphosphatemia]