Plasma phosphorus is mainly inorganic phosphate and represents a small fraction (less than 0.2%) of total body phosphate. Important determinants of plasma inorganic phosphate are renal excretion, intestinal absorption, and shift between the intracellular and extracellular spaces. The kidney is the most important regulator of the serum phosphate level. PTH decreases reabsorption of phosphate in the proximal tubule while 1,25-dihydroxyvitamin D increases reabsorption. Renal proximal tubular reabsorption of phosphate is decreased by hypertension, corticosteroids, metabolic acidosis, and proximal tubular dysfunction (as in Fanconi syndrome). Fibroblast growth factor 23 (FGF23) is a potent phosphaturic hormone. Intestinal absorption of phosphate is facilitated by active vitamin D. Cellular phosphate uptake is stimulated by various factors and conditions, including alkalemia, insulin, epinephrine, refeeding syndrome, hungry bone syndrome, and accelerated cell proliferation.
Phosphorus metabolism and homeostasis are intimately related to calcium metabolism. See sections on Metabolic Bone Disease in Chapter 26.