Phosphate (serum)

Serum inorganic phosphate reflects the balance between intestinal absorption, renal tubular reabsorption, and exchange with bone and intracellular stores, regulated principally by parathyroid hormone (PTH), fibroblast growth factor 23 (FGF-23), and calcitriol. Hyperphosphataemia is clinically significant mainly in chronic kidney disease, where impaired renal phosphate excretion drives secondary hyperparathyroidism, vascular calcification, and cardiovascular mortality. Even within the normal reference range, higher serum phosphate in community-dwelling adults has been independently associated with left ventricular hypertrophy, arterial stiffness, and increased all-cause mortality in large prospective studies. Hypophosphataemia can accompany refeeding syndrome, vitamin D deficiency, or overactive FGF-23 signalling, impairing ATP generation, red cell function, and bone mineralisation.