Calcium and phosphate homeostasis: PTH, vitamin D and FGF23

At a glance

• Serum calcium is held at 2.10 to 2.60 mmol/L total and 1.15 to 1.30 mmol/L ionised. About 45% is ionised (the active fraction), 40% is bound to albumin and 15% is complexed with citrate, phosphate and bicarbonate.
• The calcium-sensing receptor (CaSR) on parathyroid cells sets the PTH response. A fall in ionised calcium of 0.1 mmol/L can raise PTH several-fold within minutes.
• PTH raises calcium and lowers phosphate. Calcitriol (1,25-dihydroxyvitamin D) raises both. FGF23 lowers phosphate and calcitriol.