Calcium, magnesium and phosphate disorders

Calcium, magnesium and phosphate travel together. Almost all body calcium is skeletal (less than 1 percent of total calcium is in the extracellular fluid), magnesium is predominantly intracellular, and phosphate is their structural partner in hydroxyapatite, so the small serum fraction is a tightly regulated snapshot of a much larger reservoir. [3] [8] Three hormones hold the serum in range: parathyroid hormone raises calcium by resorbing bone and reabsorbing calcium in the distal tubule while it dumps phosphate; activated vitamin D (calcitriol) increases gut absorption of all three; and fibroblast growth factor 23 lowers phosphate by forcing the kidney to excrete it and by switching off calcitriol. [9] When one mineral moves, the others follow, which is why a single abnormal value almost always disturbs its neighbours.

This page treats the three minerals as one system, then splits them by the direction of the disturbance and the mechanism that drove it. Calcium sets the urgency, because a low ionised calcium can stop the heart through a prolonged QT and obstruct the airway through laryngospasm, and a high calcium can dehydrate and destabilise. [1] Magnesium is the hidden cause, because it gates both parathyroid hormone secretion and the renal potassium channel, so a deficit silently breaks calcium and potassium homeostasis at once. [7] Phosphate is the chronic bone problem and the acute refeeding and tumour lysis problem, and its value swings with age, because growing children hold far more phosphate than adults to build skeleton. [9] [12]

Overview & Definition

The first task at the bedside is to read the calcium correctly. Total calcium is reported as a concentration bound to albumin, so hypoalbuminaemia makes a normal calcium look low. About half of extracellular calcium is bound to albumin and the other half is the ionised active form, so hypoalbuminaemia makes a normal calcium look low. Order a laboratory corrected calcium (with albumin) and an ionised calcium; the ionised fraction is the value to trust in critical illness, after massive transfusion, in alkalosis and whenever the albumin is unreliable. [1] [2]

Hypocalcaemia is a total calcium below 2.2 mmol/L or an ionised calcium below 1.0 mmol/L; symptoms typically appear with a total calcium below 1.8 mmol/L, and severe features (arrhythmia, stridor, seizures) cluster below 1.6 mmol/L. Severe hypercalcaemia with dehydration or altered consciousness is an emergency. [1] Magnesium is normally 0.7 to 1.1 mmol/L, and hypomagnesaemia is a serum magnesium below 0.7 mmol/L. Phosphate is age-dependent and must be read against the local paediatric reference range, because growing children hold more phosphate than adults to build skeleton. [8]

References13Show ledgerHide ledger

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