🧪 Calcium Homeostasis & Disturbances
Hypercalcaemia and hypocalcaemia — causes, clinical features, ECG changes, and management — MRCS high-yield.
Calcium Homeostasis — Overview
Calcium is the most abundant mineral in the body and plays a critical role in neuromuscular function, cardiac rhythm, bone mineralisation, coagulation, and enzyme activation. Serum calcium is tightly regulated between 2.2–2.6 mmol/L by the interplay of PTH, calcitriol (active vitamin D), and calcitonin.
| Feature | Hypercalcaemia (>2.6 mmol/L) | Hypocalcaemia (<2.2 mmol/L) |
|---|---|---|
| Most common cause (outpatient) | Primary hyperparathyroidism | Post-operative (thyroid/parathyroid surgery) |
| Most common cause (inpatient) | Malignancy | Vitamin D deficiency / chronic kidney disease |
| Neuromuscular excitability | ↓ (depressed — muscles and nerves less excitable) | ↑ (enhanced — muscles and nerves hyperexcitable) |
| ECG | Shortened QT interval | Prolonged QT interval |
| Hallmark signs | “Stones, Bones, Groans, Psychic Moans” | Chvostek’s, Trousseau’s, perioral paraesthesia, stridor |
| Emergency treatment | IV 0.9% saline first, then bisphosphonates | IV 10% Calcium Gluconate |
Corrected Calcium — Always Adjust for Albumin
~40% of serum calcium is bound to albumin — only the ionised (free) fraction is physiologically active. Total serum calcium must be corrected for albumin before interpreting:
Corrected Ca²⁺ = Measured Ca²⁺ + 0.02 × (40 − serum albumin g/L)
Example: Albumin 20 g/L, measured Ca 2.0 mmol/L → corrected Ca = 2.0 + 0.02 × (40−20) = 2.0 + 0.40 = 2.40 mmol/L (normal). Without correction, this patient would be misdiagnosed as hypocalcaemic.
Alternatively, ionised calcium can be measured directly on a blood gas analyser — useful in critically ill patients where albumin is unreliable.