Approximately 40% of total serum calcium is bound to albumin. In hypoalbuminaemic patients (common in Indian hospitals: malnutrition, nephrotic syndrome, cirrhosis, critical illness), total calcium will be falsely low while the physiologically active ionized calcium may be normal. The correction formula: Corrected Ca = Measured Ca + 0.8 × (4.0 − Albumin). However, this formula is imprecise - whenever possible, measure ionized calcium directly, especially in ICU patients, those on albumin infusions, or in acid-base disturbances (alkalosis decreases ionized Ca by increasing protein binding).
Hypomagnesaemia causes hypocalcaemia through two mechanisms: (1) it impairs PTH secretion from the parathyroid glands, and (2) it causes skeletal resistance to PTH. The result: the body cannot mount a PTH response to correct the low calcium. Hypocalcaemia will be refractory to calcium replacement until magnesium is corrected. This is the same "magnesium gate" principle seen in hypokalaemia.
| Parameter | Calcium Gluconate 10% | Calcium Chloride 10% |
|---|---|---|
| Elemental Ca per 10 mL | 93 mg (2.3 mmol) | 272 mg (6.8 mmol) |
| Route | Peripheral or central IV | Central line ONLY |
| Extravasation risk | Low (mild irritation) | HIGH (tissue necrosis, skin sloughing) |
| Preferred setting | Ward, general use | Cardiac arrest, ICU with central access |
| Rate | 10 mL over 10 to 20 minutes | 10 mL over 10 to 20 minutes via central line |
This is the most common cause of acute symptomatic hypocalcaemia in Indian surgical wards. It occurs in 20 to 30% of total thyroidectomies. Mechanisms include: (1) inadvertent parathyroid removal, (2) devascularisation of parathyroid glands, (3) hungry bone syndrome (bones rapidly take up calcium after removal of the PTH-driven resorptive stimulus). It typically presents 24 to 72 hours post-operatively. Most cases are transient (weeks to months), but 1 to 2% become permanent.
Prophylactic protocol: Many centres now start empirical oral calcium (Calcium Carbonate 500 mg TDS or Shelcal-500) + Calcitriol 0.25 mcg BD immediately post-thyroidectomy in high-risk patients (total thyroidectomy, Graves disease, central neck dissection).
Chvostek sign: Tapping the facial nerve anterior to the ear causes ipsilateral facial muscle twitching. Present in ~10% of normocalcaemic individuals (low specificity). Trousseau sign: Inflating a BP cuff above systolic for 3 minutes causes carpopedal spasm (wrist flexion, MCP flexion, thumb adduction). More specific than Chvostek. Present in 94% of hypocalcaemic patients and only 1% of normocalcaemic individuals.
| Finding | Mechanism | Clinical Significance |
|---|---|---|
| QTc prolongation (ST segment lengthening) | Delayed phase 2 (plateau) of action potential | Most common ECG change. QTc > 500 ms increases TdP risk. |
| T wave changes (peaked or inverted) | Altered repolarisation | Less specific. Compare with prior ECGs. |
| Bradycardia / heart block | Impaired conduction | Rare but reported in severe hypocalcaemia. |
| Torsades de Pointes (TdP) | Prolonged QT-triggered arrhythmia | Life-threatening. Treat with IV Mg + IV Ca + overdrive pacing. |
| Low PTH | High PTH | Other / Multifactorial |
|---|---|---|
| Post-surgical hypoparathyroidism | Vitamin D deficiency (most common globally) | Acute pancreatitis |
| Autoimmune hypoparathyroidism | CKD (impaired 1,25-D synthesis) | Sepsis / critical illness |
| Infiltrative (haemochromatosis, Wilson) | Pseudohypoparathyroidism (PTH resistance) | Massive blood transfusion (citrate) |
| Hypomagnesaemia (impairs PTH secretion) | Hyperphosphataemia (CKD, rhabdomyolysis, TLS) | Bisphosphonate / denosumab therapy |
| DiGeorge syndrome (congenital) | Hungry bone syndrome (post-parathyroidectomy) | Alkalosis (reduces ionized Ca) |
AMA Style:
Umakanth S. Acute Hypocalcaemia Protocol. MEDiscuss. Published 2026. Accessed .
Vancouver Style:
Umakanth S. Acute Hypocalcaemia Protocol [Internet]. MEDiscuss.org; 2026 [cited ]. Available from:
