The clinical history combined with parathyroid hormone levels will reveal the cause of hypocalcaemia in the majority of cases

Causes

• vitamin D deficiency (osteomalacia)
• chronic kidney disease
• hypoparathyroidism (e.g. post thyroid/parathyroid surgery)
• pseudohypoparathyroidism (target cells insensitive to PTH)
• rhabdomyolysis (initial stages)
• magnesium deficiency (due to end organ PTH resistance)
• massive blood transfusion
• acute pancreatitis

Contamination of blood samples with EDTA may also give falsely low calcium levels.

As extracellular calcium concentrations are important for muscle and nerve function many of the features seen in hypocalcaemia seen a result of neuromuscular excitability

Features

• tetany: muscle twitching, cramping and spasm
• perioral paraesthesia
• if chronic: depression, cataracts
• ECG: prolonged QT interval

Very long QT secondary to hypocalcaemia. There are also features of hyperkalaemia. The combination of findings is not uncommon in dialysis patients - e Image used on license from Dr Smith, University of Minnesota

Trousseau's sign (tight arm)

• carpal spasm if the brachial artery occluded by inflating the blood pressure cuff and maintaining pressure above systolic