Hypocalcaemia is a lower-than-normal concentration of calcium in the blood — below about 2.2 mmol/L. Since calcium governs nerve excitability and muscle contraction, deficiency makes nerves twitchy: tingling around the mouth and in the fingers, muscle cramps, and in severe cases carpopedal spasm and seizures; the heart’s QT interval lengthens toward dangerous arrhythmia. Causes include vitamin D deficiency, hypoparathyroidism (often after thyroid surgery), kidney failure, and acute pancreatitis. Chronic insufficiency also starves bone — though blood calcium is defended at bone’s expense until late. Treatment replaces calcium intravenously in emergencies and corrects the underlying cause — usually vitamin D — for the long term.