hypocalcemia
Hypocalcemia means too little calcium in the blood. Because calcium normally dampens nerve and muscle excitability, having too little does the opposite: nerves fire too easily and muscles cramp, tingle, and spasm. In a sense, low calcium leaves the body's electrical wiring jittery and over-sensitive.
Typical signs are tingling around the mouth and fingertips, muscle cramps, and in severe cases tetany, a sustained involuntary spasm, or even seizures. Two classic bedside signs are facial twitching when the cheek is tapped (Chvostek sign) and hand spasm when a blood-pressure cuff is inflated (Trousseau sign). The most direct causes are too little PTH (hypoparathyroidism, often after neck surgery) and vitamin D deficiency, which starves the gut of absorbed calcium.
Other contributors include kidney disease, low magnesium (which is needed for PTH to work), and certain blood transfusions. Severe acute hypocalcemia is an emergency because it can stop breathing muscles or trigger heart-rhythm disturbances, and is treated with intravenous calcium. The body's first defense is to release PTH, which is why hypocalcemia that persists despite that response signals a problem with the parathyroids or vitamin D.
Hours after thyroid surgery, a patient feels tingling lips and cramping hands; the bruised parathyroid glands have stopped making enough PTH, dropping blood calcium.
Post-thyroidectomy tingling is a classic warning of hypocalcemia.
Low magnesium can both blunt PTH release and make tissues resist PTH, so persistent hypocalcemia that will not correct with calcium often needs magnesium replaced first.