Hypocalcaemia or hypocalcemia is the presence of low serum calcium levels in the blood. Physiologically, blood calcium is tightly regulated within a narrow range for proper cellular processes. Calcium in the blood exists in three primary states: bound to proteins (mainly albumin), bound to anions such as phosphate and citrate, and as free (unbound) ionized calcium. Only the ionized calcium is physiologically active. Normal blood calcium level is between 8.5 to 10.5 mg/dL (2.12 to 2.62 mmol/L) and that of ionized calcium is 4.65 to 5.25 mg/dL (1.16 to 1.31 mmol/L). Common causes of hypocalcemia include hypoparathyroidism, vitamin D deficiency, and chronic kidney disease. Symptoms of hypocalcemia include neuromuscular irritability (including tetany as manifested by Chvostek's sign or Trousseau's sign, bronchospasm), electrocardiographic changes, and seizures. Treatment is dependent upon the cause, but most commonly includes supplementation of calcium and some form of vitamin D or its analogues.