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Hypocalcaemia results in tetany (intermittent contraction of extensor muscles) and seizures when serum ionized calcium concentration is equal or lower than 0.8 mmol/l (3.2 mg/dl) or total calcium concentration is below 1.5 mmol/l (6 mg/dl) (Drobatz and Casey, 2000; Brauer et al., 2011). To convert mmol/l to mg/dl, the value in mmol/l has to be multiplied by 4 (Schenck and Chew, 2008). Total serum calcium is approximately 50% ionized, 40% protein bound (especially to albumin), and 10% chelated with anions such as citrate or phosphate. Only ionized calcium is biologically active. The proportion of ionized calcium is affected by serum protein level, acid-base status (ionized calcium levels are decreased by alkalosis and increased by acidosis) and the presence of anions that act as chelators (e.g. Mg++ or phosphate). Calcium homeostasis is regulated by parathyroid hormone (PTH), calcitriol (1,25- dihydroxyvitamin D) and calcitonin. The main organs involved in calcium metabolism are bone, kidney and small intestine.

Low calcium concentrations increase neuronal membrane permeability to sodium ions, resulting in neuronal hyperexcitability in the peripheral and central nervous system.