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Hyponatraemia occurs when plasma sodium concentration is below reference levels (usually 140 mEq/l or mmol/l in dogs, and 149 mEq/l or mmol/l in cats) (Benitah, 2010). Occurrence of clinical signs depends more on rapidity of onset of hyponatraemia than to magnitude of change. Neurologic signs may occur with sodium concentrations less than 120 mEq/l or mmol/l in dogs and less than 130 mEq/l or mmol/l in cats (de Morais and DiBartola, 2008a). Serum sodium concentration reflects the amount of sodium relative to the volume of water in the body and not total body sodium content. Hyponatraemic animals may have decreased, increased or normal total body sodium content and therefore volemic status also needs to be considered when investigating serum hyponatraemia (de Morais and DiBartola, 2008a). Hyponatraemia may result primarily from increased water gain, administration of fluids low in sodium (e.g. 5% dextrose in water, 0.45% sodium chloride), hypertonic solution without sodium (e.g. mannitol, glucose) and an excessive sodium loss (e.g. loop diuretics, thiazides). Causes of hyponatraemia classified based on plasma osmolality and hydration status are listed in Box 4.4.