Hyponatraemia is defined as a plasma sodium < 130 mmol/l.
It is the result of an excess of water in comparison to sodium or a deficit of sodium in comparison to water. If the former, the increase in electrolyte-free water (EFW) must be accompanied by the secretion of ADH in order to prevent the excretion of the excess EFW.
There is an expansion of ICF volume, unless the hyponatraemia is secondary to hyperglycaemia.
It is important to differentiate between:
and