A serious condition occurring most often in babies or elderly people, in which too much sodium is present in the blood as a result of loss of water and electrolytes through diarrhoea, excessive sweating, not drinking enough or excessive salt intake.
A serum sodium concentration that is above normal. The condition is usually caused by dehydration (either from inadequate intake or excessive loss of water while still taking in sodium); occasionally it may be caused by by an aldosterone-secreting tumour.