Hypernatraemia is defined as a serum sodium concentration exceeding 145 mmol/L. Serum sodium concentration, and hence osmolality, is normally kept from rising significantly by the release of antidiuretic hormone (ADH) or vasopressin which limits water losses, and the stimulation of thirst which increases water intake. Hypernatraemia from free water loss causes dehydration as intracellular water is drawn out of cells into the extracellular fluid (ECF) preserving the latter to a large extent; however, if sodium is lost as well as water then significant hypovolaemia (reduction in ECF volume) can also occur. Severe symptoms of hypernatraemia are usually only found with acute and large rises in sodium plasma concentration above 160 mmol/L.
This is a companion discussion topic for the original article at https://patient.info/doctor/investigations/hypernatraemia