Endocrinology & Metabolic

SIADH (Syndrome of Inappropriate Antidiuresis)

SIADH is inappropriate antidiuresis causing water retention and euvolaemic hypotonic hyponatraemia with inappropriately concentrated urine; it is a diagnosis of exclusion and management depends first on symptoms, then on the cause and risk of over-correction.

Definition

SIADH, more precisely syndrome of inappropriate antidiuresis, is euvolaemic hypotonic hyponatraemia caused by inappropriate vasopressin activity, producing water retention and inappropriately concentrated urine after other causes of hyponatraemia have been excluded.

First principles

Water excess, not simply sodium deficiency, lowers the sodium

Vasopressin (ADH) acts on collecting-duct V2 receptors despite a low plasma osmolality, so the kidney retains free water. The retained water dilutes plasma sodium; mild volume expansion triggers natriuresis, so the patient is usually clinically euvolaemic rather than oedematous.

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