SIADH Pituitary and Osmolality Disorders
SIADH
Definition and clinical scope
Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a common cause of euvolaemic, hypotonic hyponatraemia encountered in the emergency department (ED).
It results from inappropriate or continued release/action of antidiuretic hormone (ADH/arginine vasopressin), causing renal free‑water retention and dilutional hyponatraemia despite a clinically euvolaemic state. Early recognition, neurological assessment and safe, monitored correction are ED priorities (NICE CG174).
Pathophysiology
- ADH acts at V2 receptors in the collecting duct to increase aquaporin insertion and water reabsorption.
- In SIADH, ADH activity is inappropriately high for plasma tonicity; serum sodium and osmolality fall while extracellular volume remains near normal.
- The brain adapts to chronic hyponatraemia by losing osmolytes; therefore overly rapid correction risks osmotic demyelination (central pontine/extrapontine myelinolysis).
Common causes to look for (practical ED grouping)
- Drugs: SSRIs, tricyclic antidepressants, carbamazepine, cyclophosphamide, vincristine, some proton pump inhibitors, thiazide diuretics (thiazides often cause hyponatraemia by mixed mechanisms). Review medication list carefully (RCEM).
- Malignancy: classically small‑cell lung cancer; also other thoracic/abdominal and haematological cancers (paraneoplastic ADH secretion).