SIADH = Syndrome of inappropriate antidiuretic hormone secretion

Important cause of HYPOnatraemia

Diagnosis requires

  • Presence of
    • HYPOnatraemia
    • Euvolaemia (normovolemia)
    • High urinary sodium (>20 mmol/L)
    • Urine osmolality > plasma osmolality
      • Serum osmolality <280 mmol/kg (hypotonic HYPOnatremia)
      • Urine osmolality >500mmol/kg (inappropriately concentrated)
  • Absence of
    • Abnormal renal, cardiac, thyroid, adrenal hepatic function
    • Hypervolaemia (oedema)
    • NOT on a diuretic


  • Malignancy
    • Small cell lung, pancreas, prostate, leukaemia, cervical
  • Central Nervous System (CNS)
    • Tumour
    • Trauma (Head injury, SAH, SDH)
    • Infection (encephalitis, abscess)
    • Vascular (CVA, aneurysm, vasculitis)
    • Other (GBS, MS, DT)
  • Respiratory
    • Tumour
    • Infection (aspergillosis, pneumonia, abscess)
    • COPD, CF
  • Drugs
    • Opiates
    • Psychotropic (TCA, MAOI, SSRI)
    • Cytotoxic, chemotherapeutic


  • Corrected with water restriction 500-1000ml/day
  • Does not respond to Na replacement

References and Links

CCC 700 6

Critical Care


Associate Professor Curtin Medical School, Curtin University. Emergency physician MA (Oxon) MBChB (Edin) FACEM FFSEM Sir Charles Gairdner Hospital.  Passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |

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