classification of SIADH

Last reviewed 04/2022

SIADH can be divided according to the pattern of arginine vasopressin (AVP) secretion across a range of plasma osmolalities.

  • type A
    • commonest form of SIADH, is thought to be responsible for 60-70% of cases
    • characteristically seen in lung cancer and in nasopharyngeal tumors
    • patients are prone to develop severe hyponatraemia since plasma AVP concentrations are not suppressed physiologically by drinking
    • there is also a lower osmotic threshold for thirst appreciation

  • type B
    • common, seen in 20-40%
    • secretion of AVP occurs at lower plasma osmolalities than normal
    • since secretion of AVP is supressed at a lower osmotic threshold for AVP release, patients are protected against the progression to severe hyponatraemia

  • type C
    • rare
    • there is failure to suppress AVP at plasma osmolalties below the osmotic threshold, hence inappropriately high plasma AVP levels are seen even at low plasma osmolalities

  • type D
    • rare
    • low or undetectable AVP levels and no detectable abnormality in circulating AVP response o is thought to be due to nephrogenic SIADH (1)