Hypovolaemic, greatly reduced Na, reduced water

• renal:
  • diuretic therapy
  • salt losing nephritis
  • cerebral salt wasting
  • mineralocorticoid deficiency
  • renal haemorrhage
  • adrenal enzyme deficiencies (congenital adrenal hyperplasia)
  • bicarbonaturia, glucosuria, ketonuria
    
    
• non-renal:
  • vomiting
  • diarrhoea
  • pancreatitis
  • burns

Notes:

• HONK - serum sodium may be low, even if the patient is severely hyperosmolar, due to redistribution of fluid into the extracellular fluid. When the patient is very dehydrated, there may be hypernatraemia.
• DKA - the same mechanism as for HONK

Reference:

1. Adrogué HJ, Tucker BM, Madias NE. Diagnosis and management of hyponatremia: a review. JAMA. 2022 Jul 19;328(3):280-91.