Dilutional hyponatremia (SIADH)

Alternative names
Syndrome of inappropriate antidiuretic hormone secretion; SIADH

Dilutional hyponatremia is a disorder of fluid and electrolyte balance caused by excessive release of antidiuretic hormone (ADH).

Causes, incidence, and risk factors

Fluid and electrolyte imbalance in this syndrome result from the inability to excrete dilute urine, retention of water within the body, and low sodium levels. The most common cause of SIADH is a type of lung cancer called “oat cell” or small cell, which secretes excessive ADH.

Other causes are pancreatic cancer, prostate cancer, Hodgkin’s disease, central nervous system disorders, pulmonary disorders, certain drugs, thymomas, myxedema, and psychosis. SIADH may also develop in some children during the acute phase of meningitis. Risk factors are related to the causes.


  • Weight gain  
  • Loss of appetite  
  • Nausea  
  • Vomiting  
  • Headache  
  • Muscle weakness  
  • Muscle spasms or cramps  
  • Restlessness  
  • Fatigue  
  • Irritability  
  • Abnormal mental status       o Possible coma       o Hallucinations       o Consciousness, decreased       o Confusion  
  • Convulsions

Signs and tests

  • Low serum sodium  
  • Low serum osmolality  
  • High urine osmolality  
  • Adequate urine sodium excretion  
  • No edema  
  • No evidence of dehydration

The first line of treatment is to restrict fluid intake to one-half or one liter of fluid per day. The cause of the disorder must be treated, especially in the case of malignancy where radiation, chemotherapy, or surgical removal of the tumor may correct the water retention.

Expectations (prognosis)
The outcome is related to the underlying disease.


  • Possible coma  
  • Convulsions  
  • Electrolyte imbalances

Calling your health care provider
Call your health care provider if symptoms of dilutional hyponatremia occur.

Prompt treatment of causative conditions may be helpful.

Johns Hopkins patient information

Last revised: December 3, 2012
by Levon Ter-Markosyan, D.M.D.

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