Causes of SIADH (Syndrome of Inappropriate Antidiuretic Hormone)

Clinic A/P, adults, Hospital A/P, Adults

  • Malignancy: lung, brain, GI, GU, lymphoma, leukemia, thymoma, mesothelioma
  • Pulmonary conditions: Pneumonia, asthma, COPD, Pneumothorax, positive pressure ventilation, TB, aspergillosis.
  • Intracranial causes: Trauma, stroke, hemorrhage, infection, hydrocephalus, Guillain-Barre syndrome.
  • Drugs: Antipsychotics, antidepressants (esp. SSRIs), chemotherapy, AVP, MDMA
  • Other causes: Pain, nausea, postoperative state.

Source of above list: Pocket Medicine

“The root cause is the pathologic secretion of antidiuretic hormone (ADH), which can occur in response to some drugs and a variety of conditions, including infections, tumors, and dysregulation in the nervous system involving sympathetic tone and baroreflex response. Inappropriate release of ADH increases free water reabsorption, which increases circulating blood volume, dilutes sodium, and lowers hematocrit and hemoglobin. Urine output is often lowered because of this reabsorption, and the urine is more concentrated (urine osmolality > plasma osmolality) with sodium levels >20 mEq/L. Modest weight gain may be noted as a result of the increased blood volume.

Polydipsia also causes dilution of serum sodium and hemodilution but results in dilution of urine. Excessive beer drinking may result in hyponatremia and hypokalemia (beer potomania) as a result of overhydration with a fluid containing inadequate solute. Laboratory testing for sodium can be falsely reduced in a hyperglycemic state. A simple calculation can correct for this: Corrected (Na+) = Measured (Na+) + (2.4 × glucose (mg/dL) – 100 mg/dL)/100 mg/dL.” ABFM critique 2017

Patient with SIADH has these labs:
“serum sodium level of 125 mEq/L (N 135–145). His serum osmolality is 268 mOsm/kg (N 275–290). His urine sodium level is 50 mEq/L (N <20) and his urine osmolality is 300 mOsm/kg.”

 

Diabetes insipidus (too little ADH or ADH doesn’t work). 
SIADH (Syndrome of inappropriate ADH) – Too much ADH.

 

Reference

Pliquett RU, Mohr P, Obermuller N: Endocrine testing for the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Endotext website, 2014.

Braun MM, Barstow   CH, Pyzocha NJ: Diagnosis and management of sodium disorders: Hyponatremia and hypernatremia. Am Fam Physician 2015;91(5):299-307.

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