Syndrome of inappropriate antidiuretic hormone

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SIADH presents with hypotonic hyponatremia: low serum sodium and osmolality.

Pseudohyponatremia can occur due to hyperglycemia-induced water shift from the intracellular to extracellular space. This should be suspected and excluded in patients with a history of diabetes mellitus, noncompliance with diabetic therapy, polyuria, and polydipsia.

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SIADH presents with hypotonic hyponatremia: low serum sodium and osmolality.

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Usually low due to mild volume expansion.

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Values of >100 mOsm/kg H₂O indicating elevated arginine vasopressin level support the diagnosis of SIADH if present in conjunction with low serum sodium and low serum osmolality.

Low levels occur in the presence of hyponatremia due to excessive water intake.

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Urine sodium with normal dietary salt and water intake.

If patient is euvolemic, results of >30 mEq/L are consistent with SIADH.

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1 to 2 L of normal saline can be administered as a therapeutic trial if volume depletion is suspected. Serum sodium will improve in this situation, while in SIADH it will not.

A trial of normal saline should not be performed if patient is symptomatic from hyponatremia (e.g., altered mental status, seizure, coma).

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Usually not necessary.

Low serum uric acid level indicates mild volume expansion, consistent with SIADH.

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Confirms euvolemic state in people with SIADH.

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Confirms euvolemic state in people with SIADH.

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Hypothyroidism should be ruled out when diagnosing SIADH.

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Addisonism should be ruled out when diagnosing SIADH.

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AVP values vary in normal subjects with levels rising above 2.5 picograms/mL as serum sodium reaches 140 mEq/L and beyond.

AVP not routinely recommended as urinary osmolality >100 mOsm/kg H₂O is sufficient to diagnose excess AVP.[16]Ellison DH, Berl T. The syndrome of inappropriate antidiuresis. N Engl J Med. 2007 May 17;356(20):2064-72.