Syndrome of inappropriate antidiuretic hormone (SIADH)
SIADH is euvolemic hypotonic hyponatremia caused by antidiuretic hormone secretion or action that is inappropriate for the patient's osmotic and volume status. First-line treatment is fluid restriction to about 800-1,000 mL/day, with vasopressin receptor antagonists (vaptans) or hypertonic saline reserved for severe or symptomatic hyponatremia resistant to fluid restriction.
- Topic reviewThe full article, organized the way the exam asks it.
- ABIM-format MCQsVignettes with an explanation for every wrong answer.
- Key factsThe numbers and thresholds worth memorizing, up front.
- AI tutorMedBrain answers the question you're stuck on.
What this article covers
- Overview
Definition: the syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a cause of hypotonic hyponatremia in which antidiuretic hormone (ADH, vasopressin) is secreted or acts in excess of…
- Epidemiology
A common cause of hyponatremia in hospitalized patients: SIADH is one of the leading causes of hypotonic hyponatremia encountered in inpatient medicine, particularly in patients with malignancy…
- Risk factors
Central nervous system disorders: trauma, stroke, brain metastases, and CNS infection can all trigger excess release of antidiuretic hormone from the hypothalamic-pituitary axis.
- Pathophysiology
Excess ADH in the absence of a normal osmotic or volume stimulus: ADH secretion or action that persists despite hypo-osmolality, which would normally suppress ADH release, drives ongoing free water retention.
- Clinical features
Diagnostic core feature: the defining clinical pattern is euvolemic hyponatremia with hypo-osmolality of the plasma and an inappropriately concentrated urine, in a patient without an osmotic or…
- Differential diagnosis
Hypovolemic hyponatremia (subclinical hypovolemia): volume depletion can mimic SIADH biochemically, but urinary sodium below 20 mmol/L, increased serum creatinine and uric acid, and increased plasma…
- Diagnosis
Diagnostic basis: SIADH is diagnosed by demonstrating euvolemic hypotonic hyponatremia with an inappropriately concentrated urine, after excluding hyperosmolar states, adrenal insufficiency, and hypothyroidism as alternative explanations.
- Management
First, identify and treat the underlying cause: correcting the precipitating disorder, whether a pulmonary infection, an offending medication, or postoperative posterior pituitary irritation, is the…
- Complications
Osmotic central pontine myelinolysis: overly rapid correction of chronic hyponatremia, exceeding 12 mmol/L in 24 hours, can cause this often-fatal demyelinating injury, making correction-rate limits central to safe management.
- Prognosis
Favorable with identification and treatment of the underlying cause: most cases of SIADH resolve once the precipitating disorder, such as a medication, infection, or postoperative irritation, is corrected.
- Prevention
Postoperative sodium monitoring after pituitary surgery: checking serum sodium through at least the first 12 postoperative days allows early detection before severe hyponatremia develops.
Read the whole article
Subscribing to Internal Medicine opens every article, the full question bank and MedBrain, on the web, iPhone and Android.
Related articles
- Secondary adrenal insufficiency
Secondary adrenal insufficiency is ACTH deficiency from pituitary or hypothalamic disease, or most commonly from withdrawal of prolonged exogenous glucocorticoid therapy, causing isolated cortisol deficiency without hyperpigmentation or hyperkalemia because aldosterone secretion is preserved. Lifelong glucocorticoid replacement with hydrocortisone, with dose increases during illness, prevents adrenal crisis.
- Hypothyroidism
Hypothyroidism is thyroid hormone deficiency, most commonly caused by autoimmune (Hashimoto) thyroid failure in iodine-sufficient countries, diagnosed by an elevated thyroid-stimulating hormone with a low or normal free thyroxine. First-line treatment is levothyroxine dosed at 1.6 mcg/kg lean body weight orally once daily, titrated using thyroid-stimulating hormone rechecked no sooner than 6 weeks after any dose change.
- Central diabetes insipidus
Central diabetes insipidus is inability of the posterior pituitary to produce adequate antidiuretic hormone, causing free water loss, polyuria, and hypernatremia; common causes include pituitary/hypothalamic tumor, surgery, trauma, and infiltrative disease. Diagnosis is confirmed with a water deprivation test followed by a desmopressin challenge, and treatment is desmopressin (DDAVP) 10-40 mcg intranasally or 0.1-0.4 mg orally, dosed once nightly or twice daily and titrated to control polyuria while avoiding dilutional hyponatremia from over-replacement.
- Hyponatremia
Hyponatremia is a serum sodium below 135 mEq/L, worked up by serum osmolality then volume status and urine osmolality/sodium; treatment is a 3% saline bolus for severe or acute symptomatic cases (chronic correction capped at 8 mEq/L per 24 hours), isotonic saline for hypovolemic disease, and fluid restriction plus tolvaptan or demeclocycline for SIADH.