Syndrome of Inappropriate ADH (SIADH)

01Pathophysiology

SIADH occurs when antidiuretic hormone (ADH, also called arginine vasopressin) is secreted in excess despite the absence of appropriate physiological stimuli. Normally, ADH is released from the posterior pituitary in response to high serum osmolality or low effective circulating volume. In SIADH, this feedback loop is disrupted: ADH is released autonomously, either from the pituitary itself or from an ectopic source such as a small cell lung carcinoma.

The excess ADH acts on V2 receptors in the collecting ducts of the kidney, inserting aquaporin-2 channels into the luminal membrane. This causes the kidneys to reabsorb free water far beyond what the body needs. The retained water dilutes the serum sodium, producing hypotonic hyponatremia. Crucially, because the excess is pure free water (not saline), there is no significant expansion of extracellular volume detectable on exam. The body partially compensates through pressure natriuresis and suppression of aldosterone, which causes the kidneys to excrete sodium. This is why urine sodium is elevated (> 40 mEq/L) and the patient appears clinically euvolemic rather than volume-overloaded.

The neurological symptoms (confusion, seizures, coma) arise from cerebral edema. As serum osmolality drops, an osmotic gradient forms between the extracellular fluid and brain cells, driving water into neurons. The severity of symptoms depends more on the rate of sodium decline than the absolute number. A patient whose sodium drops from 140 to 118 mEq/L over 48 hours may be asymptomatic, while someone whose sodium drops the same amount in 6 hours may seize. This concept is central to both diagnosis and treatment decisions.

The low uric acid seen in SIADH results from the volume-expanded state causing increased renal clearance of uric acid. This is a useful discriminator from other causes of hyponatremia.

02Classification and Clinical Manifestation

Classification by Source

03Diagnostic Workup

The diagnostic approach begins with confirming true hyponatremia on serum sodium. The next step is checking serum osmolality to confirm the hyponatremia is hypotonic (< 280 mOsm/kg). This step excludes pseudohyponatremia from hyperlipidemia or hyperproteinemia (normal osmolality) and hypertonic hyponatremia from hyperglycemia or mannitol (elevated osmolality). For glucose-induced hyponatremia, apply the correction: for every 100 mg/dL increase in glucose above normal, add 1.6 mEq/L to the measured sodium.

Once hypotonic hyponatremia is confirmed, the next critical decision is volume status. This is a purely clinical assessment. If the patient has signs of dehydration (orthostatic hypotension, dry mucous membranes, poor skin turgor), think hypovolemic hyponatremia. If there is edema, JVD, or ascites, think hypervolemic hyponatremia (heart failure, cirrhosis, nephrotic syndrome). If the patient is euvolemic, the differential narrows to SIADH, hypothyroidism, and adrenal insufficiency.

To finalize the SIADH diagnosis, you must order urine osmolality and urine sodium. In SIADH, urine is inappropriately concentrated (> 100 mOsm/kg, often well above 300) despite the dilute serum. This makes physiologic sense: excess ADH forces the kidney to retain water, concentrating the urine when it should be maximally dilute. Urine sodium is > 40 mEq/L because the mild volume expansion triggers natriuresis.

Thyroid function (TSH) and adrenal function (morning cortisol or ACTH stimulation test) must be normal. This is non-negotiable. SIADH is a diagnosis of exclusion; hypothyroidism and cortisol deficiency both cause euvolemic hyponatremia through similar mechanisms and must be ruled out before labeling a patient with SIADH. Exam questions frequently test this requirement.

After establishing the diagnosis, pursue the underlying etiology. A chest X-ray or CT is warranted to look for a lung mass (especially in a smoker or older adult) or pulmonary infection. Brain imaging may be needed if a CNS cause is suspected.

04Management and Treatment

For acute symptomatic hyponatremia with seizures or altered consciousness, the immediate priority is 3% hypertonic saline. This is the "next best step" whenever a vignette describes a seizing patient with sodium in the low 110s-120s. The goal is a controlled, partial correction to alleviate cerebral edema, aiming for a rise of 4-6 mEq/L in the first 1-2 hours. Check sodium every 2 hours during correction. The absolute limit is 10-12 mEq/L in any 24-hour period (some guidelines use 8 mEq/L as the safer target). Overcorrection risks osmotic demyelination syndrome (previously called central pontine myelinolysis), which causes irreversible damage to the pons with quadriplegia, dysphagia, and "locked-in syndrome." If you overshoot the correction rate, administer desmopressin (DDAVP) 1-2 mcg IV with D5W to lower sodium back down. This "rescue" step is high-yield.

For chronic, mildly symptomatic SIADH, the cornerstone is fluid restriction to 800-1000 mL/day. The logic is straightforward: if the problem is excess free water retention, limit intake. If fluid restriction alone fails (sodium does not improve after 24-48 hours, or the patient cannot comply), escalate to oral salt tablets combined with a loop diuretic. Furosemide works here not as a volume reducer but because it interferes with the medullary concentration gradient, making the kidney less able to concentrate urine and thereby promoting free water loss.

Tolvaptan is a selective vasopressin V2 receptor antagonist that blocks ADH action at the collecting duct. It directly addresses the pathophysiology. It must be started in an inpatient setting because it can cause rapid sodium correction. Do not use tolvaptan in patients with liver disease (it carries an FDA black box warning for hepatotoxicity). Demeclocycline, a tetracycline derivative, is an older alternative that works by making the collecting duct resistant to ADH (inducing nephrogenic diabetes insipidus). Its onset is slow and it carries risks of nephrotoxicity and photosensitivity, so it is generally reserved for when vaptans are unavailable or contraindicated.

Always address the underlying cause. Discontinue the offending medication (switch SSRI, stop carbamazepine), treat the pneumonia, or pursue oncologic management for small cell carcinoma.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common way students lose points on SIADH questions is misidentifying volume status. The entire diagnostic algorithm hinges on whether the patient is hypovolemic, euvolemic, or hypervolemic. Cerebral salt wasting is the classic trap: a post-neurosurgical patient with hyponatremia and high urine sodium looks identical to SIADH on labs, but the vignette will embed signs of volume depletion (tachycardia, orthostatic drop, clinical dehydration) that students overlook. The treatment is opposite: CSW needs volume, SIADH needs restriction.

The second major trap is correcting sodium too quickly. When a vignette describes a patient with chronic hyponatremia (onset > 48 hours or unknown duration), the test-writer is checking whether you understand osmotic demyelination syndrome. The answer is never "give hypertonic saline and normalize the sodium." The answer is controlled correction at no more than 10-12 mEq/L per 24 hours, with frequent monitoring. If the question asks "what is the most dangerous complication of treatment," the answer is osmotic demyelination, not persistent hyponatremia.

Third, remember that SIADH is a diagnosis of exclusion. If the vignette does not mention that thyroid and adrenal function have been checked (or if TSH/cortisol results are abnormal), you cannot confirm SIADH. Questions that present all the classic findings but include an unchecked cortisol level are testing whether you will jump to SIADH prematurely.

Finally, know the urine osmolality cutoff. If urine osmolality is < 100 mOsm/kg in a hyponatremic patient, ADH is appropriately suppressed and the problem is excess water intake (psychogenic polydipsia) or inadequate solute (beer potomania), not SIADH. A concentrated urine in the face of dilute serum is the hallmark finding that makes SIADH the diagnosis.