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Summary Table

Category	Details
Risk Factors	Small cell lung cancer, CNS disorders (stroke, hemorrhage, meningitis, trauma), pulmonary disease (pneumonia, TB, positive-pressure ventilation), medications (SSRIs, carbamazepine, cyclophosphamide, vincristine, oxytocin, NSAIDs, ecstasy/MDMA), postoperative state, pain, nausea
Etiology	Excessive ADH release (or ADH-like substance) independent of normal osmotic/volume stimuli, leading to free water retention and dilutional hyponatremia
Patient Presentation	Confusion, lethargy, headache, nausea/vomiting, seizures, or coma in severe cases. Often found incidentally on labs. The patient is euvolemic with no edema, no orthostatic changes, and no signs of dehydration
Classic Physical Exam	Euvolemia: no peripheral edema, no JVD, no ascites, no dry mucous membranes, normal skin turgor. Neurological symptoms correlate with the severity and rapidity of sodium decline
Key Diagnostic Results	Serum sodium < 135 mEq/L, serum osmolality < 280 mOsm/kg (hypotonic), urine osmolality > 100 mOsm/kg (inappropriately concentrated), urine sodium > 40 mEq/L, low serum uric acid, normal thyroid and adrenal function
Management	Mild/chronic: fluid restriction (first-line, 800-1000 mL/day). Moderate: add salt tablets + loop diuretic. Refractory: tolvaptan (vasopressin V2 receptor antagonist) or demeclocycline. Severe/symptomatic (seizures): hypertonic 3% saline with correction rate not exceeding 10-12 mEq/L per 24 hours
Keywords	Euvolemic hyponatremia, concentrated urine with dilute serum, small cell carcinoma, "confused elderly patient on SSRI," low serum osmolality with high urine osmolality, fluid restriction

1. Pathophysiology

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.

2. Classification and Clinical Manifestation

Serum Sodium (mEq/L)	Severity	Typical Clinical Manifestation
130-134	Mild	Often asymptomatic, may have subtle cognitive impairment or gait instability
125-129	Moderate	Nausea, headache, mild confusion, difficulty concentrating
120-124	Severe	Vomiting, marked confusion, drowsiness, disorientation
< 120	Critical/Life-threatening	Seizures, obtundation, coma, respiratory arrest, risk of brainstem herniation

Classification by Source

Category	Examples	Mechanism
Ectopic ADH production	Small cell lung cancer (most classic), pancreatic cancer, lymphoma, thymoma	Tumor cells synthesize and secrete ADH autonomously
CNS disorders	Stroke, subarachnoid hemorrhage, meningitis, encephalitis, traumatic brain injury	Direct irritation or damage to hypothalamic-pituitary axis increases ADH release
Pulmonary disease	Pneumonia, tuberculosis, lung abscess, positive-pressure ventilation	Altered intrathoracic pressure or inflammatory signaling stimulates ADH
Drug-induced	SSRIs, carbamazepine, oxcarbazepine, cyclophosphamide, vincristine, MDMA, oxytocin, NSAIDs, desmopressin	Potentiates ADH action at the kidney or stimulates central ADH release
Postoperative / Pain / Nausea	Any surgical procedure, particularly in elderly patients	Strong non-osmotic stimuli for ADH secretion

3. Diagnostic Workup

Test	Role	Expected Finding in SIADH
Serum sodium	Best initial test when hyponatremia is suspected	< 135 mEq/L
Serum osmolality	Confirms hypotonic state	< 280 mOsm/kg
Urine osmolality	Demonstrates inappropriate concentration	> 100 mOsm/kg (often > 300)
Urine sodium	Differentiates from hypovolemic causes	> 40 mEq/L
Serum uric acid	Supportive finding	Low (< 4 mg/dL)
TSH, cortisol (or ACTH stimulation test)	Required to exclude hypothyroidism and adrenal insufficiency	Must be normal to diagnose SIADH
Volume status assessment	Clinical exam to classify hyponatremia	Euvolemic: no edema, no orthostasis
Chest imaging	Identify underlying etiology	May reveal lung mass (small cell carcinoma), pneumonia, or other pulmonary pathology

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.

4. Management and Treatment

Clinical Scenario	Treatment	Key Details
Severe symptomatic (seizures, coma, obtundation)	3% hypertonic saline IV	Bolus 100-150 mL over 10-20 minutes, may repeat up to 3 times. Goal: raise sodium by 4-6 mEq/L in the first few hours. Do NOT exceed 10-12 mEq/L in 24 hours
Mild-to-moderate, chronic	Fluid restriction	First-line therapy. Restrict to 800-1000 mL/day (must be less than urine output)
Refractory to fluid restriction	Salt tablets + furosemide	Oral NaCl 1-3 g TID with furosemide 20-40 mg daily. Furosemide impairs the kidney's concentrating ability, promoting free water excretion
Persistent or drug-refractory	Tolvaptan (15 mg PO daily, titrate to max 60 mg)	V2 receptor antagonist ("vaptan"). Must be initiated in hospital. Monitor sodium closely in first 24 hours. Contraindicated in liver disease
Alternative for refractory	Demeclocycline (300-600 mg PO BID)	Induces nephrogenic diabetes insipidus. Slow onset (2-5 days). Avoid in liver disease and renal failure
Underlying cause	Treat or remove the trigger	Discontinue offending drug, treat infection, treat malignancy

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.

5. Differential Diagnosis and Distractors

Diagnosis	Why It Looks Similar	Key Discriminator
Cerebral salt wasting (CSW)	Both cause hyponatremia with high urine sodium and occur in CNS disease. This is the single most tested SIADH mimic	CSW patients are hypovolemic (orthostatic hypotension, tachycardia, poor skin turgor). SIADH patients are euvolemic. CSW shows negative fluid balance; SIADH shows positive or neutral fluid balance. CSW is treated with saline replacement, not fluid restriction
Adrenal insufficiency	Euvolemic hyponatremia with concentrated urine. Cortisol deficiency removes tonic suppression of ADH	Low morning cortisol or failed ACTH stimulation test. Often accompanied by hyperkalemia (from aldosterone deficiency), hypoglycemia, and hyperpigmentation (if primary). SIADH has normal potassium
Hypothyroidism	Can cause euvolemic hyponatremia through impaired free water excretion	Elevated TSH, low free T4. Must be checked before diagnosing SIADH
Psychogenic polydipsia	Hyponatremia with dilute serum, often in psychiatric patients	Urine is maximally dilute (osmolality < 100 mOsm/kg) because ADH is appropriately suppressed. In SIADH, urine is inappropriately concentrated
Beer potomania / Tea-and-toast diet	Hyponatremia from low solute intake, dilute serum	Urine osmolality is very low (< 100 mOsm/kg). Low urine sodium. History of poor dietary solute intake
Heart failure / Cirrhosis	Hyponatremia with elevated ADH and concentrated urine	These patients are hypervolemic with peripheral edema, JVD (heart failure), or ascites (cirrhosis). SIADH is euvolemic

6. Traps 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.

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