SIADH - Syndrome of Inappropriate Antidiuretic Hormone Secretion

Definition

• Goodman & Gilman's Pharmacological Basis of Therapeutics, p. 599
• Harrison's Principles of Internal Medicine 22E, p. 782

Pathophysiology

1. ADH acts on V2 receptors in the renal collecting ducts, inserting aquaporin-2 water channels and causing water reabsorption.
2. In SIADH, ADH is secreted autonomously - the normal feedback inhibition (where low osmolality suppresses ADH) does not occur.
3. Water retention expands extracellular fluid volume slightly, but this subtle volume expansion triggers urinary sodium loss (natriuresis), keeping the patient clinically euvolemic despite water excess.
4. The net result: dilutional hyponatremia with concentrated urine.

• Costanzo Physiology 7th Edition
• Goodman & Gilman's, p. 599

Causes (Etiologies)

• Symptom to Diagnosis 4th Ed.
• Goodman & Gilman's, p. 599
• Rosen's Emergency Medicine

Clinical Features

• Mild/Chronic: Often asymptomatic, discovered incidentally; falls, weakness, mild confusion (especially in elderly)
• Moderate: Lethargy, anorexia, nausea/vomiting, muscle cramps, headache
• Severe/Acute: Depressed mental status, seizures, coma, death

• Harrison's 22E, p. 781
• Goldman-Cecil Medicine, p. 1201

Diagnostic Criteria

• Hypothyroidism (TSH)
• Secondary adrenal insufficiency / glucocorticoid deficiency (cortisol, ACTH stim test) - found in 3-4% initially suspected SIADH
• Psychogenic polydipsia
• Hypervolemic states (heart failure, cirrhosis, renal failure)
• Cerebral salt wasting (distinguished by high fractional excretion of uric acid in CSW vs. SIADH)

Important pitfall: Only 33-41% of patients suspected of SIADH are actually evaluated for adrenal insufficiency, which can perfectly mimic SIADH.

• Symptom to Diagnosis 4th Ed.
• Goldman-Cecil Medicine, p. 1201

Reset Osmostat (SIADH Variant)

• Hyponatremia is not progressive
• Patients retain the ability to excrete a water load at the new equilibrium
• Very dilute urine osmolality (<100 mOsm/L) may be seen after water loading
• Etiology similar to SIADH
• Treatment directed at underlying disorder

Treatment

1. Treat the Underlying Cause

• Review and discontinue offending medications
• Treat underlying infection, malignancy, or CNS disease

2. Fluid Restriction

• First-line for most patients
• Restrict fluid intake to less than urine output plus insensible losses
• Strict monitoring of oral and IV fluid intake required
• Most effective for mild-to-moderate chronic SIADH

3. Hypertonic Saline (3% NaCl)

• Reserved for severe hyponatremia (Na+ <115 mEq/L) or symptomatic patients (altered mental status, seizures)
• Can also give 100 mL bolus of 3% saline (repeated up to 2 more times) for rapid correction
• Rate of correction: 0.5-1 mEq/L per hour - slow correction is essential
• Furosemide can be added to enhance free water clearance

4. Vasopressin Receptor Antagonists (Vaptans)

• Mechanism: Block V2 receptors in renal collecting ducts → selective water excretion (aquaresis) without sodium loss
• Tolvaptan 15 mg PO daily - particularly effective in combination with fluid restriction
• Conivaptan 20-120 mg PO BID or 10-40 mg IV
• Especially useful in euvolemic hyponatremia
• Recent evidence: A 2025 meta-analysis (PMID 40288608) confirms low-dose tolvaptan is effective and safe for SIADH-associated hyponatremia

5. Demeclocycline

• 150-300 mg PO 3-4 times daily
• Mechanism: Induces nephrogenic DI, inhibiting vasopressin action on distal tubule
• Largely supplanted by vaptans; onset slow (1-2 weeks)
• Avoid in liver disease

6. Urea

• A 2025 systematic review/meta-analysis (PMID 39362395) supports oral urea as an effective treatment option for SIADH-associated hyponatremia - an emerging alternative, especially in resource-limited settings

7. Salt Tablets / Normal Saline

• Not helpful unless volume depletion is also present

Correction Rate Warning

Osmotic demyelination syndrome (central pontine myelinolysis) occurs with overly rapid correction. Never correct Na+ faster than 8-10 mEq/L in 24 hours (or ~0.5-1 mEq/L per hour). Rapid correction causes brain dehydration and irreversible neurologic damage.

• Harrison's 22E, p. 782
• Goldman-Cecil Medicine
• Goodman & Gilman's, p. 599

Key Distinguishing Points: SIADH vs. Cerebral Salt Wasting

Recent Evidence

• Low-dose tolvaptan meta-analysis (2025) - confirms clinical effectiveness and safety of tolvaptan for SIADH hyponatremia [PMID 40288608]
• Urea meta-analysis (2025) - oral urea is a valid, effective option for SIADH hyponatremia [PMID 39362395]
• CSW vs SIADH in children (2026) - FE-urate and urinary phosphate have diagnostic utility [PMID 41058069]

Hypokalemic Paralysis with Persistent Potassium Loss

Understanding the Key Distinction

In the Brenner & Rector clinical case: a 76-year-old Asian man presented with acute flaccid paralysis, serum K+ of 1.8 mmol/L, urine K+ of 26 mmol/L (inappropriately high for hypokalemia), metabolic alkalosis (pH 7.55, HCO3- 45), and hypertension. The combination of acute transcellular shift (post-exercise catecholamine surge + post-carbohydrate insulin release) ON TOP of chronic renal K+ wasting is the paradigm for this presentation.

Causes of Hypokalemia with Persistent Potassium Loss

A. RENAL POTASSIUM WASTING

1. Mineralocorticoid Excess (with Hypertension)

• Primary aldosteronism (Conn's syndrome) - adrenal adenoma (most common) or adrenal hyperplasia
  • Persistent muscular weakness in 73% of patients; intermittent paralysis in 21%; tetany in 21%
  • Predominantly affects women (unlike familial HypoKPP)
  • Associated with hypertension, hypernatremia, polyuria, metabolic alkalosis
  • Low renin, HIGH aldosterone

• Cushing's syndrome / ectopic ACTH
• Congenital adrenal hyperplasia (17α-hydroxylase deficiency)
• Licorice ingestion / glycyrrhizic acid - acts as potent mineralocorticoid; LOW renin, LOW aldosterone
• Liddle syndrome (constitutively active ENaC) - LOW renin, LOW aldosterone
• Geller syndrome
• Exogenous mineralocorticoids (fludrocortisone)

2. Diuretics

• Loop diuretics (furosemide, bumetanide) - most potent
• Thiazide diuretics - very common cause; Gitelman's syndrome mimics thiazide effect genetically
• Amphotericin B - distal tubule toxicity

3. Renal Tubular Acidosis (RTA)

• Type 1 (Distal RTA) - impaired H+ secretion; hypokalemia + non-anion gap metabolic acidosis + urine pH >5.5
  • Causes: Sjögren's syndrome, toluene inhalation, amphotericin B, lithium
• Type 2 (Proximal RTA) - Fanconi syndrome; proximal tubule wasting of HCO3-, K+, phosphate, glucose, urate, amino acids
  • Causes: multiple myeloma, Wilson's disease, heavy metals

Note: RTA type 4 (hypoaldosteronism) causes HYPERkalemia - opposite.

4. Tubular Syndromes (Genetic)

• Gitelman syndrome (SLC12A3 mutation - thiazide-sensitive NaCl transporter) - more common (1:40,000)
  • Hypokalemia + metabolic alkalosis + hypomagnesemia + hypocalciuria
  • Often milder; may present in adulthood
• Bartter syndrome (various mutations in loop-equivalent transporters) - rarer (1:1,000,000)
  • Hypokalemia + metabolic alkalosis + normal/high urine calcium + normal BP
• EAST syndrome (epilepsy, ataxia, sensorineural deafness, tubulopathy)

5. Osmotic Diuresis

• Hyperglycemia (DKT, HHS) - increased Na+ and water delivery to distal tubule promotes K+ exchange
• Mannitol, urea

6. Hypomagnesemia

• Coexistent hypomagnesemia impairs K+ repletion - K+ repletion will fail if Mg2+ is not corrected first
• Causes: cisplatin, alcoholism, diuretics, diarrhea

7. Thyrotoxic Periodic Paralysis (TPP)

• Involves renal K+ wasting component combined with massive intracellular shift
• Predominantly young Asian/Hispanic males despite higher thyrotoxicosis in women
• 8.9% of men with thyrotoxicosis in Japan; 13% in China
• Mutation in KCNJ18 (Kir2.6, muscle-specific thyroid hormone-responsive K+ channel)
• Low K+ restored by KCl; prevented by treating hyperthyroidism (propranolol acutely blocks β-adrenergic-mediated K+ shift)

B. EXTRARENAL POTASSIUM LOSS

Mechanism of Paralysis

1. Low extracellular K+ hyperpolarizes the muscle cell membrane (resting membrane potential becomes more negative)
2. A more negative resting potential means a larger stimulus is needed to reach the action potential threshold
3. At K+ <2.5 mEq/L: weakness; at K+ <2.0 mEq/L: flaccid paralysis
4. Voltage-gated Na+ channels become largely inactivated at the hyperpolarized membrane potential - contributing to inexcitability (paradoxical depolarization block)
5. In genetic HypoKPP, a "gating pore current" through mutant S4 voltage sensor domains causes K+-dependent paradoxical depolarization
6. Acute triggers in secondary forms: rest after vigorous exercise (catecholamine surge → K+ into cells), large carbohydrate meal (insulin → K+ into cells) - these shifts superimpose on chronic depletion to precipitate acute paralysis

Clinical Features

• Distribution: Proximal > distal; legs before arms; trunk relatively spared
• Spared muscles: Ocular, facial, pharyngeal, laryngeal, diaphragm, sphincters (usually)
• Reflexes: Absent/reduced tendon reflexes during attack (areflexia)
• No myotonia - presence of myotonia essentially excludes HypoKPP
• Cardiac: Risk of arrhythmias, especially in thyrotoxic form
• Duration: Hours to days; weakness may be persistent (not episodic) when K+ depletion is severe and sustained

Diagnostic Approach

Hypokalemic paralysis
        |
        ├─ Urine K+ <20 mEq/L → EXTRARENAL LOSS
        │       (GI, sweat, inadequate intake)
        │
        └─ Urine K+ >20-30 mEq/L → RENAL LOSS
                |
                ├─ Blood pressure?
                │
                ├─ HYPERTENSIVE
                │       → Check Renin + Aldosterone
                │       ├─ High Aldo, Low Renin → Primary aldosteronism (Conn's)
                │       ├─ High Aldo, High Renin → Secondary hyperaldosteronism
                │       └─ Low Aldo, Low Renin → Apparent mineralocorticoid excess
                │               (licorice, Cushing's, CAH, Liddle syndrome)
                │
                └─ NORMOTENSIVE
                        → Check acid-base status
                        ├─ Metabolic ALKALOSIS
                        │       → Bartter / Gitelman / vomiting / diuretics
                        │       → Check Mg2+ (Gitelman: low Mg, low urine Ca)
                        └─ Metabolic ACIDOSIS
                                → RTA type 1 or 2
                                → Check urine pH, FE-bicarbonate

Treatment

1. Acute Attack with Severe Hypokalemia

• Oral KCl: 0.25 mEq/kg - first choice if tolerated
• IV KCl (if not tolerated or severe symptoms):
  • 0.05-0.1 mEq/kg initial bolus at a safe rate
  • Then 20-40 mEq KCl in 5% mannitol (NOT glucose, NOT NaCl - these can worsen paralysis)
  • Rate: generally ≤10-20 mEq/hour peripherally; cardiac monitoring required for faster infusion
• Cardiac monitoring throughout (hypokalemia + K+ repletion both carry arrhythmia risk)

2. Correct Hypomagnesemia First

• K+ repletion will fail if Mg2+ is not corrected simultaneously

3. Treat the Underlying Cause

4. Prevention of Attacks (when underlying cause cannot be eliminated)

• Dichlorphenamide (carbonic anhydrase inhibitor) - for late-progressive polymyopathy from repeated attacks
• Spironolactone - K+-sparing; particularly for mineralocorticoid excess causes
• Acetazolamide - useful in primary genetic HypoKPP (but not in secondary forms with acidosis)
• Avoid precipitants: strenuous exercise followed by rest, high-carbohydrate meals

Key Points to Remember

1. Persistent hypokalemia (not episodic) with paralysis always points to true K+ depletion from renal or GI loss - not primary genetic HypoKPP
2. Urine K+ is the single most important test to localize the source
3. Hypomagnesemia must always be checked - it blocks K+ repletion
4. In thyrotoxic PP (Asian/Hispanic young males), check TSH before diagnosing idiopathic renal K+ wasting
5. Never use glucose/NS as carrier for IV K+ in acute attack - worsens paralysis
6. The combination of acute transcellular shift + chronic depletion is what precipitates most acute attacks of secondary hypokalemic paralysis