👤 By the CinnaRN Clinical Content Team🕐 Updated 2026-08-23🏷️ Physiological Adaptation🔖 Free to read, print, and share
Also known as: too much ADH · water retention syndrome · SIADH · inappropriate ADH · dilutional hyponatremia
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Use this quick-reference guide to spot, treat, and prevent SIADH (Syndrome of Inappropriate Antidiuretic Hormone) on the NCLEX. Keep it handy during review and on exam day!
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📒 The 1-minute cheat sheet
📌 Mechanism
↑ ADH → water retention
Dilutional hyponatremia results
Causes: SCLC, head injury, SSRIs
🧪 Labs
↓ serum Na <135, ↓ osmolality
↑ urine Na, concentrated urine
🩺 Signs
Wt gain, NO edema, ↓ urine
Headache, confusion, seizures
✅ Do
Fluid restrict 800-1000 mL/day
3% saline SLOW if severe
Tolvaptan, seizure precautions
📚 SIADH (Syndrome of Inappropriate Antidiuretic Hormone) — full study notes
The cheat sheet above is your quick recall card. These notes go deeper — what it is, what to do first, what must be reported, and what to teach.
SIADH is excess antidiuretic hormone (ADH/vasopressin) causing the body to retain water, which dilutes the blood and produces hyponatremia. Common causes include small cell lung cancer, CNS disorders, certain medications, and surgery. The body holds onto water it should excrete, leading to fluid overload and low sodium. Memory aid: SIADH = 'Soaked Inside' (too much water held in); it is the mirror image of diabetes insipidus.
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Key points
Understand these first
Water retention causes dilutional hyponatremia with concentrated urine and dilute, low-volume serum.
Patients have fluid overload but typically without obvious edema; weight gain occurs without thirst.
Urine output is low and urine is highly concentrated with high specific gravity and high urine sodium.
Neurologic symptoms of low sodium include headache, confusion, lethargy, muscle cramps, and seizures.
Serum osmolality is low while urine osmolality is inappropriately high.
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Nursing priorities
What to do, in order
Restrict fluids (often 500-1000 mL/day) as the primary intervention and monitor strict intake and output.
Monitor serum sodium, neurologic status, and daily weights closely.
Administer hypertonic 3% saline cautiously and only for severe symptomatic hyponatremia, correcting sodium slowly.
Institute seizure precautions and maintain a safe, low-stimulation environment.
Administer ADH antagonists (vaptans) or demeclocycline as ordered.
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Red flags — report now
Escalate immediately
Rapid sodium correction can cause osmotic demyelination (central pontine myelinolysis); raise sodium slowly.
Report seizures, marked decline in level of consciousness, or worsening confusion immediately.
Report sodium below 120 mEq/L or a rapidly falling sodium level.
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Labs & values
Numbers to know
Serum sodium: low (normal 135-145 mEq/L)
Serum osmolality: low (normal 275-295 mOsm/kg)
Urine specific gravity: high (normal 1.005-1.030)
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Patient teaching
What patients must know
Follow the prescribed fluid restriction carefully and measure all intake.
Report headache, confusion, nausea, or muscle weakness, which may indicate worsening low sodium.
Weigh yourself daily and report rapid weight gain.
❓ SIADH (Syndrome of Inappropriate Antidiuretic Hormone): NCLEX FAQs
What are the priority nursing interventions for SIADH (Syndrome of Inappropriate Antidiuretic Hormone)?
Restrict fluids (often 500-1000 mL/day) as the primary intervention and monitor strict intake and output. Monitor serum sodium, neurologic status, and daily weights closely. Administer hypertonic 3% saline cautiously and only for severe symptomatic hyponatremia, correcting sodium slowly. Institute seizure precautions and maintain a safe, low-stimulation environment.
What are the warning signs of SIADH (Syndrome of Inappropriate Antidiuretic Hormone) a nurse must report?
Rapid sodium correction can cause osmotic demyelination (central pontine myelinolysis); raise sodium slowly. Report seizures, marked decline in level of consciousness, or worsening confusion immediately. Report sodium below 120 mEq/L or a rapidly falling sodium level.
What do I need to know about SIADH (Syndrome of Inappropriate Antidiuretic Hormone) for the NCLEX?
Water retention causes dilutional hyponatremia with concentrated urine and dilute, low-volume serum. Patients have fluid overload but typically without obvious edema; weight gain occurs without thirst. Urine output is low and urine is highly concentrated with high specific gravity and high urine sodium. Neurologic symptoms of low sodium include headache, confusion, lethargy, muscle cramps, and seizures.
What patient teaching is important for SIADH (Syndrome of Inappropriate Antidiuretic Hormone)?
Follow the prescribed fluid restriction carefully and measure all intake. Report headache, confusion, nausea, or muscle weakness, which may indicate worsening low sodium. Weigh yourself daily and report rapid weight gain.
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Quick Tip
Water retention causes dilutional hyponatremia with concentrated urine and dilute, low-volume serum.