SIADH vs Diabetes Insipidus: Water, Sodium and ADH
What it is
Antidiuretic hormone (ADH, vasopressin) from the posterior pituitary makes the kidneys reabsorb free water. In the syndrome of inappropriate ADH (SIADH), excess ADH retains water, producing scant concentrated urine, weight gain and dilutional hyponatremia. In diabetes insipidus (DI), ADH is missing (central DI) or the kidneys do not respond to it (nephrogenic DI), producing large volumes of dilute urine, thirst, dehydration and hypernatremia. Both follow head injury, neurosurgery, some cancers and some drugs, so the nurse tracks urine output, specific gravity, sodium and weight.
What the NCLEX focuses on
- SIADH: low urine output with high specific gravity, weight gain without edema, hyponatremia and low serum osmolality; causes include small cell lung cancer, head injury, meningitis and drugs such as carbamazepine and SSRIs.
- As sodium falls: headache, nausea, confusion, muscle cramps, then seizures and coma; institute seizure precautions.
- SIADH treatment: fluid restriction of 800 to 1,000 mL/day, hypertonic 3% saline for severe neurologic symptoms, and tolvaptan or demeclocycline for chronic SIADH.
- Correct sodium slowly: a rise faster than the prescribed limit causes osmotic demyelination syndrome (dysarthria, dysphagia, paralysis).
- DI: sudden high output of urine with specific gravity below 1.005, intense thirst, hypernatremia and dehydration; watch for it after pituitary surgery or head injury. Lithium causes nephrogenic DI.
- Central DI responds to desmopressin; nephrogenic DI does not and is treated with a thiazide diuretic and a low-sodium diet.
- Desmopressin works when urine output falls and specific gravity rises; weight gain, headache and confusion signal water intoxication.
Priority nursing actions
- Assess neurologic status and airway; institute seizure precautions with severe hyponatremia.
- Measure hourly intake and output, urine specific gravity and daily weight on the same scale.
- SIADH: restrict fluids as prescribed and give hypertonic saline only by infusion pump at the prescribed rate.
- DI: replace fluids as prescribed and give desmopressin; report urine output that stays high.
- Recheck serum sodium as prescribed and report a rise faster than the prescribed limit or new neurologic changes.
Numbers to know
| Value | SIADH (too much ADH) | Diabetes insipidus (too little ADH effect) |
|---|---|---|
| Serum sodium (136 to 145 mEq/L [136 to 145 mmol/L]) | Low, dilutional | High |
| Serum osmolality (285 to 295 mOsm/kg [285 to 295 mmol/kg]) | Low | High |
| Urine specific gravity | High, concentrated | Below 1.005, dilute |
| Urine output | Low | High, several liters a day |
| Weight | Gain without edema | Loss |
| Main treatment | Fluid restriction 800 to 1,000 mL/day | Fluid replacement plus desmopressin (central DI) |
Common exam traps
- Choosing 0.45% sodium chloride for SIADH: wrong because hypotonic fluid adds free water and worsens hyponatremia.
- Choosing to restrict fluids in DI: wrong because the client cannot conserve water and becomes severely dehydrated and hypernatremic.
- Choosing to raise sodium to normal as fast as possible: wrong because rapid correction causes osmotic demyelination syndrome.
- Choosing to give the next desmopressin dose to a client with weight gain, headache and a low sodium: wrong because these are water intoxication signs; hold and report.
- Choosing desmopressin for lithium-induced DI: wrong because nephrogenic DI does not respond to ADH.
What to teach
- SIADH: count every liquid toward the allowance, including soup, ice cream and ice chips; use sugar-free gum, hard candy and frequent oral care for thirst.
- Weigh every morning and report the gain or loss limit the provider sets; report headache, confusion, nausea or muscle cramps.
- Desmopressin: drink only when thirsty, never double a dose, and report nasal congestion, which reduces absorption of the nasal spray.
- Wear medical alert identification for DI.
Try 2 NCLEX-style questions on this topic
From the NCLEXPace question bank. Pick your answers, then check them inside NCLEXPace.
The nurse cares for a client 12 hours after a supratentorial craniotomy. Vital signs are stable. The nurse notes the following over the last 3 hours: urine output 350 mL/hr, 400 mL/hr, and 425 mL/hr; urine specific gravity 1.001; serum sodium 149 mEq/L (reference 136 to 145). Which action should the nurse take first?
- A. Notify the provider and anticipate a prescription for desmopressin
- B. Restrict oral and intravenous fluids to 1,000 mL daily to limit further urinary losses and correct the rising sodium
- C. Continue hourly urine measurements and repeat the sodium and urine specific gravity after another hour of observation
- D. Elevate the head of the bed to 30 degrees and complete a focused neurologic assessment before changing the treatment plan
A client with chronic SIADH associated with small cell lung cancer is discharged with a fluid allowance of 1,000 mL per day and instructions to report a 1-kg weight gain in 24 hours. The nurse evaluates the discharge teaching.
For each client statement, click to specify whether it indicates effective teaching or a need for further teaching.
| Effective teaching | Needs further teaching | |
|---|---|---|
| “I will keep my fluids under 1 liter a day and count soup and ice cream as fluid.” | ||
| “I will use sugar-free gum for dry mouth and count any ice chips by their melted volume within my daily fluid allowance.” | ||
| “I will weigh myself every morning and call if I gain 1 kilogram in a day.” | ||
| “If I get a bad headache or feel confused, I will lie down and wait for it to pass.” | ||
| “I can stop the restriction on days when my urine looks clear.” | ||
| “I will take the salt tablets and the medication exactly as prescribed.” |
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- Harding MM, Kwong J, Hagler D, Reinisch C. Lewis's Medical-Surgical Nursing: Assessment and Management of Clinical Problems. 12th ed. Elsevier; 2023; Chapter: Endocrine Problems.
- Harding MM, Kwong J, Hagler D, Reinisch C. Lewis's Medical-Surgical Nursing: Assessment and Management of Clinical Problems. 12th ed. Elsevier; 2023; Chapter: Fluid, Electrolyte, and Acid-Base Imbalances.
- Verbalis JG, Goldsmith SR, Greenberg A, et al. Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. Am J Med. 2013;126(10 Suppl 1):S1-S42.
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