Hyponatremia and Hypernatremia: What the NCLEX Tests

Medical-Surgical · Renal & Genitourinary·Updated 2026-10-28

What it is

Sodium is the main extracellular cation and the main determinant of serum osmolality, so most sodium imbalances are really water imbalances. In hyponatremia, water moves into cells and brain cells swell, causing headache, confusion and seizures; in hypernatremia, water leaves cells and brain cells shrink, causing thirst, restlessness and lethargy. Because brain cells adapt over time, correcting either imbalance too fast injures the brain: a rapid rise in sodium causes osmotic demyelination, and a rapid fall causes cerebral edema.

What the NCLEX focuses on

Hyponatremia with fluid excess: SIADH, heart failure, excess water or hypotonic IV fluid; with fluid loss: vomiting, diarrhea, diuretics, adrenal insufficiency.
Hyponatremia signs: headache, nausea, muscle cramps, confusion and lethargy, progressing to seizures and coma when sodium falls below 120 mEq/L or falls quickly.
SIADH: low serum osmolality, inappropriately concentrated urine and weight gain without edema; the first treatment is fluid restriction.
Severe symptomatic hyponatremia: hypertonic 3% sodium chloride on an infusion pump with frequent sodium checks; overly rapid correction causes osmotic demyelination (dysarthria, dysphagia, paralysis).
Hypernatremia causes: water loss (fever, diarrhea, diabetes insipidus), poor water intake (older or confused clients) and sodium gain (tube feedings without water flushes, hypertonic saline).
Hypernatremia signs: intense thirst, dry sticky mucous membranes, restlessness, agitation, muscle twitching, lethargy, seizures and coma.
Diabetes insipidus after head injury or neurosurgery: large volumes of dilute urine with a rising serum sodium; central diabetes insipidus is treated with desmopressin.

Priority nursing actions

Assess level of consciousness, airway and neurologic status; initiate seizure precautions for severe hyponatremia or any new neurologic change.
Treat the cause as prescribed: fluid restriction for SIADH, isotonic saline for volume loss, 3% saline for severe neurologic symptoms, hypotonic fluid for hypernatremia from water loss.
Check serum sodium as often as prescribed; stop hypertonic saline and notify the provider when sodium rises faster than the prescribed limit.
Monitor intake and output, daily weight, urine specific gravity and serum osmolality.
Maintain safety: fall precautions, frequent oral care and reorientation for confused clients.

Numbers to know

Value	Normal or target	Why it matters
Serum sodium	136 to 145 mEq/L (136 to 145 mmol/L)	Below 136 is hyponatremia; above 145 is hypernatremia
Severe hyponatremia	Below 120 mEq/L (120 mmol/L)	High risk of seizures and coma
Serum osmolality	285 to 295 mOsm/kg (285 to 295 mmol/kg)	Low with water excess; high with hypernatremia
Rate of correction	Rise of no more than 8 mEq/L (8 mmol/L) in 24 hours	Faster correction causes osmotic demyelination
SIADH fluid restriction	800 to 1,000 mL per day	Removes the free water that dilutes sodium
Urine specific gravity in diabetes insipidus	Below 1.005	Dilute urine and water loss drive sodium up

Common exam traps

Choosing IV 0.9% sodium chloride for hyponatremia from SIADH: wrong because the problem is retained water; fluid restriction raises the sodium.
Choosing to speed up hypertonic saline so sodium reaches normal quickly: wrong because rapid correction causes osmotic demyelination; the goal is a slow, limited rise.
Choosing fluid restriction for a client with polyuria, dilute urine and a rising sodium after craniotomy: wrong because diabetes insipidus is causing water loss, and restriction worsens hypernatremia.
Choosing a rapid infusion of 5% dextrose in water to lower a high sodium: wrong because a fast fall shifts water into brain cells and causes cerebral edema.

What to teach

Fluid restriction: spread the allowance over the day, count soup, ice chips and ice cream as fluid, and use sugar-free gum or hard candy for thirst; weigh daily and report rapid weight gain, headache or confusion.
Older adults and caregivers: offer fluids on a schedule, because thirst declines with age, and report dry mouth, confusion or reduced urine output.
Desmopressin: drink only when thirsty and report headache, confusion or weight gain, which signal water excess and a low sodium.

Try 2 NCLEX-style questions on this topic

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Question 1 · QID 1793ADEE

A client with severe hyponatremia from SIADH is receiving 3% sodium chloride at 25 mL/hr through a central line. Baseline sodium was 116 mEq/L at 0600. At 1200, the repeat sodium is 125 mEq/L. Which action by the nurse is most appropriate?

A. Stop the infusion and notify the provider of the rate of rise
B. Increase the hypertonic saline rate so serum sodium reaches the normal range before the end of the day
C. Continue the prescribed 3% saline rate because the sodium remains below the normal range despite clinical improvement
D. Reduce the hypertonic saline rate by half and repeat the sodium level after another 6 hours of treatment

Question 2 · QID 7ECD2DFA

A client with heart failure has gained 6 lb (2.7 kg) in 4 days. Assessment reveals crackles, pitting edema, and a serum sodium level of 128 mEq/L. Which prescription should the nurse expect?

Select the best answer.

A. Oral sodium chloride tablets
B. IV 0.9% sodium chloride
C. IV 3% sodium chloride
D. IV furosemide

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Sources

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.
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.
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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