Hyponatremia

Clinical Snapshot

Hyponatremia — a serum sodium concentration below 135 mmol/L — is the most common electrolyte abnormality encountered in clinical practice, present in as many as 28% of hospitalized patients and associated with increased length of stay, higher costs, and a roughly twofold increase in mortality.^[1] It is frequently drug-induced, which makes it a genuinely pharmacist-relevant condition rather than one to leave entirely to the prescriber (see Risk Factors). Severity is staged by the absolute sodium level — mild 130-134 mmol/L, moderate 125-129 mmol/L, severe under 125 mmol/L^[1] — but severity by level is only half of what determines how urgently, and how carefully, it should be corrected.

The second, equally important classification is by acuity: acute hyponatremia develops over under 48 hours, while chronic hyponatremia develops over 48 hours or longer, or is of unknown duration.^[1] This distinction exists because the brain adapts to sustained hypotonicity by extruding intracellular organic osmolytes to limit cell swelling, a process that takes roughly 48 hours to occur meaningfully. A brain that has not yet adapted (acute hyponatremia) is at higher risk of life-threatening cerebral edema from the hyponatremia itself; a brain that has adapted (chronic hyponatremia) is instead put at risk by the sudden loss of those protective osmolytes if serum sodium is corrected too quickly, producing osmotic demyelination syndrome (ODS) — a serious, potentially irreversible neurologic injury.^[2]^[3] This acute-versus-chronic distinction, more than the absolute sodium level, is what should govern how aggressively hyponatremia is corrected, and it is the central safety teaching point of this chapter (see Choose Initial Treatment and the Clinical Decision Algorithm).

Hyponatremia is also classified by the patient's volume status — hypovolemic, euvolemic, or hypervolemic — which is the standard framework for identifying the underlying cause and choosing initial treatment (see Recognize the Condition).^[1]^[4]

Recognize the Condition

The volume-status framework — the backbone of diagnosis and treatment

Once true hypotonic hyponatremia is confirmed (see Assessment & Diagnosis), the extracellular fluid volume assessed on history and physical examination sorts every case into one of three categories, each with a different set of typical causes and a different first-line treatment.^[1]^[4]

Hyponatremia by volume status

Hypovolemic

True total-body sodium and water loss, with sodium loss exceeding water loss.

GI losses — vomiting, diarrhea, small bowel obstruction.
Diuretic use, especially thiazides (see Risk Factors).
Primary adrenal insufficiency and salt-losing nephropathy.
Burns, third-spacing, cerebral or renal salt wasting.

Euvolemic

Water excess without a significant corresponding change in total-body sodium. SIADH is the most common cause in this category, and is itself frequently drug-induced.

SIADH — from a causative drug, pain, nausea, postoperative state, pulmonary disease, or malignancy (classically small cell lung cancer).
Primary polydipsia.
Low solute intake ("tea and toast," or "beer potomania" in the context of alcohol use disorder).
Secondary adrenal insufficiency (cortisol deficiency) and hypothyroidism.

Hypervolemic

Total-body sodium and water are both increased, but water more than sodium.

Heart failure.
Cirrhosis and severe liver disease.
Nephrotic syndrome.
Advanced chronic kidney disease.

Symptoms — a spectrum driven by both the sodium level and the rate of decline

Symptom severity correlates with the absolute sodium level and, independently, with how quickly it fell — a given sodium level reached slowly may be nearly asymptomatic, while the same level reached rapidly can be a genuine neurologic emergency.^[1]^[2]

Mild: nausea, vomiting, weakness, headache, and mild neurocognitive deficits — often subtle enough to be missed or attributed to another cause, especially in older adults.^[1]
Moderate: gait disturbance, unsteadiness, and falls — a genuinely under-recognized presentation, particularly in older adults, where even mild-to-moderate chronic hyponatremia is associated with attention deficits and gait instability that increase fall and fracture risk.^[1]
Severe: delirium, confusion, impaired consciousness, ataxia, seizures, and — rarely — brain herniation and death.^[1]
Chronic hyponatremia with only mild-to-moderate sodium reduction can be entirely asymptomatic, which is why it is so often found on routine bloodwork rather than through symptoms.^[1]

Risk Factors

Hyponatremia is frequently drug-induced, and identifying a causative medication is one of the most valuable contributions a pharmacist can make to a patient's workup.^[1]

Drug-induced causes — the pharmacist's central focus

Thiazide diuretics — the classic and most common drug-induced cause

Hydrochlorothiazide, chlorthalidone, indapamide, and thiazide-containing combination products cause hyponatremia genuinely more often than loop diuretics. Thiazides act on the cortical diluting segment of the nephron, impairing the kidney's ability to dilute urine while leaving its ability to concentrate urine intact — so with continued free-water intake, water is retained. Loop diuretics instead blunt the medullary concentrating gradient itself, which if anything works against dilutional hyponatremia.^[1] Risk is highest in the first several weeks after starting or increasing the dose, and in older, thinner women — see the elderly-specific outcome data below.^[5]

SSRIs and other antidepressants

Sertraline, citalopram, escitalopram, fluoxetine, paroxetine, venlafaxine, duloxetine, tricyclic antidepressants, and other antidepressant classes are recognized causes of a SIADH-like syndrome, through enhanced central ADH release and/or increased renal sensitivity to ADH. Onset is typically within the first 2-4 weeks of starting or increasing the dose. This is particularly relevant in elderly patients, both because of age-related susceptibility and because antidepressants are frequently combined with other hyponatremia-inducing drugs in this population (see Special Populations).^[1]

Carbamazepine and oxcarbazepine

Both stimulate ADH release and/or increase renal sensitivity to it; SIADH incidence is reported to be meaningfully higher with oxcarbazepine than with carbamazepine.^[1]

Desmopressin

A genuinely distinct mechanism from the drugs above: desmopressin is a direct synthetic vasopressin (V2-receptor) agonist, prescribed for central diabetes insipidus, nocturnal enuresis, nocturia, and some bleeding disorders. Because its therapeutic mechanism — renal water retention — is mechanistically identical to the pathology being treated in this chapter, iatrogenic hyponatremia from desmopressin is a genuinely common and easily overlooked cause, and warrants an active dose/necessity review whenever hyponatremia develops in a patient taking it.^[1]

NSAIDs

NSAIDs potentiate the renal action of ADH at the collecting duct by inhibiting the prostaglandins that normally blunt that effect, and can precipitate or worsen hyponatremia on their own or in combination with any of the drugs above.^[1]

Other recognized drug causes

Loop diuretics — a less common cause than thiazides, but still relevant, especially at high doses or with other risk factors present.
Barbiturates, clozapine, opioid analgesics ("narcotic medications"), oxytocin, and vincristine — all recognized SIADH-inducing agents.^[1]
Proton pump inhibitors and antipsychotics — reported, less common associations.

Non-drug risk factors

Older age — see Special Populations for the physiologic and polypharmacy-related reasons this population is disproportionately affected.
Any cause of GI fluid loss (vomiting, diarrhea), heart failure, cirrhosis, nephrotic syndrome, or advanced chronic kidney disease.
Malignancy, particularly small cell lung cancer, and pulmonary disease (pneumonia, COPD exacerbation) — both recognized SIADH triggers independent of medication.^[1]
Postoperative state, significant pain, and nausea — all nonspecific physiologic stimuli for ADH release.^[1]
Primary or secondary adrenal insufficiency and hypothyroidism.^[1]
Excessive free-water intake relative to solute intake — endurance exercise with aggressive water (rather than electrolyte-containing fluid) replacement, primary polydipsia, or a very low-solute diet.^[1]

Red Flags & Refer

Either of the following moves hyponatremia from routine outpatient management to a true medical emergency:

Severe symptomatic hyponatremia — seizure, marked confusion, obtunded or impaired consciousness, or any other severe neurologic symptom in a hyponatremic patient — regardless of the exact sodium level. This requires immediate emergency-department assessment and urgent, carefully monitored correction with hypertonic saline (see Choose Initial Treatment); do not wait for a full diagnostic workup before beginning treatment.^[1]
Any sign of overly rapid correction, or new neurologic symptoms developing during treatment of chronic hyponatremia — new dysarthria, dysphagia, weakness, spastic quadriparesis, altered mental status, or seizure appearing 2-6 days after correction began is a red flag for osmotic demyelination syndrome and requires immediate reassessment of the correction rate and urgent neurology involvement.^[2]^[3]
A rapid, unplanned drop in sodium in a hospitalized patient, or a sodium correction that has exceeded the intended 24-hour or 48-hour ceiling (see Choose Initial Treatment) — flag immediately rather than waiting for the next scheduled recheck.
Hyponatremia in a patient with a known or suspected underlying malignancy, especially with a new presentation of SIADH — this can be the presenting sign of an occult cancer and needs investigation beyond simple sodium correction.

Assessment & Diagnosis

Confirm true hypotonic hyponatremia first

Not every low sodium result reflects a true water-excess-relative-to-sodium state. Obtain serum osmolality before proceeding.^[1]^[4]

Normal or high serum osmolality (pseudohyponatremia or translocational hyponatremia) — pseudohyponatremia occurs with severe hyperlipidemia or hyperproteinemia, a laboratory-measurement artifact rather than a true sodium disturbance. Translocational hyponatremia occurs with hyperglycemia: glucose draws water out of cells into the extracellular space, diluting measured sodium without a true free-water excess; a corrected-sodium calculation should be applied before concluding true hyponatremia is present, and the corrected value normalizes as glucose is treated.^[4]
Low serum osmolality confirms true hypotonic hyponatremia — proceed to volume-status assessment.^[1]

Volume status assessment

Assess extracellular fluid volume directly on history and examination — skin turgor, mucous membranes, jugular venous pressure, orthostatic vital signs, and the presence or absence of edema — to sort the case into hypovolemic, euvolemic, or hypervolemic (see Recognize the Condition). This is the pivotal step that determines both the differential and the initial treatment.^[1]^[4]

Urine studies

Urine osmolality and urine sodium help confirm the volume-status impression and specifically identify SIADH.

Urine osmolality under 100 mOsm/kg indicates appropriately suppressed ADH — suggests primary polydipsia or a very low solute intake.^[1]
Urine osmolality over 100 mOsm/kg indicates ADH is active, either appropriately (hypovolemic hyponatremia, where ADH release is a normal physiologic response to volume depletion) or inappropriately (SIADH).^[1]
Urine sodium helps distinguish further — generally low (under 20-30 mEq/L) in hypovolemic hyponatremia from extrarenal losses, and generally over 20 mEq/L in SIADH.^[1]

SIADH — a diagnosis of exclusion

SIADH is confirmed by the combination of decreased serum osmolality, urine osmolality over 100 mOsm/kg, clinical euvolemia, urine sodium over 20 mEq/L (with a normal salt and water intake), and the absence of thyroid disease, hypocortisolism, diuretic use, and significant kidney impairment.^[1] It remains fundamentally a diagnosis of exclusion — ruling out the other causes of euvolemic hyponatremia is required, not optional, before labeling a patient's hyponatremia as SIADH and before considering vasopressin-antagonist therapy.^[1]

Further testing

Additional testing is directed at suspected comorbidities once the above framework has narrowed the differential — B-type natriuretic peptide if heart failure is suspected, TSH for hypothyroidism, morning cortisol or an ACTH stimulation test for adrenal insufficiency, and a review of the full medication list (see Risk Factors) in every case.^[1]

Treatment Targets

Correct the sodium deficit to a safe level while strictly respecting the correction-rate ceilings that prevent osmotic demyelination syndrome (see Choose Initial Treatment) — the rate of correction, not just the endpoint, is itself a treatment target.
In severe, acute, or symptomatic hyponatremia, achieve a prompt initial rise sufficient to reverse or prevent cerebral edema, without exceeding the same-day cumulative ceiling once the emergency has been addressed.
Identify and treat the underlying cause and volume status, not the sodium number in isolation.
Identify and adjust, where clinically appropriate, any causative medication.
Avoid both under-treatment (ongoing risk of neurologic injury from persistent hyponatremia) and over-treatment (osmotic demyelination syndrome from overly rapid correction) — both are genuine, serious harms.

Choose Initial Treatment

Direct treatment rule

The central safety rule: acuity determines how fast you may correct

Before selecting a treatment, determine two things: (1) is this severe and/or acute (developed under 48 hours) hyponatremia with significant symptoms, or chronic (48 hours or longer, or unknown duration) hyponatremia with mild or no symptoms; and (2) does the patient have any risk factor for osmotic demyelination syndrome (ODS) — a serum sodium under 105 mmol/L, hypokalemia, malnutrition, alcohol use disorder, or advanced liver disease.^[1]^[2]

Correction-rate ceilings — do not exceed these once any emergency bolus phase has ended:

Any ODS risk factor present, or acuity/risk status unknown: limit total correction to 8 mmol/L over 24 hours.^[1]
Confirmed low-risk chronic hyponatremia: limit total correction to 12 mmol/L over 24 hours and 18 mmol/L over 48 hours.^[1]

These ceilings apply to the total cumulative rise over the stated window — including any rapid initial bolus given for acute symptomatic treatment (below). A rapid initial increment is not exempt from the ceiling; it is simply front-loaded within it, and correction must be slowed or held once the ceiling is approached.^[2]^[3]

Severe, acute, or symptomatic hyponatremia — treat the emergency first

For a sodium under 125 mmol/L with severe symptoms (seizure, marked confusion, obtunded consciousness), or hyponatremia clearly developing over under 48 hours with significant symptoms, the immediate risk is cerebral edema, not ODS — a more urgent initial correction is appropriate and should not be withheld out of concern for the 24-hour ceiling above.^[1]^[2]

Emergency treatment up to three 100 mL boluses of 3% hypertonic saline IV, given at 10-minute intervals (or a single 150 mL infusion over 20 minutes), with an initial goal of raising sodium by 4-6 mmol/L — enough to meaningfully reduce cerebral edema and improve or resolve severe symptoms.^[1] Monitor sodium after each bolus, and at least every 6 hours through the first 24 hours.^[1] Adjunct a low dose of a loop diuretic (e.g., furosemide) is sometimes co-administered with hypertonic saline to promote free-water excretion and limit volume expansion, particularly in a patient with any element of volume overload.

Management by etiology, once any emergency has been addressed

Hypovolemic hyponatremia — isotonic (0.9%) saline for volume repletion, plus treatment of the underlying cause (stop a causative diuretic, treat GI losses, address adrenal insufficiency). Correcting the volume deficit alone is often sufficient — restoring volume removes the physiologic stimulus for ADH release, and free-water excretion can then proceed appropriately; monitor closely, since sodium can rise faster than expected once volume is restored.^[1]

Euvolemic hyponatremia (including SIADH) — fluid restriction is first-line: 500 mL/24h is the traditional target, though adherence is difficult and a 1 L/24h target has also been shown effective and may improve adherence.^[1] Stop or reduce any causative medication (see Risk Factors). Adjunct oral salt tablets have limited evidence but are sometimes added; oral urea (typically initiated at 15-60 g/day) increases solute excretion and free-water clearance and is a reasonable second-line option where fluid restriction alone is inadequate or not tolerated.^[1]

Hypervolemic hyponatremia — treat the underlying disorder (optimize heart failure therapy, manage cirrhosis) plus fluid restriction (roughly 1 L/24h) and, where appropriate, diuretic therapy; free-water restriction and sodium correction are secondary to managing the underlying condition itself.^[1]

Vasopressin receptor antagonists — tolvaptan

Select cases tolvaptan is an option for euvolemic (SIADH) hyponatremia not adequately controlled by fluid restriction, and for select hypervolemic hyponatremia (e.g., heart failure) under specialist direction; it produces a selective aquaresis (electrolyte-free water excretion) by blocking the V2 receptor, with proven efficacy in its pivotal placebo-controlled trials.^[6]

Tolvaptan must be started as an inpatient, with fluid restriction discontinued at initiation — the same aquaretic effect that treats the hyponatremia can also drive overly rapid correction: in one cohort of SIADH patients treated with tolvaptan, 25% corrected faster than 12 mmol/L in 24 hours.^[7] Monitor sodium closely (per institutional protocol, typically every 6-8 hours) for at least the first 24-48 hours after starting or up-titrating the dose.^[7]^[2] Tolvaptan also carries an FDA boxed warning for rare but potentially serious and irreversible hepatotoxicity: avoid use beyond 30 days, avoid in patients with underlying liver disease, and discontinue promptly with any sign of hepatic injury (fatigue, anorexia, right upper quadrant discomfort, dark urine, jaundice).^[8]

If correction is progressing faster than the safe target

Reassess immediately. Stop hypertonic saline or any other active correction measure, and consider giving electrolyte-free water (dextrose 5% in water, D5W) to lower sodium back toward the intended rate; adding desmopressin is a recognized rescue strategy specifically because it halts further aquaresis-driven correction and gives the clinical team room to re-lower sodium in a controlled way.^[3] This "re-lowering" approach is well described for preventing and reversing inadvertent overcorrection and should be considered proactively, not only after neurologic symptoms of ODS have already appeared.^[3]

Medication review — the pharmacist-specific action item

In every case of hyponatremia, review the full medication list for a causative or contributing drug (see Risk Factors) and discuss with the prescriber whether it can be safely stopped, reduced, or switched to a lower-risk alternative — a thiazide switched to a different antihypertensive class, an SSRI switched to an agent with a lower reported hyponatremia signal, or a desmopressin dose reassessed against its original indication. This is frequently the single most effective long-term intervention, particularly in an older adult on multiple contributing medications at once (see Special Populations).^[1]

Non-Pharmacologic Treatment

Fluid restriction is the mainstay of non-pharmacologic management for euvolemic (SIADH) and hypervolemic hyponatremia — typically 500 mL-1 L per 24 hours, counting all fluid intake (including that in food) rather than beverages alone; involve a dietitian for a patient struggling with adherence.^[1]
Counsel patients and caregivers on practical fluid restriction: measuring intake explicitly rather than estimating, spacing allowed fluids through the day, and using thirst-relief strategies that do not add free water (ice chips in small quantity, sugar-free hard candy, frequent mouth care) rather than simply telling a patient to "drink less" with no concrete plan.
For hypovolemic hyponatremia, address the source of ongoing fluid loss directly (antiemetic for vomiting, antidiarrheal review, stopping an unnecessary diuretic) alongside fluid repletion.
Review and reduce unnecessary free-water intake in a patient with a very low-solute diet or heavy beer/alcohol intake with minimal food intake ("beer potomania") — nutritional counseling and addressing the underlying alcohol use are the actual treatment, not simply restricting further fluid intake in someone who is already under-eating.
For hypervolemic hyponatremia, reinforce standard heart failure or cirrhosis self-management (daily weight, dietary sodium restriction as separately directed by the care team, adherence to diuretic therapy) as part of the same visit — fluid restriction alone rarely succeeds without addressing the underlying disease process.
Educate every patient and caregiver on the specific symptoms that should prompt an urgent call or return to care: new confusion, unsteadiness/falls, severe headache, or any seizure — and, for a patient actively being corrected in hospital, that new weakness, difficulty speaking or swallowing, or worsening confusion during treatment must be reported immediately rather than assumed to be part of the original illness (see Red Flags).^[2]^[3]

Pharmacologic Treatment Options

Drug / intervention	Role	Dose / use	Important adverse effects	Teaching points
Sodium Chloride 3% Injection (hypertonic saline) Special situation	Severe/acute/symptomatic hyponatremia, emergency correction	Up to three 100 mL IV boluses over 10 minutes (or 150 mL over 20 minutes); target initial rise 4-6 mmol/L	Overly rapid correction if not carefully monitored; fluid overload; infusion-site irritation	Monitor sodium after each bolus and at least every 6 hours for 24 hours. The rapid initial rise is appropriate for acute/severe symptoms but still counts toward the 24-hour ceiling.
Sodium Chloride Injection USP 0.9% (normal saline) Preferred	Hypovolemic hyponatremia, volume repletion	Isotonic infusion, rate individualized to the degree of volume depletion	Can unmask a faster-than-expected sodium rise once volume is restored and the ADH stimulus resolves	Monitor sodium closely as volume repletion progresses — correction can accelerate once the hypovolemic stimulus for ADH release is removed.
Tolvaptan Preferred	Select euvolemic (SIADH) or hypervolemic hyponatremia inadequately controlled by fluid restriction	15 mg PO daily initially, titrated per protocol to a maximum of 60 mg/day	Overly rapid correction; thirst, dry mouth, polyuria; rare but serious hepatotoxicity	Must be initiated as an inpatient with fluid restriction discontinued; do not use beyond 30 days; avoid in underlying liver disease.
Furosemide Alternative	Adjunct to hypertonic saline to limit volume expansion; hypervolemic hyponatremia (heart failure/cirrhosis) with diuretic therapy otherwise indicated	Low dose as an adjunct to hypertonic saline; otherwise per standard heart failure/cirrhosis dosing	Volume depletion if overused; electrolyte disturbance (hypokalemia — itself an ODS risk factor)	Correct or monitor potassium closely, since concurrent hypokalemia is itself a risk factor for osmotic demyelination syndrome.
Desmopressin (overcorrection rescue) Special situation	Rescue therapy when sodium is correcting faster than the intended rate	Per institutional overcorrection-rescue protocol, typically combined with electrolyte-free water (D5W)	The same water-retaining effect that treats hyponatremia when used therapeutically — dosed here specifically to halt further correction	A recognized strategy for preventing/reversing inadvertent overcorrection; consider proactively rather than only after neurologic symptoms appear.
Dextrose 5% in Water (D5W) Special situation	Overcorrection rescue, paired with desmopressin	Per institutional overcorrection-rescue protocol	Hyperglycemia with rapid/large-volume infusion; fluid overload	Electrolyte-free water used specifically to lower sodium back toward the intended correction rate.
Hydrochlorothiazide (representative thiazide) Avoid	Causative — review and reconsider whenever hyponatremia is identified	N/A — medication review, not a dosing decision	Hyponatremia risk highest in the first several weeks after starting/increasing the dose	Discuss switching to a non-thiazide antihypertensive with the prescriber where clinically appropriate.
Sertraline (representative SSRI) Avoid	Causative — review and reconsider whenever hyponatremia is identified, especially in an older adult	N/A — medication review, not a dosing decision	SIADH-like syndrome, typically within 2-4 weeks of starting/increasing the dose	Do not stop an antidepressant unilaterally — coordinate the medication review with the prescriber.
Carbamazepine Avoid	Causative — review and reconsider whenever hyponatremia is identified	N/A — medication review, not a dosing decision	Direct stimulation of ADH release/renal sensitization	Reassess necessity and dose with the prescriber; do not adjust anticonvulsant therapy unilaterally.
Oxcarbazepine Avoid	Causative — review and reconsider whenever hyponatremia is identified	N/A — medication review, not a dosing decision	Higher reported SIADH incidence than carbamazepine	Reassess necessity and dose with the prescriber; do not adjust anticonvulsant therapy unilaterally.
Ibuprofen (representative NSAID) Avoid	Causative/potentiating — review whenever hyponatremia is identified, especially alongside another causative drug	N/A — medication review, not a dosing decision	Potentiates ADH action at the collecting duct via prostaglandin inhibition	Acetaminophen is a reasonable default analgesic/antipyretic alternative that does not carry this mechanism.

Special Populations

Older adults — the population most affected by drug-induced hyponatremia

Older adults are disproportionately affected by hyponatremia for several compounding reasons: age-related reductions in the kidney's free-water excretory capacity, a higher baseline prevalence of the conditions listed in Risk Factors (heart failure, malignancy, hypothyroidism), and — critically for a pharmacist — polypharmacy, which means an older adult is more likely to be taking two or more hyponatremia-inducing drugs simultaneously (e.g., a thiazide plus an SSRI), each independently increasing risk and together compounding it.^[1] Chronic hyponatremia in older adults carries an increased risk of all-cause mortality, certain cancers, gait instability, osteoporosis, falls, and fractures, and these risks are more pronounced in this population than in younger adults.^[1] Thiazide-associated hyponatremia specifically, in patients over 65, has been associated with a significantly higher risk of major adverse cardiovascular events (adjusted HR 1.29) and all-cause death (adjusted HR 1.39), with a numerically higher but not statistically significant risk of stroke, in a large Taiwanese cohort study — a concrete illustration of why the thiazide-hyponatremia link matters clinically, not just biochemically, in older patients.^[5] A structured medication review specifically targeting thiazides, SSRIs, carbamazepine/oxcarbazepine, and desmopressin is a genuinely high-value pharmacist intervention in this population.

Pregnancy and breastfeeding

Mild hyponatremia can occur in normal pregnancy from physiologic changes in osmoregulation and does not automatically require treatment; significant hyponatremia in pregnancy should be managed by the same volume-status framework used outside pregnancy, in conjunction with the patient's obstetric provider. Data on tolvaptan in pregnancy and breastfeeding are limited; avoid unless specifically directed by a specialist managing a complex case, and involve obstetric/maternal-fetal medicine input.

Pediatrics

Hyponatremia in children is managed under the same acute-versus-chronic correction-rate principle, but pediatric fluid management, hypertonic saline dosing, and fluid-restriction targets must be individualized under pediatric or pediatric nephrology direction — the adult mmol- and mL-based dosing in this chapter's drug tables must not be extrapolated directly to a child. Not established in this source for routine pediatric dosing.

Renal impairment

Reduced ability to both concentrate and dilute urine narrows the safe margin for both hypovolemic and euvolemic hyponatremia management; individualize fluid choice and rate closely, and involve nephrology for anything beyond straightforward, mild hyponatremia.

Hepatic impairment / cirrhosis

Hyponatremia is common in advanced cirrhosis and is managed primarily by treating the underlying liver disease and fluid restriction; tolvaptan and other vasopressin antagonists must be avoided or used only under specialist direction in significant hepatic impairment, given the class's own hepatotoxicity signal on top of the underlying liver disease.^[8]

Heart failure

Hyponatremia in decompensated heart failure is managed by optimizing heart failure therapy and fluid restriction; avoid over-aggressive correction with hypertonic saline outside a genuine acute symptomatic emergency, since rapid volume expansion is poorly tolerated in this population.

Monitoring & Follow-Up

During active correction of severe/acute hyponatremia: recheck sodium after each hypertonic saline bolus, and at least every 6 hours through the first 24 hours.^[1]
During correction of chronic hyponatremia at a controlled rate: recheck sodium at least every 4-6 hours until the trend is clearly tracking the intended ceiling, then per the treating team's protocol.^[2]
After initiating or up-titrating tolvaptan: monitor sodium closely for at least the first 24-48 hours, typically every 6-8 hours per institutional protocol, given the meaningful proportion of SIADH patients who correct faster than intended.^[7]
Watch specifically for any new neurologic symptom during treatment (see Red Flags) — this can signal either inadequate treatment of the original hyponatremia or the earliest signs of osmotic demyelination syndrome from overcorrection, and either possibility requires immediate reassessment rather than a "wait and see" approach.^[3]
Once a causative medication has been identified and held or adjusted, confirm sodium has genuinely normalized (not just improved) before considering the issue resolved, and document the causative drug clearly so it is not unintentionally restarted.
For chronic SIADH managed with fluid restriction or tolvaptan, arrange outpatient follow-up sodium checks — the interval should be individualized, but a patient newly stabilized on tolvaptan needs closer follow-up than one on stable long-term fluid restriction.
Recheck liver function periodically in any patient on tolvaptan beyond a brief course, and confirm therapy has not exceeded 30 days without a specific specialist-directed reason.^[8]

Treatment Failure / Next Step

Fluid restriction not achieving the expected rise in sodium: confirm actual adherence (measured intake, not estimated) before escalating therapy — this is the most common reason fluid restriction appears to fail.
Genuine fluid-restriction failure despite confirmed adherence: consider adding oral urea or salt tablets, or escalating to tolvaptan under the inpatient-initiation safeguards described in Choose Initial Treatment.
Correction proceeding faster than the intended ceiling: stop the active correction measure immediately and consider the electrolyte-free water plus desmopressin rescue strategy described in Choose Initial Treatment — do not simply continue and "recheck later."
Correction proceeding more slowly than clinically needed in a still-symptomatic patient: reassess the diagnosis — confirm the volume-status classification was correct, and confirm a causative medication was actually held rather than only flagged.
Recurrent or relapsing hyponatremia in the same patient: this is a signal to revisit the medication list in full (a causative drug may have been restarted, or a new one added) and to reconsider whether an underlying cause (malignancy, adrenal insufficiency, hypothyroidism) was adequately excluded the first time.
New neurologic symptoms during or after correction that do not fit the expected trajectory: treat as possible osmotic demyelination syndrome until proven otherwise — urgent neurology involvement and neuroimaging are appropriate, not a "wait and monitor" approach.{C(verbalis)}

Common Errors to Avoid

Correcting chronic hyponatremia faster than the safe 24-hour/48-hour ceiling — the single most consequential error in this chapter, and one that can cause irreversible neurologic injury.^[2]^[3]
Treating a low sodium result without first checking serum osmolality, and missing pseudohyponatremia or translocational (hyperglycemia-driven) hyponatremia as a result.^[4]
Withholding urgent hypertonic saline from a severely symptomatic patient out of excessive concern for the 24-hour ceiling — the immediate risk in that situation is cerebral edema, not ODS.^[1]
Missing a causative medication — especially a thiazide, an SSRI, carbamazepine/oxcarbazepine, or desmopressin — and treating only the sodium number without addressing why it is low.^[1]
Initiating tolvaptan outside an inpatient setting, or continuing fluid restriction alongside it — both meaningfully increase overcorrection risk.^[7]
Continuing tolvaptan beyond 30 days, or using it in a patient with significant underlying liver disease, without specialist direction — the hepatotoxicity boxed warning is not a theoretical concern.^[8]
Infrequent sodium monitoring during any active correction — overcorrection can occur silently between checks if the interval is too long.
Assuming new confusion or falls in an older adult reflect baseline cognitive decline rather than considering hyponatremia (or overcorrection, if being actively treated) as a contributing or causative factor.^[1]
Discontinuing a causative psychiatric or anticonvulsant medication unilaterally rather than coordinating the change with the prescriber.

Clinical Decision Algorithm

Algorithm A — Confirm true hyponatremia and classify by volume status

Serum sodium under 135 mmol/L

Check serum osmolality to confirm true hypotonic hyponatremia — rule out pseudohyponatremia (hyperlipidemia/hyperproteinemia) and translocational hyponatremia (hyperglycemia)^[1]^[4]

Assess extracellular fluid volume status on history and exam; obtain urine osmolality and urine sodium^[1]^[4]

Hypovolemic — GI losses, diuretics (esp. thiazides), adrenal insufficiency

Isotonic (0.9%) saline for volume repletion; treat/stop the underlying cause

Euvolemic — most commonly SIADH; also primary polydipsia, low solute intake, hypothyroidism, cortisol deficiency

Fluid restriction (500 mL-1 L/24h) first-line; confirm SIADH is a diagnosis of exclusion before labeling it

Hypervolemic — heart failure, cirrhosis, nephrotic syndrome, advanced CKD

Treat the underlying disorder plus fluid restriction (~1 L/24h) and diuretic therapy where appropriate

Regardless of etiology: determine acuity (under vs. over 48 hours) and symptom severity before selecting a correction rate — see Algorithm B^[1]

Algorithm B — The correction-rate safety rule (the central teaching point of this chapter)

Hyponatremia requiring active correction — determine acuity and symptom severity

Severe symptoms (seizure, marked confusion, obtunded consciousness), or clearly acute (under 48 hours) with significant symptoms

Emergency: up to three 100 mL boluses of 3% hypertonic saline IV over 10 minutes each (or 150 mL over 20 minutes); target an initial rise of 4-6 mmol/L^[1]

This rapid initial increment is appropriate — the immediate risk is cerebral edema, not ODS — but it still counts toward the same-day cumulative ceiling below^[2]^[3]

Chronic (48 hours or longer, or unknown duration), mild-moderate symptoms or asymptomatic

Treat by etiology per Algorithm A, at a controlled rate — do not bolus hypertonic saline

Any ODS risk factor present (Na under 105 mmol/L, hypokalemia, malnutrition, alcohol use disorder, advanced liver disease), or risk status unknown

Limit total correction to 8 mmol/L over 24 hours^[1]

Confirmed low-risk chronic hyponatremia

Limit total correction to 12 mmol/L over 24 hours and 18 mmol/L over 48 hours^[1]

Recheck sodium after each hypertonic bolus and at least every 6 hours through the first 24 hours; more often during tolvaptan initiation^[1]^[7]

Correcting faster than the target rate: stop active correction; give electrolyte-free water (D5W), and consider adding desmopressin to halt further correction and re-lower sodium toward the safe rate^[3]

Algorithm C — Medication review

Any new or worsening hyponatremia — review the full medication list for a causative or contributing drug

Thiazide diuretic — the most common drug-induced cause

Discuss holding or switching to a non-thiazide agent with the prescriber; risk is highest in the first few weeks after a dose start/increase^[1]

SSRI/antidepressant, carbamazepine, or oxcarbazepine

Discuss reassessing necessity/dose or switching with the prescriber — do not stop a psychiatric or anticonvulsant medication unilaterally^[1]

Desmopressin or an NSAID

Reassess desmopressin against its original indication/dose; NSAIDs potentiate ADH action and should be reviewed alongside any other causative drug^[1]

Older adults are disproportionately affected given polypharmacy and age-related susceptibility — a structured medication review is a high-value, genuinely pharmacist-led intervention in this population^[5]

Drug Tables

How to use this table: Doses are adult doses and require patient-specific confirmation (renal/hepatic function, weight, allergy history). Pediatric dosing is weight-based per the current product monograph unless stated. Drug pricing is intentionally omitted. Select any drug name to open its complete Pharmasist Drug Database page.

IV Fluids for Hyponatremia

Selection depends on volume status, severity, and acuity — see Choose Initial Treatment.

Drug	Dosage	Adverse effects	Drug interactions	Comments
Sodium Chloride 3% Injection (hypertonic saline)	Adults, emergency: Up to three 100 mL IV boluses over 10 minutes each, or 150 mL over 20 minutes; target initial rise 4-6 mmol/L Children: Not established for routine pediatric use — individualize under pediatric/nephrology-directed dosing.	Overly rapid correction if not carefully monitored; fluid overload; infusion-site irritation with peripheral administration	Additive volume-overload risk with other IV fluids; incompatible with several drugs in the same line — check compatibility before co-infusion	Reserve for severe, acute, or symptomatic hyponatremia. Monitor sodium after each bolus and at least every 6 hours for 24 hours. Does not count as exempt from the 24-hour correction ceiling.
Sodium Chloride Injection USP 0.9% (normal saline)	Adults: Isotonic infusion, rate individualized to the degree of volume depletion Children: Not established for routine pediatric use — individualize under pediatric/nephrology-directed dosing.	Can unmask a faster-than-expected sodium rise as volume repletion removes the hypovolemic stimulus for ADH release	None significant at standard use	First-line for hypovolemic hyponatremia. Monitor sodium closely as repletion progresses.
Dextrose 5% in Water (D5W)	Adults, overcorrection rescue: Per institutional overcorrection-rescue protocol Children: Not established for routine pediatric use — individualize under pediatric/nephrology-directed dosing.	Hyperglycemia with rapid/large-volume infusion; fluid overload	None significant at standard use	Electrolyte-free water, used specifically as a rescue measure to lower sodium back toward the intended correction rate when correction is proceeding too fast.

Vasopressin Receptor Antagonists

For select euvolemic (SIADH) or hypervolemic hyponatremia inadequately controlled by fluid restriction. Must be initiated as an inpatient.

Drug	Dosage	Adverse effects	Drug interactions	Comments
Tolvaptan	Adults, initial: 15 mg PO once daily Adults, titration: May be titrated at intervals of at least 24 hours to 30 mg, then a maximum of 60 mg PO once daily, guided by serum sodium response Children: Not established for routine pediatric use — individualize under pediatric/nephrology-directed dosing.	Overly rapid correction; thirst, dry mouth, polyuria, constipation; rare but serious hepatotoxicity (FDA boxed warning)	CYP3A4 inhibitors/inducers significantly affect exposure; avoid combining with other sodium-raising therapy without close monitoring	Mandatory inpatient initiation with fluid restriction discontinued during initiation/up-titration. Monitor sodium closely for at least 24-48 hours after any dose change. Do not use beyond 30 days; avoid in significant underlying liver disease.

Loop Diuretics

Adjunct role in hyponatremia management — not a primary correction therapy on its own.

Drug	Dosage	Adverse effects	Drug interactions	Comments
Furosemide	Adults, adjunct to hypertonic saline: Low dose per institutional protocol Adults, hypervolemic hyponatremia: Per standard heart failure/cirrhosis dosing Children: Not established for routine pediatric use — individualize under pediatric/nephrology-directed dosing.	Volume depletion if overused; hypokalemia — itself a risk factor for osmotic demyelination syndrome	Additive electrolyte disturbance with other diuretics; monitor potassium closely alongside sodium	Sometimes co-administered at low dose with hypertonic saline to limit volume expansion; also used as part of standard heart failure/cirrhosis management in hypervolemic hyponatremia.

Overcorrection Rescue Therapy

Use when serum sodium is rising faster than the intended 24-hour/48-hour ceiling. Consider proactively, not only after neurologic symptoms of osmotic demyelination syndrome appear.

Drug	Dosage	Adverse effects	Drug interactions	Comments
Desmopressin (overcorrection rescue)	Adults, overcorrection rescue: Per institutional overcorrection-rescue protocol Children: Not established for routine pediatric use — individualize under pediatric/nephrology-directed dosing.	The same water-retaining effect that causes hyponatremia when used therapeutically for its approved indications — dosed here specifically to halt further sodium correction	Additive water-retention risk with any other ADH-active drug or SIADH-inducing medication	Given together with electrolyte-free water (D5W, above) to actively re-lower sodium toward the intended rate. A recognized, described strategy for preventing and reversing inadvertent overcorrection.

Causative Medications to Review

Identify and discuss with the prescriber whenever hyponatremia is diagnosed. Do not stop a psychiatric or anticonvulsant medication unilaterally — coordinate the change.

Drug	Dosage	Adverse effects	Drug interactions	Comments
Hydrochlorothiazide (representative thiazide diuretic)	Review: N/A — medication review, not a dosing decision	Hyponatremia, most often within the first few weeks of starting or increasing the dose; also hypokalemia, hypomagnesemia	Additive hyponatremia risk with any other causative drug in this table	The classic and most common drug-induced cause — genuinely more so than loop diuretics. Applies equally to chlorthalidone, indapamide, and thiazide-containing combination products.
Sertraline (representative SSRI/antidepressant)	Review: N/A — medication review, not a dosing decision	SIADH-like syndrome, typically within 2-4 weeks of starting or increasing the dose	Additive hyponatremia risk with a thiazide or other causative drug — a common combination in older adults	Applies broadly across SSRIs and other antidepressant classes (citalopram, escitalopram, fluoxetine, paroxetine, venlafaxine, duloxetine, tricyclic antidepressants).
Carbamazepine	Review: N/A — medication review, not a dosing decision	Direct stimulation of ADH release/renal sensitization	Additive hyponatremia risk with other causative drugs	Reassess necessity and dose with the prescriber.
Oxcarbazepine	Review: N/A — medication review, not a dosing decision	Direct stimulation of ADH release/renal sensitization — higher reported incidence than carbamazepine	Additive hyponatremia risk with other causative drugs	Reassess necessity and dose with the prescriber.
Desmopressin (causative use)	Review: N/A — medication review, not a dosing decision	Direct V2-receptor agonism — the same water-retaining mechanism as the pathology being treated in this chapter	Additive hyponatremia risk with any other causative drug	Prescribed for central diabetes insipidus, nocturnal enuresis, nocturia, and some bleeding disorders. Reassess dose against the original indication whenever hyponatremia develops.
Ibuprofen (representative NSAID)	Review: N/A — medication review, not a dosing decision	Potentiates ADH action at the collecting duct via prostaglandin inhibition	Additive/potentiating hyponatremia risk with other causative drugs	Acetaminophen is a reasonable default analgesic/antipyretic alternative that does not carry this mechanism.

References

Diagnosis and Management of Sodium Disorders: Hyponatremia and Hypernatremia — American Family Physician (Miller NE, Rushlow D, Stacey SK). Am Fam Physician 2023;108(5):476-486
Clinical practice guideline on diagnosis and treatment of hyponatraemia — European Journal of Endocrinology (Spasovski G, Vanholder R, Allolio B, et al.). Eur J Endocrinol 2014;170(3):G1-G47
Diagnosis, Evaluation, and Treatment of Hyponatremia: Expert Panel Recommendations — American Journal of Medicine (Verbalis JG, Goldsmith SR, Greenberg A, Korzelius C, Schrier RW, Sterns RH, Thompson CJ). Am J Med 2013;126(10 Suppl 1):S1-S42
The hyponatremic patient: a systematic approach to laboratory diagnosis — CMAJ (Milionis HF, Liamis GL, Elisaf MS). CMAJ 2002;166(8):1056-1062
Thiazide-associated hyponatremia increases the risk of major adverse cardiovascular events among elderly Taiwanese patients — BMC Geriatrics (Yang H, Hsu JL, Kuo YH, Liao KF). BMC Geriatr 2023;23(1):862
Tolvaptan, a Selective Oral Vasopressin V2-Receptor Antagonist, for Hyponatremia — New England Journal of Medicine (Schrier RW, Gross P, Gheorghiade M, et al.; SALT Investigators). N Engl J Med 2006;355(20):2099-2112
Rapidity of Correction of Hyponatremia Due to Syndrome of Inappropriate Secretion of Antidiuretic Hormone Following Tolvaptan — American Journal of Kidney Diseases (Morris JH, Bohm NM, Nemecek BD, Crawford R, Kelley D, Bhasin B, Nietert PJ, Velez JCQ). Am J Kidney Dis 2018;71(6):772-782
Tolvaptan — LiverTox: Clinical and Research Information on Drug-Induced Liver Injury, National Institute of Diabetes and Digestive and Kidney Diseases. LiverTox [Internet], NCBI Bookshelf NBK548503

Professional reference only. Apply clinical judgment and verify patient-specific decisions against current authoritative guidance and product information.