Hyponatremia

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]

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]

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]

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]

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]

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]

• 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.

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]

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]

• 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]

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.

• 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]

1. 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.
2. 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.
3. 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."
4. 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.
5. 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.
6. 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)}

• 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.

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.

Vasopressin Receptor Antagonists

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

Loop Diuretics

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

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.

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.