Syndrome of inappropriate antidiuresis

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

SIAD occurs when AVP action persists despite low plasma tonicity and adequate effective circulating volume, leading to water retention and dilutional hyponatraemia. The kidney continues to excrete sodium, so total-body sodium is not necessarily deficient even though its measured concentration is low. First confirm plasma hypotonicity and assess onset and neurological symptoms. Examination then looks for hypovolaemia or oedema, but clinical volume assessment is imperfect. Paired urine osmolality and urine sodium obtained before major therapy add physiological information. Review all medicines, recent surgery, pain and nausea, chest and neurological symptoms, cancer history and fluid intake. Exclude cortisol deficiency explicitly because it closely mimics SIAD and glucocorticoid replacement can provoke a rapid water diuresis. Severe hypothyroidism and advanced kidney dysfunction are alternative contexts, while thiazides can generate mixed patterns.

Treatment separates emergency cerebral protection from chronic water-balance management. Society for Endocrinology guidance uses small measured boluses of hypertonic sodium chloride for severe or moderately severe symptoms, with repeated neurological and sodium review. The goal is symptom improvement through a limited initial rise, not normal sodium immediately. Chronic or minimally symptomatic SIAD requires removal of the driver, stopping an implicated medicine when safe, and a feasible fluid plan that includes all drinks and liquid feeds. Restriction may fail when urine is highly concentrated, intake cannot be reduced safely or nutrition is compromised. Specialist alternatives include increasing solute excretion or antagonising V2 receptors, each with monitoring and jurisdictional restrictions. Once nausea, cortisol deficiency or another stimulus resolves, spontaneous aquaresis can accelerate sodium correction, so urine output is an early warning signal.

Key points

SIAD is a diagnosis of hypotonic hyponatraemia with inappropriately concentrated urine after effective-volume depletion, adrenal insufficiency, severe hypothyroidism, kidney failure and diuretics are addressed.
Confirm true hypotonicity because hyperglycaemia, exogenous osmoles and pseudohyponatraemia create different physiology and treatment.
Urine osmolality shows whether AVP activity is present; maximally dilute urine points towards excess intake rather than persistent antidiuresis.
Interpret urine sodium with volume status, dietary intake, kidney function and recent diuretics; no single urine threshold proves SIAD in isolation.
Common drivers include pulmonary disease, central nervous system disease, malignancy, pain, nausea, postoperative stress and medicines such as antidepressants or antiseizure drugs.
Severe symptomatic hyponatraemia receives controlled hypertonic saline regardless of whether SIAD has yet been confirmed.
For chronic stable SIAD, fluid restriction is often first-line, but likely response depends on electrolyte-free water clearance and ongoing obligatory intake.
Tolvaptan, oral urea and other second-line strategies require specialist selection and local policy; tolvaptan can overcorrect and must never substitute for emergency hypertonic saline.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Pulmonary or neurological disease

Chest infection, lung pathology, stroke, haemorrhage and other cerebral disease can stimulate vasopressin despite low plasma tonicity.

Malignancy-related secretion

Some tumours produce or provoke vasopressin activity, making persistent unexplained SIAD a reason to assess the appropriate cancer context.

Medicine, pain and nausea

Several medicines, postoperative pain and nausea sustain non-osmotic vasopressin release and may create a reversible hospital-associated syndrome.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Vasopressin action persists
The collecting duct continues to reabsorb water even though plasma is already hypotonic and effective circulating volume is adequate.
2
Body water increases
Retained water dilutes extracellular sodium while compensatory urinary sodium excretion limits overt oedema, producing the typically euvolaemic appearance of SIAD.
3
Plasma becomes hypotonic
Lower effective osmolality drives water into brain cells, producing neurological symptoms when onset is rapid or the fall is severe.
4
Spontaneous aquaresis may emerge
When pain, nausea, medicine effect or cortisol deficiency resolves, vasopressin falls and brisk water excretion can overcorrect sodium.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Acute cerebral oedema

Seizure, vomiting, reduced consciousness, severe confusion or rapidly developing neurological symptoms suggest acute brain swelling and require emergency treatment irrespective of the final cause.

Chronic subtle phenotype

Gait instability, attention difficulty, falls, nausea or mild confusion can accompany chronic hyponatraemia and should not be dismissed as normal ageing or malignancy burden.

Euvolaemic pattern

Absence of convincing dehydration or oedema with hypotonic plasma and urine that remains concentrated is compatible with SIAD only after endocrine and renal mimics are excluded.

Medicine-associated disease

A temporal link with an SSRI, carbamazepine, oxcarbazepine, antipsychotic, opioid or cytotoxic drug raises suspicion but does not remove the need to assess tonicity and cortisol.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Measured serum osmolalityFirst step
Why
Confirm that low sodium represents hypotonic hyponatraemia rather than a non-hypotonic state.
Interpretation and limitations
Low osmolality supports excess water relative to solute. Normal or high osmolality directs evaluation towards pseudohyponatraemia, glucose or another effective osmole.
02
Urine osmolality
Why
Assess whether renal water excretion is appropriately maximised at low plasma tonicity.
Interpretation and limitations
Persistently concentrated urine indicates AVP effect; very dilute urine suggests primary polydipsia or low-solute intake, although timing and recent therapy matter.
03
Urine sodium with paired bloods
Why
Support assessment of effective volume and renal sodium handling before treatment changes the pattern.
Interpretation and limitations
A non-low urine sodium can fit SIAD, but diuretics, kidney disease, adrenal deficiency and dietary solute alter it; integrate rather than use a standalone cut-off.
04
Morning cortisol or acute adrenal assessment
Why
Exclude glucocorticoid deficiency, an important treatable mimic of SIAD.
Interpretation and limitations
Apply current assay and clinical thresholds. If adrenal crisis is possible, draw a sample if immediate and give hydrocortisone without waiting; water diuresis after replacement needs close sodium monitoring.
05
TSH and free thyroxine
Why
Identify severe hypothyroidism or central pituitary disease contributing to impaired water excretion.
Interpretation and limitations
Mild TSH changes rarely explain marked hyponatraemia. Low free thyroxine with a non-raised TSH suggests pituitary disease and requires cortisol-safe endocrine review.
06
Cause-directed chest and neurological assessment
Why
Find pulmonary, malignant, infectious or intracranial stimuli sustaining antidiuresis.
Interpretation and limitations
Choose radiography, CT or other investigations from symptoms, examination and cancer risk rather than routinely scanning every mild medicine-associated episode.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Adrenal insufficiency

Cortisol deficiency increases vasopressin and closely mimics SIAD; hypotension, steroid history, hypoglycaemia or hyperkalaemia provide clues.

Hypovolaemic hyponatraemia

Postural symptoms, dry mucosa, sodium losses and physiological antidiuresis indicate volume depletion rather than inappropriate secretion.

Heart failure or cirrhosis

Oedema and low effective arterial volume drive appropriate neurohormonal water retention despite increased total-body fluid when supported by the history and examination.

Primary polydipsia

Very high intake produces maximally dilute urine, whereas SIAD retains an inappropriately concentrated urine despite plasma hypotonicity.

Additional chapter-specific clues

Hidden alternative

Postural hypotension, hyperkalaemia, steroid exposure, vomiting, diuretic use, heart failure, cirrhosis or renal impairment points away from uncomplicated SIAD and changes treatment.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01EmergencyTreat severe symptomsFirst stepHypotonic hyponatraemia accompanies seizure, reduced consciousness or another severe neurological syndrome.

1Begin monitored ABCDE care, check glucose, stop hypotonic fluid and obtain serum and urine samples if this causes no treatment delay.
2Give protocol-defined boluses of hypertonic saline, reassessing symptoms and sodium after each rather than prescribing an open-ended infusion.
3Set correction limits, monitor urine output and sodium frequently, involve critical care or endocrinology, and use a specialist reversal plan if the rise becomes excessive.

02DiagnosisEstablish SIAD by exclusionStable hypotonic hyponatraemia has no immediately obvious volume-depleted or oedematous cause.

1Review onset, fluid and solute intake, examination, medicines, renal function, glucose and paired serum and urine osmolality with urine sodium.
2Exclude cortisol deficiency and clinically relevant thyroid or kidney disease, accounting for recent diuretic exposure and treatment already given.
3Search proportionately for a pulmonary, neurological, malignant, postoperative or pharmacological driver and label SIAD only when the physiology coheres.

03Chronic careReduce water excess sustainablyConfirmed or strongly supported SIAD persists without emergency neurological symptoms.

1Treat the precipitant and stop an implicated medicine when benefit-risk permits, replacing necessary therapies rather than withdrawing abruptly.
2Agree a quantified fluid restriction if appropriate, counting medication vehicles and feeds while protecting nutrition and comfort, then measure early response.
3Refer persistent or poorly tolerated disease for specialist solute or vaptan options under local policy, with a planned start setting, sodium schedule and discontinuation criteria.

Key medicines and prescribing safety2 treatments · regimens, roles and cautions

Raises plasma sodium promptly enough to reduce acute cerebral oedema in severe or moderately severe symptomatic hyponatraemia.

Hypertonic sodium chloride

Administer the current Society for Endocrinology protocol-defined bolus in a monitored acute setting, repeat only after symptom and sodium reassessment, and stop when the limited initial correction goal is achieved.

It is not routine treatment for asymptomatic chronic SIAD. Overcorrection risks osmotic demyelination; verify concentration, use controlled administration, account for potassium and spontaneous aquaresis, and have a reversal plan.

Blocks renal V2 receptors, producing electrolyte-free water excretion in selected persistent euvolaemic hyponatraemia.

Tolvaptan

Initiate only under an experienced specialist and local commissioned protocol at the approved starting regimen, with free access to water and closely timed sodium checks; adjust according to response rather than routine escalation.

Can cause thirst, dehydration and dangerously rapid sodium correction, particularly when baseline sodium is low or SIAD resolves. It is not emergency therapy and requires liver, interaction and licence review.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Cerebral oedema

Rapid hypotonicity drives water into brain cells, causing headache, vomiting, confusion, seizure and reduced consciousness through cerebral swelling.

Falls and fracture

Chronic milder hyponatraemia impairs attention and gait and may add bone vulnerability, increasing falls and injury.

Osmotic demyelination

Over-rapid correction after cerebral adaptation can cause delayed dysarthria, weakness, movement disorder and profound neurological disability.

Nutritional compromise

Rigid fluid restriction can reduce food and nutrient intake when drinks, feeds and medicine fluids are not planned together.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

During active correction, chart sodium against time, neurological state and urine output so spontaneous aquaresis is recognised before an excessive rise.
For fluid restriction, document the actual total intake, urine volume, weight and daily sodium response rather than recording only that advice was given.
Recheck cortisol or pituitary conclusions when glucocorticoids, acute illness or assay timing made initial exclusion uncertain.
Monitor nutrition and medicine delivery because severe restriction may reduce protein, calories or essential oral treatment.
After the trigger is removed, continue observation long enough to detect overcorrection and define whether long-term SIAD therapy is still required.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

SIAD is not a sodium result

The diagnosis describes inappropriate water retention after exclusions. Euvolaemia plus low sodium alone cannot distinguish cortisol deficiency, thiazide effect or low-solute intake.

Urine output predicts acceleration

A sudden water diuresis may precede a steep sodium rise when nausea resolves or cortisol is replaced. Hourly output can warn earlier than the next blood result.

Restriction has a physiological ceiling

When urine remains very concentrated, obligatory urinary solute carries little water; a modest fluid restriction may therefore be ineffective even with perfect adherence.

Cancer does not prove SIAD

Malignancy increases probability but patients still develop dehydration, adrenal metastases, kidney failure and medicine effects. Paired physiology prevents anchoring.

Correction is a cumulative exposure

Potassium replacement, hypertonic saline and spontaneous water loss all raise sodium. The total trajectory matters more than any single prescribed source.

11Common pitfallsFrequent interpretation and management errors.

01
Calling any apparently euvolaemic hyponatraemia SIAD without measuring osmolality or excluding cortisol deficiency.
02
Giving normal saline reflexively to SIAD and worsening sodium when concentrated urine excretes its sodium while retaining water.
03
Attempting to normalise severe chronic hyponatraemia in the first hours rather than seeking limited symptom relief.
04
Prescribing fluid restriction without counting feeds, drug diluents and all drinks or checking whether the urine pattern predicts failure.
05
Starting tolvaptan without a monitored sodium schedule, water access and a plan for rapid correction.
06
Failing to watch for aquaresis after nausea resolves or glucocorticoid deficiency is treated.