SIADH: causes, safe sodium correction, and the ODS rule

SIADH is the most common single cause of euvolaemic hyponatraemia, accounting for roughly 30–40% of all hyponatraemia in hospitalised Australians and a high proportion of community presentations in older adults (eTG; Hoorn & Zietse JASN 2017). Low sodium itself is frequently dismissed as a minor finding — but even mild chronic hyponatraemia at 130–134 mmol/L carries a two to three times excess mortality, contributes to falls and hip fractures, and impairs cognition in older people (Renneboog Am J Med 2006; Waikar Am J Med 2009). This article covers the GP approach to diagnosis and the management pathway from first-line fluid restriction through to specialist escalation.

A. Core clinical — diagnosing SIADH in general practice

The diagnostic criteria

SIADH is a diagnosis of exclusion. The ESE 2014 guideline updates the original Bartter-Schwartz criteria and requires all of the following:

• Effective serum osmolality below 275 mOsm/kg (true hypotonic hyponatraemia — not pseudohyponatraemia from severe hyperlipidaemia or paraproteinaemia, and not hyperosmolar hyponatraemia from hyperglycaemia or mannitol).
• Urine osmolality above 100 mOsm/kg during hypotonicity — the kidneys are failing to maximally dilute the urine when they should be.
• Clinical euvolaemia — no postural blood-pressure drop, no oedema, no elevated jugular venous pressure, no dry mucous membranes; weight stable.
• Urine sodium above 30 mmol/L on a spot sample collected while the patient is eating and drinking normally (not after intravenous fluids, which falsely elevate urine sodium; not during active diuretic use).
• Normal thyroid function (TSH and free T4) and normal adrenal function (9 am cortisol or short Synacthen test) — both must be confirmed before the label SIADH is applied.

The supporting test that is often overlooked is serum uric acid — in SIADH it is typically low (below 240 µmol/L) because intracellular volume expansion promotes urate excretion. An elevated uric acid favours hypovolaemic hyponatraemia instead.

Key investigations (MBS items)

Order the following as a standard first-pass workup for any unexplained hyponatraemia:

• Paired serum and urine osmolality — MBS item 66602. This is the diagnostic anchor for SIADH.
• Urine sodium and potassium — MBS item 73529. Collect on a normal diet.
• Full biochemistry panel including urea, creatinine, uric acid, glucose — MBS item 66500 or 65070.
• TSH and free T4 — MBS 66716 + 66717.
• 9 am cortisol — MBS 66825. Mandatory. Isolated adrenal insufficiency — secondary AI from pituitary disease — can present as euvolaemic hyponatraemia indistinguishable from SIADH; missing it is a preventable death.

Differential diagnosis at a glance

The three broad categories of hyponatraemia are distinguished by volume status:

• Hypovolaemic (low total body sodium — GI loss, thiazide within 24 hours, salt-wasting nephropathy): postural blood-pressure drop, dry mucosae, elevated urea-to-creatinine ratio, uric acid high. Urine sodium low if extra-renal loss, high if renal salt-wasting.
• Hypervolaemic (high total body sodium and water — heart failure, cirrhosis, nephrotic, advanced CKD): oedema, ascites, elevated JVP. Treat the underlying condition, not the sodium.
• Euvolaemic (SIADH, hypothyroidism, adrenal insufficiency, reset osmostat, primary polydipsia): normal volume; SIADH most common after thyroid and adrenal excluded. Cerebral salt wasting is the key SIADH mimic — post-neurosurgical or subarachnoid haemorrhage context, looks identical biochemically but is actually hypovolaemic and requires saline, not fluid restriction (Sterns Curr Opin Crit Care 2008).

B. The ODS rule — correcting sodium without causing brain injury

Osmotic demyelination syndrome (ODS) is the most feared complication of SIADH management — and it is caused by treatment, not by the disease itself. When sodium has been low for more than 48 hours, brain cells adapt by actively expelling organic osmolytes (myo-inositol, taurine, glutamate) to reduce intracellular volume. Correcting the sodium too quickly then strips these osmolytes from cells that no longer have them, dehydrating the myelin-forming oligodendrocytes in the pons and elsewhere — producing spastic quadriparesis, dysarthria, dysphagia, and in severe cases locked-in syndrome (Sterns NEJM 2015; Adrogué NEJM 2000).

The hard ceiling is a sodium rise of no more than 10 mmol/L in the first 24 hours, and no more than 18 mmol/L in the first 48 hours (ESE 2014; Gankam-Kengne Kidney Int 2010).

For patients at highest risk of ODS, the ceiling should be further lowered to six to eight mmol/L per 24 hours. These patients are: alcohol use disorder, malnutrition (including anorexia and frailty), hypokalaemia, liver disease, and baseline sodium below 105 mmol/L.

If sodium overshoots the target, the protocol is to actively re-lower it by giving dextrose 5% in water (six mL/kg IV) with or without DDAVP 2–4 µg IV — a specialist inpatient decision. This counter-intuitive manoeuvre genuinely prevents ODS and is endorsed by the ESE guideline and Sterns 2015.

For community patients with chronic mild hyponatraemia (Na 130–134 mmol/L), sodium should be corrected gradually over weeks, not days.

C. Treatment ladder — from fluid restriction to tolvaptan

Step 1: Treat the underlying cause

This is the highest-leverage intervention and should happen simultaneously with the investigation. Remove or treat:

• The offending drug — thiazide diuretic (switch to amlodipine or an ACE inhibitor; do not re-challenge after a symptomatic event); SSRI or SNRI (switch to agomelatine, mirtazapine, or vortioxetine — lower SIADH risk); carbamazepine or oxcarbazepine (consider lamotrigine or levetiracetam with the treating specialist).
• Infection — antibiotics for pneumonia or urosepsis; SIADH typically resolves within one to two weeks of treatment.
• Malignancy — chemotherapy or radiotherapy for small-cell lung cancer often resolves paraneoplastic SIADH within two to four weeks; involve oncology early.
• Pain and nausea — both independently trigger non-osmotic ADH release; adequate analgesia and anti-emetics alone may resolve perioperative SIADH.

Step 2: Fluid restriction

Fluid restriction to under 800 mL to one litre per day is the first-line pharmacological treatment for chronic SIADH once the cause has been addressed (ESE 2014; Lehrich CJASN 2010). All fluid sources count — tea, soup, fluid in food, oral medications.

Calculate the Furst index (urine [Na + K] ÷ serum Na) on a spot urine before committing to restriction:

• Below 0.5 — restrict to one litre per day; this alone will usually succeed.
• 0.5 to 1 — restrict to 500 mL; loop diuretic (furosemide 20 mg daily) plus two to three grams of dietary sodium may help.
• Above 1 — restriction is unlikely to work; escalate to urea or tolvaptan.

Recheck sodium at one to two weeks.

Step 3: Oral urea

For refractory chronic SIADH where fluid restriction has failed or is intolerable, oral urea 15–60 g daily is the preferred second-line option in Australian practice (Decaux CJASN 2018). Urea induces a solute diuresis — the kidney must excrete the urea load dissolved in free water, lowering body water volume and raising serum sodium.

Urea is compounded by Australian pharmacies as a sachet dissolved in 100 mL of juice (lemon or orange reduces the bitter taste). Typical regimen: 15 g once daily, titrated up to 30–60 g if response is inadequate. Cost is approximately $30–60 per month. It is not PBS-listed; the script is private. It is not appropriate in liver failure (increases azotaemia). Palatability is the main limiting factor.

Step 4: Tolvaptan (specialist-initiated)

Tolvaptan (Samsca) is a selective vasopressin V2-receptor antagonist that blocks ADH at the kidney, producing a pure aquaresis — water loss without sodium loss. The SALT-1 and SALT-2 trials (Schrier NEJM 2006, n=448) demonstrated significant sodium rise at days four and 30 versus placebo; the SALTWATER extension (Berl JASN 2010) confirmed sustained efficacy over four years of open-label use.

In Australia, tolvaptan is PBS Authority Required (Streamlined) for hyponatraemia secondary to SIADH (Samsca formulation); verify current authority criteria at PBS online at the time of prescribing. Key restrictions:

• Specialist initiation — endocrinology or nephrology.
• In-hospital monitoring for the first 24 hours — sodium must be checked frequently; liberal water intake is required during therapy (fluid restriction plus tolvaptan risks over-rapid correction).
• Hepatotoxicity warning — the FDA issued a black-box warning in 2013 after the TEMPO 3:4 trial in polycystic kidney disease; LFTs must be monitored during therapy. Limit to 30 days for the SIADH indication where possible.
• Start dose 15 mg daily; titrate to 30 mg or 60 mg as needed.

D. Australian operations

Chronic disease planning: SIADH from any cause lasting more than three months qualifies as a chronic condition for the GP Chronic Condition Management Plan (GPCCMP) preparation item 965 and review item 967 (the post-July 2025 replacement for GPMP/TCA). Allied health referrals under the GPCCMP can include: pharmacist (Home Medicines Review for polypharmacy and SSRI/thiazide deprescribing), dietitian (fluid restriction plan, sodium and protein intake), and psychologist (CBT for fluid-restriction adherence, primary polydipsia).

MBS pathology items: Paired serum and urine osmolality 66602 is the diagnostic anchor; urine Na/K/osmolality 73529 for Furst index; 9 am cortisol 66825 mandatory. CXR 58503 and CT chest 56301 for unexplained SIADH in a smoker.

PBS prescribing: Tolvaptan is Authority Required (specialist-initiated). Oral urea requires a private compounding script. Hypertonic 3% saline is hospital-only supply. SSRI alternatives — agomelatine (Valdoxan) and mirtazapine are general schedule; vortioxetine (Brintellix) is Authority Required Streamlined for treatment-resistant depression. Antiepileptic alternatives — lamotrigine is Authority Required Streamlined; levetiracetam is general schedule.

DVA: Full SIADH workup including paired osmolality, specialist referral, and tolvaptan or urea is covered under Gold and White cards.

E. Special populations

Older adults deserve particular attention. The falls and fracture risk from even mild hyponatraemia (Na 130–134 mmol/L) is substantial and independent of other fall risk factors — correcting sodium to 135 mmol/L measurably improves gait and reduces fall frequency (Renneboog Am J Med 2006). The highest-risk prescribing combination is an SSRI plus a thiazide in a woman over 70 — reconsider both at every review.

Small-cell lung cancer is the malignant cause to look for in any smoker with unexplained SIADH. SIADH may be the presenting symptom before the tumour is visible on plain X-ray — CT chest is mandatory. Chemotherapy or radiotherapy resolves paraneoplastic SIADH in most patients within four weeks, but fluid restriction and urea may be needed during and after treatment.

SSRI-induced SIADH is commonest in the first two to four weeks of treatment and peaks in the first month. The risk is five times baseline in older adults. If sodium falls below 130 mmol/L and the SSRI is implicated, switch rather than dose-reduce. Agomelatine and mirtazapine carry a substantially lower hyponatraemia signal.

Post-operative patients develop SIADH primarily because pain, nausea, vomiting, and opioids all trigger non-osmotic ADH release. The specific preventable iatrogenic contribution is the use of hypotonic maintenance fluids (0.18% saline in 4% dextrose) in adults and elderly patients — this practice should never occur; use isotonic 0.9% saline as the default maintenance fluid for adult inpatients.

When to escalate

Refer urgently to hospital emergency department when:

• Sodium is below 125 mmol/L acutely (developing over less than 48 hours).
• Sodium is below 120 mmol/L at any duration or rate.
• The patient has neurological symptoms: confusion, vomiting, drowsiness, seizure, or a fall in GCS.
• Adrenal insufficiency is suspected (postural hypotension, hyperpigmentation, known pituitary disease).

Refer routinely to endocrinology or nephrology when:

• Confirmed SIADH without a reversible cause.
• Fluid restriction has failed (Furst index above 1).
• Urea or tolvaptan initiation is needed.
• The SIADH is associated with suspected malignancy — involve respiratory medicine or oncology.

What this article is and is not

This article is clinical educational information about SIADH and how it is managed in Australian general practice. It is not a substitute for assessment and management by a qualified clinician. Individual decisions depend on full clinical assessment, investigation results, comorbidities, and medication interactions. Patients with low sodium and any concerning symptoms — confusion, headache, vomiting, or difficulty walking — should seek same-day or emergency medical assessment.

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Sources cited

• eTG complete — Hyponatraemia / SIADH / disorders of water balance
• Australian Medicines Handbook (AMH) — tolvaptan, urea
• ESE / ESICM / ERA-EDTA Clinical Practice Guideline on Hyponatraemia 2014
• Sterns RH — Disorders of Plasma Sodium. NEJM 2015
• Verbalis JG et al. — Hyponatremia Expert Panel. Am J Med 2013
• Adrogué HJ, Madias NE — Hyponatremia. NEJM 2000
• Schrier RW et al. — SALT-1 / SALT-2 tolvaptan trials. NEJM 2006
• Berl T et al. — SALTWATER long-term tolvaptan extension. JASN 2010
• Decaux G et al. — Urea for long-term treatment of SIADH. CJASN 2018
• Furst H et al. — Predictive index for fluid restriction in SIADH. Am J Med Sci 2000
• Gankam-Kengne F et al. — Overcorrection and ODS risk. Kidney Int 2010
• Renneboog B et al. — Mild hyponatraemia and falls / gait. Am J Med 2006
• Waikar SS et al. — Mortality and hyponatraemia. Am J Med 2009
• Sterns RH — Cerebral salt wasting vs SIADH. Curr Opin Crit Care 2008
• Hoorn EJ, Zietse R — Diagnosis and treatment of hyponatraemia: compilation of the guidelines. JASN 2017
• Lehrich RW — Tolvaptan and fluid restriction review. CJASN 2010
• Endocrine Society of Australia
• Kidney Health Australia
• HealthDirect — Hyponatraemia
• PBS — tolvaptan, agomelatine, mirtazapine, lamotrigine
• MBS item 66602 — paired serum and urine osmolality
• MBS item 73529 — urine Na, K, osmolality
• MBS item 66825 — cortisol