AKT · Endocrine/Pituitary & ADH

Syndrome of inappropriate ADH (SIADH)

Excess ADH → euvolaemic hyponatraemia with concentrated urine

Overview

Inappropriate ADH secretion causing water retention and dilutional, EUVOLAEMIC hyponatraemia with inappropriately concentrated urine. Causes: malignancy (small-cell lung — paraneoplastic), CNS disease, lung disease, drugs (SSRIs, carbamazepine, etc.). A diagnosis of exclusion (euvolaemic, normal thyroid/adrenal). Treated by fluid restriction and the cause.

Recognise

EUVOLAEMIC hyponatraemia — no oedema, no dehydration; symptoms of low sodium (confusion, nausea, seizures if severe/rapid)
Low serum osmolality with INAPPROPRIATELY concentrated urine (urine osmolality high, urine sodium >20–30)
Causes: small-cell lung cancer (paraneoplastic), CNS (stroke, meningitis), pneumonia, drugs (SSRIs, carbamazepine, NSAIDs)

Red flags

Severe/rapid hyponatraemia with seizures/coma → hypertonic saline (specialist; avoid overcorrection → osmotic demyelination)
Underlying malignancy (small-cell lung) → investigate

Differentials & how to tell them apart

Hypovolaemic hyponatraemiasigns of dehydration, low urine sodium (<20) — D&V, diuretics; treat with saline

Cerebral salt-wasting syndromehyponatraemia after brain injury/SAH but the patient is HYPOvolaemic (renal salt+water loss) — treat with SALT and fluid, the OPPOSITE of SIADH fluid restriction

Hypervolaemic hyponatraemiaoedema — heart failure, cirrhosis, nephrotic — fluid restrict + treat cause

Hypothyroidism / Addison'smust be excluded — they cause hyponatraemia and mimic SIADH

Hyperglycaemia/pseudohyponatraemiacorrect for glucose; lipids/paraproteins

Investigations

Confirm true hypo-osmolar hyponatraemia; EUVOLAEMIA clinically; urine osmolality >100 (inappropriately concentrated) and urine sodium >20–30; NORMAL thyroid and adrenal function (exclude hypothyroidism/Addison's). Find the cause (CXR/CT for malignancy, drug review).

Management

Fluid restriction + treat the cause (hypertonic saline if severe symptoms)

1Confirm euvolaemic hypo-osmolar hyponatraemia with concentrated urine and normal thyroid/adrenal function. First-line is FLUID RESTRICTION and treating the cause (drug, malignancy, CNS/lung disease).Gate: Correct sodium SLOWLY (≤8–10 mmol/L/24 h) — overcorrection causes osmotic demyelination (central pontine myelinolysis); reserve hypertonic saline for severe symptoms (seizures/coma) under specialist care
2Persistent/severe → tolvaptan or demeclocycline; investigate and treat the underlying cause (often small-cell lung cancer — see paraneoplastic).

Fluid restriction — first-line for chronic/mild-moderate SIADH

Treat the cause — stop the drug, treat the tumour/infection

Tolvaptan / demeclocycline / hypertonic saline (severe) — specialist — vaptan or hypertonic saline for severe symptomatic disease, with careful correction

Key points

Euvolaemic hyponatraemia + concentrated urine + normal thyroid/adrenal = SIADH (a diagnosis of exclusion). Fluid restrict, treat the cause, correct slowly. Think small-cell lung cancer (paraneoplastic) and drugs (SSRIs, carbamazepine).

Monitor & prognosis

Sodium (rate of correction), fluid balance.

Depends on cause; correction risk if too fast.

Source: Society for Endocrinology; CKS Hyponatraemia