# Syndrome of Inappropriate Antidiuretic Hormone > SIADH for NEET-PG Medicine: euvolaemic hyponatraemia, inappropriately concentrated urine, urine sodium over 30, fluid restriction and tolvaptan. - Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/siadh - Exam / course: NEET-PG · Subject: Medicine - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Syndrome of Inappropriate Antidiuretic Hormone", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/siadh ## Direct answer Unsuppressed vasopressin action — water retained, sodium secondarily lost — produces the euvolaemic hyponatraemia of the syndrome of inappropriate antidiuretic hormone secretion (SIADH), classically due to small-cell lung carcinoma, central nervous system disease, pneumonia or drugs such as carbamazepine and SSRIs. Diagnosis requires hypotonic plasma with inappropriately concentrated urine (osmolality above 100 mOsm per kg) and urine sodium above 30 mmol per litre in a euvolaemic patient with normal thyroid, adrenal and renal function and no diuretic use. Treatment is fluid restriction, with salt tablets, loop diuretics or vasopressin antagonists (tolvaptan) when needed. ## What you must remember - Essential criteria: plasma sodium below 135 mmol per litre, plasma osmolality below 275 mOsm per kg, urine osmolality above 100 mOsm per kg, urine sodium above 30 mmol per litre and clinical euvolaemia. - Supporting features: low uric acid (typically below 4 mg per dL), low urea, normal thyroid-stimulating hormone and cortisol, and correction of sodium with fluid restriction; a urine osmolality above 500 suggests a strong vasopressin drive. - Causes worth memorising: small-cell lung cancer (the classic exam cause), other malignancies, meningoencephalitis, stroke, subarachnoid haemorrhage, head injury, pneumonia and tuberculosis, pain or nausea postoperatively, and drugs — carbamazepine and oxcarbazepine, SSRIs, cyclophosphamide, vincristine and NSAIDs. - Treatment ladder: restrict fluid to about 800 to 1,000 mL per day; increase solute intake (salt tablets with or without a loop diuretic); tolvaptan for refractory cases with careful sodium monitoring. - Severe symptomatic hyponatraemia (seizures, coma): hypertonic 3 per cent saline — about 100 to 150 mL boluses — but correct by no more than 8 to 10 mmol per litre in 24 hours to avoid osmotic demyelination syndrome. - Do not rapidly normalise sodium; over-correction causing central pontine myelinolysis (confusion, quadriparesis, dysphagia) is the most feared complication of therapy, not of the disease. - Reset osmostat, glucocorticoid deficiency and hypothyroidism must be excluded before the label is applied. ## The composite case the exam keeps reusing A 66-year-old smoker with a lung mass becomes confused over a week; sodium is 118 mmol per litre, plasma osmolality 255 mOsm per kg, urine osmolality 540 (a strong vasopressin drive), urine sodium 65, and the patient is clinically euvolaemic — the composite MCQ whose answer is SIADH from small-cell carcinoma through ectopic vasopressin. Before committing, clear the exclusions the label demands: normal thyroid-stimulating hormone and cortisol, because glucocorticoid deficiency and hypothyroidism mimic SIADH exactly; normal renal function; no diuretics, which confound urine sodium interpretation; and not a reset osmostat, in which sodium sits stably mildly low. The supporting biochemistry seals it — low uric acid typically below 4 mg per dL and low urea — and correction with fluid restriction confirms retrospectively. Management then climbs the ladder: increase solute with salt tablets plus a loop diuretic to force excretion of dilute urine if restriction fails; tolvaptan for refractory cases with careful sodium monitoring; and treat the cause, which in the malignancy case is the tumour itself. The emergency clause sits above the ladder: if the patient seizes or slips toward coma, hypertonic 3 per cent saline in about 100 to 150 mL boluses for a prompt rise — but never more than 8 to 10 mmol per litre in 24 hours, because over-correction producing central pontine myelinolysis (confusion, quadriparesis, dysphagia) is the most feared complication of therapy, not of the disease. ## How the exam frames SIADH The criteria list is asked verbatim — sodium below 135, plasma osmolality below 275, urine osmolality above 100, urine sodium above 30, euvolaemia — and the drug list (carbamazepine and oxcarbazepine, SSRIs, cyclophosphamide, vincristine, NSAIDs) plus the cause list (small-cell cancer above all, meningoencephalitis, stroke, subarachnoid haemorrhage, pneumonia, tuberculosis, postoperative pain and nausea) supply the stems. The three contrasts carry the harder marks: the oedematous hyponatraemias, hypervolaemic with low urine sodium; and low uric acid as the supporting feature examinees forget. The correction ceiling of 8 to 10 mmol per day is the number the viva exists to hear. ## Frequently asked questions ### What are the essential diagnostic criteria for SIADH? Hypotonic hyponatraemia, urine osmolality above 100 mOsm per kg, urine sodium above 30 mmol per litre, clinical euvolaemia, and normal renal, thyroid and adrenal function off diuretics. ### Which malignancy classically causes SIADH? Small-cell carcinoma of the lung through ectopic vasopressin production. ### Which drugs commonly precipitate SIADH? Carbamazepine and oxcarbazepine, SSRIs, cyclophosphamide, vincristine, and NSAIDs. ### What is first-line treatment? Fluid restriction to roughly 800 to 1,000 mL daily; salt tablets with a loop diuretic or tolvaptan if restriction fails. ### How fast should sodium be corrected? No more than 8 to 10 mmol per litre in 24 hours; hypertonic saline is reserved for severe symptoms such as seizures or coma. ### What is cerebral salt wasting? A hypovolaemic hyponatraemia of brain injury with high urine sodium, treated with saline replacement rather than fluid restriction.