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SIADH Causes & Management Algorithm

TL;DR

Endo: SIADH: Causes & Management Algorithm is a core MRCP Part 1 topic focused on euvolaemic hyponatraemia with inappropriately concentrated urine. Diagnose using hypotonic hyponatraemia, urine osmolality >100 mOsm/kg, urine sodium >30 mmol/L, and clinical euvolaemia after excluding thyroid and adrenal disease. Management is severity-based: fluid restriction first-line, hypertonic saline for severe symptoms, and treatment of the underlying cause. Avoid rapid correction to prevent osmotic demyelination.


Why this matters

Hyponatraemia is one of the most frequently examined electrolyte disturbances in MRCP Part 1, and SIADH is the classic euvolaemic cause. Questions typically test your ability to interpret labs, exclude endocrine mimics, and manage safely—especially the rate of sodium correction. A structured algorithm improves both accuracy and speed.

Start with the MRCP Part 1 overview, then reinforce with Free MRCP MCQs and timed practice via Start a mock test.


Core sections

1) Definition & Pathophysiology (high-yield)

  • SIADH = inappropriate ADH secretion despite low plasma osmolality

  • Leads to water retention → dilutional hyponatraemia

  • Total body sodium is normal; excess free water is the problem

  • Urine remains inappropriately concentrated

2) Diagnostic Criteria (exam framework)

Use a checklist approach:

  1. Plasma osmolality <275 mOsm/kg (hypotonic)

  2. Urine osmolality >100 mOsm/kg

  3. Urine sodium >30 mmol/L

  4. Clinical euvolaemia

  5. Normal TSH and cortisol

  6. No recent diuretic use

3) Causes of SIADH (must-memorise)

Group them for recall:

  • Malignancy

    • Small cell lung carcinoma (classic)

  • CNS causes

    • Stroke, haemorrhage, infection, tumour

  • Pulmonary disease

    • Pneumonia, tuberculosis

  • Drugs

    • SSRIs

    • Carbamazepine

    • Cyclophosphamide

    • Vincristine

    • MDMA

  • Other

    • Post-operative state

    • Pain, nausea

4) Differentials of Euvolaemic Hyponatraemia

Always exclude:

  • Hypothyroidism

  • Adrenal insufficiency

  • Primary polydipsia (urine osmolality <100)

  • Low solute intake (beer potomania)

5) SIADH Management Algorithm (exam-critical)

Scenario

Clinical Features

Management

Mild (Na 130–135)

Asymptomatic

Fluid restriction (≤1–1.5 L/day)

Moderate (Na 125–129)

Nausea, confusion

Fluid restriction ± salt tablets

Severe (Na <125 or symptoms)

Seizures, coma

3% hypertonic saline

Chronic/refractory

Persistent SIADH

Demeclocycline or vaptans (specialist)

Key rules:

  • Correct sodium ≤8–10 mmol/L in 24 hours

  • Avoid osmotic demyelination syndrome (ODS)

  • Treat the underlying cause

6) Five Most Tested Subtopics

  1. Lab pattern recognition

    • Hypotonic hyponatraemia + high urine osmolality

  2. Urine sodium interpretation

    • 30 mmol/L = SIADH

  3. Exclusion of endocrine causes

    • Always check TSH and cortisol

  4. Hypertonic saline indications

    • Severe symptomatic cases only

  5. Safe correction limits

    • Prevent ODS (high-yield exam trap)


Practical examples / mini-cases

Case: A 70-year-old man with pneumonia presents with confusion .Na 118 mmol/L, plasma osmolality 260 mOsm/kg, urine osmolality 420 mOsm/kg, urine sodium 50 mmol/L. He is clinically euvolaemic.

Question: Best initial management?

Answer: Fluid restriction

Explanation: Classic SIADH (pulmonary trigger + concentrated urine). No seizures or coma → fluid restriction is first-line. Hypertonic saline is reserved for severe neurological symptoms.


Common pitfalls (5 bullets)

  • Failing to exclude adrenal insufficiency

  • Misclassifying volume status

  • Overusing hypertonic saline

  • Ignoring drug-induced SIADH

  • Correcting sodium too rapidly → ODS

Doctor analysing hyponatraemia lab results for SIADH diagnosis

Practical study-tip checklist

  • Memorise the diagnostic triad

  • Always mention TSH and cortisol

  • Learn drug causes (frequent MCQ theme)

  • Practise sodium correction questions using Free MRCP MCQs

  • Simulate exam conditions with Start a mock test

  • Revise alongside a hyponatraemia algorithm (see suggested sibling post)


FAQs

1) What is the key diagnostic feature of SIADH?

Hypotonic hyponatraemia with inappropriately concentrated urine (>100 mOsm/kg) despite low serum osmolality.

2) When should hypertonic saline be used?

Only in severe symptomatic hyponatraemia (e.g., seizures, coma) with careful monitoring.

3) Why must cortisol be checked?

Adrenal insufficiency mimics SIADH by increasing ADH; missing it leads to incorrect treatment.

4) What is the safe correction rate?

No more than 8–10 mmol/L in 24 hours to prevent osmotic demyelination.

5) Which cancer is classically linked to SIADH?

Small cell lung carcinoma—a classic MRCP Part 1 association.


Ready to start?

Strengthen your exam performance with structured revision: start with the MRCP Part 1 overview, consolidate with Free MRCP MCQs, and test yourself under pressure using Start a mock test. Pair this with a dedicated hyponatraemia approach to maximise scoring.


Sources

 
 
 

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