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Health condition

SIADH deep dive

SIADH - the syndrome of inappropriate antidiuresis - is the commonest cause of low sodium in hospital and clinic alike: the body retains water inappropriately, diluting sodium, with causes ranging from medications to lung and brain disease.

Why trust this guide

  • 01

    Editorial team

    Written by our editorial team from the UK sources listed on each page.

  • 02

    Sourced from guidance

    Checked against current NHS and UK specialist society guidance at the time of review.

  • 03

    Current for 2026

    Reviewed on 2026-09-27 with the next review scheduled for 2027-09-27.

Key facts

The essentials at a glance.

The symptoms to recognise, when to seek help, which specialist to see, and what treatment usually involves.

  • The pattern: low sodium with concentrated urine (urine osmolality >100), urine sodium typically >30, in someone with normal fluid volume - and normal thyroid and adrenal function

  • Cortisol deficiency and severe hypothyroidism must be excluded before the label is applied - a missed Addison's masquerading as SIADH is a classic error

  • Common culprits: SSRIs and other antidepressants, carbamazepine, PPIs, chest infections, brain disorders and some cancers (notably small-cell lung cancer)

  • First-line treatment is fluid restriction; resistant cases have options including urea and tolvaptan under specialist care

  • Correction speed matters: overly rapid sodium rise can injure the brain (osmotic demyelination), so severe cases belong in hospital

Understanding the condition

What it is and what happens next.

What causes it, how it is diagnosed, and the treatment routes available in the UK.

What it is and how it feels

Antidiuretic hormone tells the kidneys to hold water; in SIADH it keeps signalling when it should stop, so ingested water dilutes the blood. Mild chronic hyponatraemia is far from harmless - it subtly impairs balance and cognition and increases falls and fractures in older people - while rapid or severe falls cause headache, confusion, seizures and coma. The diagnosis is one of exclusion and pattern: paired blood and urine osmolalities and urine sodium, taken before treatment blurs the picture.

Diagnosis and treatment routes

The workup runs: confirm true hypotonic hyponatraemia, assess volume status honestly, check 9am cortisol (or synacthen) and thyroid function, review every medication, and then hunt the cause - chest imaging and further tests guided by the story, since SIADH can be the first sign of underlying disease. Treatment starts with fluid restriction and correcting the cause; specialist options exist for resistance. Persistent unexplained low sodium deserves endocrine review; privately, typically within about two weeks, against NHS waits often of 20-45+ weeks.

Seeing a specialist

Who to see, and how quickly you can be seen.

The specialist for this condition is usually a consultant endocrinologist. Your GP can refer you on the NHS, or you can arrange a private consultation directly.

A quiet reminder

This guide is for information, not medical advice.

Your GP knows your history and can tell you which parts apply to you. If in doubt, see them - and seek urgent care for any of the emergency symptoms described above.

Frequently asked

Common questions about SIADH deep dive.

Quick, plain-English answers to what people ask us most.

  • What is SIADH?

    SIADH - the syndrome of inappropriate antidiuresis - is the commonest cause of low sodium in hospital and clinic alike: the body retains water inappropriately, diluting sodium levels in the blood.

  • What causes SIADH?

    Causes include certain medications (particularly some antidepressants), lung conditions, some cancers, and disorders of the brain or pituitary gland, all of which can trigger inappropriate release of antidiuretic hormone.

  • What are the symptoms of SIADH?

    Symptoms relate to low sodium and can include nausea, headache, confusion, and in severe or rapidly developing cases, seizures, though mild cases may cause few or no symptoms.

  • Is SIADH dangerous?

    Significant or rapidly developing low sodium from SIADH can be dangerous, particularly affecting brain function, so identifying and correcting the cause, along with careful sodium correction, is important.

  • How is SIADH treated?

    Treatment includes fluid restriction, treating the underlying cause where identifiable (such as stopping an offending medication), and in some cases specific medications to help correct sodium levels.

  • How is SIADH diagnosed?

    Diagnosis involves blood and urine tests showing the specific pattern of diluted blood sodium alongside inappropriately concentrated urine, and a search for an underlying trigger.

  • When should I see a specialist about SIADH?

    See a doctor if blood tests show low sodium, so the cause can be investigated. A private consultant endocrinologist can typically be seen within about two weeks for further assessment.