21st May 2025

Syndrome of Inappropriate Anti-Diuretic Hormone Secretion (SIADH)

SIADH is the most common cause of hyponatraemia in hospitalised patients. It is a condition in which the body is unable to suppress anti-diuretic hormone (ADH) secretion from the posterior pituitary gland causing water retention and eventually hyponatraemia.

Aetiology

The causes of SIADH are various. A useful mnemonic for remembering these causes is, “SIADH” 

Surgery – pituitary/ neurosurgery

Intracranial – head injury, intracranial infections, stroke, pituitary apoplexy, hypopituitarism

Alveolar causes – small cell lung cancer

Drugs – thiazide diuretics, cyclophosphamide, TCAs, carbamazepine, SSRIs, opioids

Hormonal – low cortisol, hypothyroidism, Addison’s disease 

Other causes include HIV infection, Guillain-Barre syndrome, multiple sclerosis, tuberculosis, pneumonia (particularly if caused by legionella or mycoplasma)

Diagnosis

The criteria for diagnosing SIADH include:

• Clinically euvolaemic
• Serum osmolality <275 mOsmol/kg 
• Urine Sodium >15 mmol/L
• Significantly raised urine osmolality >100 mOsmol/kg
• No evidence of other causes including adrenal, thyroid, pituitary or renal disease

Note: There is often (but not always) evidence of low serum uric acid/ urea

It is important to interpret these values with caution if a patient is on diuretic therapy.

The cause of hyponatraemia is often multifactorial and therefore it is important to thoroughly assess a patient and to rule out other potential causes.

Management

If the patient meets the criteria for SIADH, then they should be managed as follows:

1. Fluid restriction
2. Investigate for a cause – if this is not obvious a CT chest, abdomen, pelvis and MRI head should be organised to rule out a possible malignancy
3. Tolvaptan should be considered