5th April 2024

Investigations

Initial investigations include renal function tests; serum and urine osmolality, urine sodium and thyroid function tests (TFT). The investigation algorithm and different causes of hyponatraemia are shown in the image below.  

Syndrome of inappropriate antidiuretic hormone 

Syndrome of inappropriate antidiuretic hormone (SIADH) secretion causes hyponatraemia, characterised by low plasma osmolality with inappropriate urinary response. Diagnosis requires elevated urine sodium, euvolemic fluid status and for other causes of euvolemic hyponatremia to be excluded. Causes of SIADH can be divided into:

• Neurological: stroke, trauma, epilepsy and haemorrhage
• Respiratory: COPD, sarcoidosis, asthma and acute respiratory failure
• Medications: carbamazepine, phenytoin, risperidone, quetiapine, duloxetine, gabapentin
• Malignancy: lung cancer, genitourinary and gastrointestinal cancers

Figure 1: Algorithm for investigations in hyponatraemia