Lab Case 12 – Interpretation

48 year old with severe hyponatraemia, at risk of seizures

FBC normal

Severe hyponatraemia (112), requires investigation

severe hypochloraemia – ?salt losing (no history of vomiting), dilutional

Normal renal function and potassium

Osmolality 227.3 (glucose not given) – low osmolality

Urine osmolality high

therefore, severe hyponatraemia with low plasma osmolality and high urine osmolality – SIADH

Elevated GGT, ALP and ALT with normal bilirubin – obstructive picture (gallstones, tumours, no history of alcohol use given)

Interpretation – 48 year old man presents with severe hypoNa (low plasma osmolality, high urine osmolality) likely to be due to SIADH. He requires fluid restriction (daily weight), observation for seizures, consider ivi Na to increase Na to 120 (beware CPM). Further investigation for cause of SIADH (tumours -lung, liver, pancreas, drugs- SSRI, NSAIDs, TCA, infection – pneumonia, TB, meningitis/encephalitis).

Other causes of high urine osmolality – dehydration (may be contributing factor), adrenal insufficiency, glycosuria, hypernatraemia, high protein diet.

His spot urine Na concentration was 9mmol/l which was not helpful in terms of SIADH.

CXR showed wedge shaped opacity directed inferolaterally from hilum with blunting of rt costophrenic angle. This was interpreted as likely to be infective rather than neoplastic.

The patient respiratory status deteriorated in the ward, he required ICU admission and intubation. He subsequently had a bronchoscopy and biopsy of a mass in the right lower lobe which was small cell carcinoma on histology.



Diagnostic criteria:

1. Hyponatraemia

2. clinical euvolaemia

3. low plasma osmolality

4. high urine osmolality

5. urine Na concentration >20mmol/l

6. Normal TFT, renal and adrenal function


Tumours – small cell Ca, bronchogenic ca, pancreatic ca, Hodgkins Lymphoma

Pulmonary – pneumonia, lung abscess, TB

Medication – SSRI, Carbamazepine, TCA, NSAID

CNS – tumour, encephlitis, SAH, trauma




1. water restriction

2. salt restriction

3. daily weight

4. loop diuretic – not carbonic anhydrase inhibitors

5. Urea (rarely used)

6. Treat the cause