Intended for healthcare professionals

Practice Rational Testing

Investigating hyponatraemia

BMJ 2011; 342 doi: (Published 07 March 2011) Cite this as: BMJ 2011;342:d1118

This article has a correction. Please see:

  1. Ammar Wakil, locum consultant, diabetes and endocrinology1,
  2. Jen Min Ng, clinical research fellow1,
  3. Stephen L Atkin, head, academic endocrinology, diabetes and metabolism1
  1. 1Hull Royal Infirmary, Hull HU3 2RW
  1. Correspondence to: A Wakil ammar.wakil{at}
  • Accepted 15 November 2010

The commonest causes of hyponatraemia are syndrome of inappropriate antidiuretic hormone (SIADH) and drugs, usually thiazides

Learning points

  • History, especially drug history, and focused examination of heart rate, blood pressure, venous filling, peripheral oedema, and cognitive function should identify possible causes, severity, and the fluid status in patients with hyponatraemia

  • Hyponatraemia usually has multiple causes with SIADH and drugs (especially thiazides) the commonest

  • The criteria to diagnose SIADH are hyponatraemia, urine osmolality >100 mOsm/kg and urinary sodium ≥30 mmol/L in the absence of hypovolaemia and hypervolaemia

The patient

A 70 year old woman with chronic obstructive pulmonary disease who smoked 30 cigarettes daily was admitted to the emergency department after a fall and a fracture of the left neck of femur. Her daughter stated that her mother had lately seemed confused and unsteady, and that she had lost weight. She was using a β agonist inhaler and taking no other drugs. On examination, she looked emaciated, had nicotine stained fingers, and was disorientated in time. Blood pressure was 140/85 mm Hg with no postural drop. Chest examination found scattered wheeze; no cardiac, abdominal, or other neurological abnormalities were found. Results of biochemistry were sodium 122 mmol/L (normal range 136-146 mmol/L), potassium 4.8 mmol/L (3.5-5.3 mmol/L), urea 1.7 mmol/L (2.1-7.6 mmol/L), and creatinine 55 mmol/L (51-107 mmol/L). Liver function tests, plasma glucose concentration, and lipids were normal. Computed tomography of the brain was normal.

What is the next investigation?

Hyponatraemia (a serum sodium concentration <136 mmol/L) is found in up to 42% of inpatients.1 Hyponatraemia may be asymptomatic if it is mild to moderate (>125 mmol/L) and chronic (>48 hours). Symptoms and signs due to brain oedema occur in severe (≤125 mmol/L) or acutely …

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