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Clinical biochemistry: sodium & water disorders

Disorders of sodium and water are common in clinical practice and often difficult to investigate and manage appropriately. These resources will take you through the underlying pathophysiology, clinical features, tests used in investigation, and management of hypo- and hyper-natraemia.


  1. The following YouTube videos outline sodium homeostasis. They are helpful, and the diagrams of the feedback loops are a good feature. However, some of the effects of sodium imbalance mentioned, such as those on the heart, are not particularly important clinically. The web pages detailing these (later) are a better reference for these aspects.
  2. Sodium Homeostasis: Part 1
  3. Sodium Homeostasis: Part 2
  4. Next is a short video on water homeostasis. This completes the overview of sodium and water homeostasis.
  5. We now come on to the tests used in the investigation of sodium and water disorders. Clinically, important features to pay attention to are the volume status of the patient (ie hypo-, eu-, or hypervolaemic), urine output and treatments (eg diuretics, IV fluids). The tests that are ordered are typically serum urea and electrolytes, plasma glucose, serum osmolality, urine sodium and urine osmolality. The blood samples and urine samples should be taken at the same time - this is called "paired" sampling. The links below explain the sodium and osmolality tests.
  6. Hyponatraemia is a common problem, particularly in primary care. The two resources below from the professional pages of and Prodigy are excellent, and give good overviews of the investigation and management of hyponatraemia in a readable format. Prodigy is well referenced and gives plenty of detail. The key to investigating many cases is to remember to gather all the appropriate clinical information before investigating biochemically - the clinical context often suggests a cause and can be very helpful in directing which investigations to request.
  7. Hypernatraemia can be problematic, particularly in hospitalised patients. Diagnosis can be difficult in some cases, and the hypernatraemia page below from outlines how to investigate in clear terms.
  8. And finally, to put this all in context, the case report on water intoxication here provides a good insight into how investigating these disorders occurs in practice. There is also an important lesson - that if a neonate is found to have severe hyponatraemia soon after birth, think about investigating the mother.