A 55 yo male with a history of significant alcohol intake presents with a 2-week history of lethargy. He takes no regular medications and has no other medical disorders. Clinically, he appears malnourished and euvolaemic. Investigations reveal:
a) What is the most likely cause of the hyponatraemia?
This is a hypoosmolar hyponatremia with low urine osmolality and low urine sodium.
I.e. the kidneys are making an excellent effort to excrete copious amounts of water while preserving sodium, which means that not only is the renal response appropriate, but that the whole body systems are actively trying to defend tonicity. This is the sort of response one would expect from a sudden massive excess of water.
So. how did this happen?
Well: the differentials in this scenario would include
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