SIADH vs. psychogenic polydipsia

[dorian24]

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Just did a UWorld problem related to primary polydipsia, which I hadn't learned before. I wanted to make sure I understood the difference between these two, since both present with hyponatremia. Here's what I reasoned...

Water deprivation test increases initially low urine osmolality in primary polydipsia, whereas the urine osmolality is already high in SIADH due to ↑↑ ADH sucking water back in.

Does this make sense?

 

[sahell]

Absolutely right. In primary polydipsia you will be trying to lose that excess water but without the intake it will almost normalise.