SIADH and urine specific gravity

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Hi all,

I'm sure this is really obvious but I can't get my head around it and can't find the answer/explanation anywhere I search....

Why/how does the urine specific gravity go up in SIADH?? In my head it should go down because the urine would be more diluted due to less water output. I know this is wrong but can't work out why?!

Thanks,

Jill

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Why/how does the urine specific gravity go up in SIADH?? In my head it should go down because the urine would be more diluted due to less water output.

"less water output" equals more concentrated urine, not more dilute urine. The same amount of solutes is voided in a smaller volume of liquid, thereby increasing the concentration of solutes in the urine (USG).

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Specializes in Emergency, Telemetry, Transplant.

What is specific gravity (SG)? What does it tell you about a solution (such as urine)? If SG is very close to 1 what does this mean about the solute content of the urine? If it is higher (what would a high SG for urine be?), what does it tell you about the solute content of the urine?

So, what happens in the kidney with SIADH? What can you then say about the nature of the urine--specifically solute content of the urine? Given that solute content, what would you expect the SG to be?

What about the opposite of SIADH? What is this condition called? What would SG be here?

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Ha, of course!! I was (stupidly) thinking of the fluid in the body instead of excreted urine. I knew it was something simple. Thank you!!

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Good answer, get me thinking! I have it straight in my head now. I was having a really silly mind block. Thank you :)

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Oh great.

Now I have got a question on Kaplan regarding low sodium (122) and low specific gravity (1.008) and the answer states that it is due to SIADH, which "results in increased water absorption and dilutional hyponatremia". But the specific gravity should be high!?!?! :down:

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The only way I can make sense of a low specific gravity in the presence of SIADH is if they're actually talking about the blood's specific gravity rather than the urine's. However, I did find this old thread: https://allnurses.com/nursing-student-assistance/siadh-low-specific-385300.html that may offer a more reasonable explanation.

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Specializes in Emergency, Telemetry, Transplant.
Oh great.

Now I have got a question on Kaplan regarding low sodium (122) and low specific gravity (1.008) and the answer states that it is due to SIADH, which "results in increased water absorption and dilutional hyponatremia". But the specific gravity should be high!?!?! :down:

SIADH is caused by excessive antidiuretic hormone. So the pt. will over "anti" diurese--i.e., the kidneys will resorb too much water. So, there will be a greater than normal level of water in the blood. This excess water dilutes the blood, so the sodium level will be lower than normal.

At the same time, the water came from the urine, so the urine is more concentrated than normal. More concentrated urine equals higher SG.

What is going to be the first line treatment for SIADH? Don't overcomplicate it!!

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