How much resolution decrease as we raise the ISO ?

Hi,
I curious if someone of you has been calculate this.

As we know, if we keep the noise constant by applying NR, then there would be a decrease in resolution as we raise the ISO.
But by how much from the native resolution ?

For example we had 20 MP camera.
Say we are able to get the full resolution of 20 MP is on ISO 100.

As we raise the ISO, say ISO 1600, then we keep the noise similar to the ISO 100 by applying NR. How much resolution is left ? Surely is not 20 MP anymore.
Would be it 5 MP only ?

I think we could compare it with a 5 MP camera and then shot with ISO 100.
If the picture is identical then the resolution is the same.

And I think the decrease is not linear between ISO 100 and ISO 400.

Opinions are welcome.

Sorry, English is my third language
I hope I state my question clearly.

Thanks for the reply.

-
Brian