If you wanted to rule out addisons just as a guess for safety. would a serum cortisol, potassium and sodium be ok?

Addisons: Morning serum cortisol should be the first step.

Maybe: If you're sick enough to think you have Addison's, hopefully you're already had your serum potassium assay; it's neither sensitive nor specific. Serum sodium varies a lot and isn't helpful. There's a cutoff value for cortisol that the lab will share that rules out Addison's, but many people fall below and will require further study, such as an ACTH stimulation test.