I had a dog with hypothyroidism (another autoimmune), diabetes, and cushings. I actually had to convince the vet to run the tests for cushings. The diabetes diagnosis had been made, but the blood Sugar levels weren't coming down. I was doing research online about dogs with diabets, and I came across info on cushings.
It took me a couple weeks to convince the vet to finally run the tests. I was young at the time and actually thought that vets knew "everything". Now I know better and would force the test immediately.
I have bloodwork done on my current dog twice a year because of all of previous dog's issues. By doing this, I found that her BUN levels were high and was able to get it back into the normal range with a supplement, rather than waiting until it got so bad that she displayed symptoms of kidney failure, when it is almost too late.
I advise everyone to trust their instincts when it comes to their pets' health!!!