As you are already taking the dog to your vet, that is the first step.
See what they are testing for and why. Do some net research to have a better idea of what could be happening, better understanding of test results, and better underrstanding of what the symptoms are. Allows you to ask better questions. Some of the more sophisticated tests require sending off for results. Often there is long time lag, and some distant labs are better than others.
I have lost 2 rescues in the past 4 years dealing with a small country vet that depends on outside labs for most results. Both dogs ended up, my call, with a emergency vet practice that can do most anything, but you pay for the sophistication! One dog a 7 yo cocker, ended up in surgery to try to find enough liver to keep him functioning, but to no avail.
The other a 5yo shih tzu was diagnosed with Addison's disease, but the test results after a 3 week delay showed Cushing's disease. Same type disease but one low in results the other high! Delays through the small vet aggregated me. He went into a severe decline so back to the emergency vet at night. They tested his salt and potassium levels and they were off the scale, one low and one high, putting his heart at risk!
He passed away that night due to heart failure.
Our present shih tzu we got as a puppy, now 5months old, and we drive to the emergency vet, and he is seen through their regular vet practice for everything! Regular vet practice costs care similar to the country vet. There is always somebody(s) available, with a complete team, immediate tests for almost everything, ICU type monitoring if necessary, very good communication, complete explanations, etc. Several of the vets there are 2-3 years out of vet school, so along with the experienced vets, nice balance of latest and more common treatments and diagnoses are there! You also get a complete summary of potential costs to approve before anything, and afterwards if something changed!
Best of luck with your 4 legged family member !