I think a lot of vets underestimate the importance of thyroid levels. When I had a dog with a borderline initial test, I had my vet follow up with a full panel through Hemopet (Dr. Dodds' lab) -- they offer a very comprehensive state-of-the-art "Thyroid Panel 5" panel, which your vet can have done by overnighting the sample to them. Dr. Dodds personally reviews the results and is available to consult with the vet if needed (from the website: "Each sample includes a personal interpretation from Dr. Dodds plus consultation and follow up questions if desired.") Dr. Dodds is THE authority on thyroid research in dogs -- if you need an expert, this is the easiest way to get access to a really good one. I think it was under $200 when I had it done, including the overnighting costs--I paid my vet clinic, and they paid Hemopet.
Hemopet Hemolife Thyroid Testing
You might also want to work with a veterinary endocrinologist at your state vet school, if it's within driving distance. Sometimes consulting with a local expert is enormously helpful.