Thompson hadn't eaten anything since Sunday so yesterday morning I brought him into the vet. As heard in this video, he wasn't very happy. His thyroid level is on the high side of normal. His kidneys and liver tested fine. So at this point we still don't know what's going on. The vet explained that tests for cancer aren't always that accurate since to detect cancer you need to get a sample of the tissue. Much of the further testing will be pretty invasive and if the tests are positive it's usually for something that can't be treated effectively.
I'm not sure what we are going to do next. The good news was whenever one of the boyz visits the vet they all get treats when we get home and they have all three learned this is the case. Today when Thompson and I got back home and his carrier was opened, he ran to the kitchen and snarfed down the treats I gave him. He wouldn't however, touch his normal food. The next step will be seeing if there are different foods he might be more willing to eat.