My old (now passed away) dog developed hypothyroid some years ago and the advice and information I received on the forum, especially from Nikita, was very helpful. The first thing to make sue is that the vet carries out a **complete** panel on the sample, as they tend to look for only two or three factors, out of, I think, six. (I’m very sorry, I can’t remember which is which as my old boy’s note are in storage, and I can’t find my posts on here - they were in about 2015/6 - as the search function is limited to 365 days, and google won’t find it either.) I had to insist firmly that my vet test for everything thyroid and it showed that some of the elements (the ones that weren’t going to include!) were low. He was on thyroid medicine for the rest of his life, and lived a normal lifespan (14 yrs 11 months at death) very happily.
Nikita does still post on here, although infrequently now, and has a lot of experience of thyroid problems. If you can find her previous posts on the topic, or get in touch with her, you will find her information very helpful.
I’m sorry I can’t give you the full information, but hope you will be able to build on this.