
Definitely go with Dr Dodds - she has more precise ranges than our vets, and where behavioural symptoms are concerned sometimes treatment is warranted if the results are still within range, but in the bottom 1/4. So well worth having that expert interpretation of the levels.
Anxiety is one thing that can increase with low thyroid and indeed I know someone whose dog's separation anxiety was not getting better - she'd make progress then lose it again, and it was only when the dog (who was in that bottom 1/4 of the range) was put on soloxine that she finally improved long term. He does sound very anxious generally, if he's not sleeping in the day - if everything checks out medically, definitely get someone in (making sure they don't use punishment or aversives, not necessary with any dog but definitely the last thing an anxious dog needs). It may be that there are other things stressing him out that you've not picked up on, which are contributing to the separation issue/his anxiety in general.
What sort of other issues have you had? If you can give us an idea it might point to something else, often things can be related or one symptom can be exacerbated by several different causes.