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I think that you should have the vet check his heart. Passing out with excitement is usually a sign of a heart problem. The dog passes out because the heart is not pumping effectively.
One of the vets on site might want to comment on the thyroid condition but I really think you should look more into any heart issues that might be occurring.
Let me know if you have more questions.
Disc disease usually won't cause the fainting that you are seeing.
I think that you should really see that specialist that you have been referred to about his heart.
Sorry that I can't be more help.