I do know that it is hereditary. My yellow lab had it and he wouldn't pull on his leash, he was well trained. I have never used harsh correction on any of my dogs with the choke collar, he would just get a quick correction and release of the choke chain...he never needed a good, strong correction. He was a rescue, and although he looked like a pure bred lab, we were told that he was a lab/golden mix. Poor breeding stock passing on the genes is the cause as far as I know. Being that he was a "mixed" breed, and he and his siblings were in the shelter, led me to believe that he was the product of irresponsible breeding.

That is an interesting though, about getting hypo, hyper thyroidism from yanking too hard on the choke collar. If my yellow labbie were not as well trained as he was and he pulled on his leash, or he needed harsh corrections with the choke collar, I would think that might apply to him, but I am afraid it didn't.