A bunch of stuff could be effecting your thyroid, mercury amalgams, fluoride in the water .. etc. You can't say for sure tooth X is causing the problem with your thyroid. That said, all root canals are toxic, if you care about your health it makes sense to remove them.
You just need to find a holistic dentist that does a biological extraction, ie one where they clean the bone surrounding the tooth socket.
Some root canals actually have mercury at the end of them, which is fairly insane.