Why do you have to put him down for a thyroid problem?

Personally, I do not agree on putting pets to sleep at all unless they are in a life-threatening situation, or suffering too much. From what I now, in humans, thyroid problems only cause your hormones to get kind of messed up and can easily be controlled by medicine. Is it the same in dogs? I hope you won't have to put him down. Poor Chewy. Hope he's alright.