Okay, you didn't say how long you've been vegan, so I assumed you'd been doing it awhile.

You really need to get more thorough thyroid testing done. STTM has good advice about that. If your current doctor won't do it you might have to get it down yourself.

There's always the possibility that it isn't thyroid, or only thyroid. You could have other autoimmune issues going on, adrenal fatigue, and so on.