FWIW, I started a similar thread
on another forum and that thread had many replies. Many vegans with compromised thyroids, on thyroid medication, still regularly eat soy and their doctors don't have a problem with it at all. The only concern of their doctors is that they don't eat anything
until an hour has passed after taking their medication.
Maybe this whole thing thyroid disease necessitating abstention from soy consumption is Internet folklore.