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Yeah, that's pretty horrible. I would be crushed, quite honestly, AND I would have done something horrible to her food. And maybe followed her out of the restaurant and kicked her in the shin.
I do agree (generally) that culturally, it's much more acceptable to be, shall we say, frank in European cultures. And I know many countries like to harp on how fat the American populace is, so maybe she... I don't know. I don't know what would possess someone to say that.
I'm sorry you are having such trouble with your thyroid, not that it would ever be okay for people to speak that way to you, no matter what your weight or what the reason for it.
If you ever see her again, pull out the old standby "well, you're ugly and I can lose weight."