I have a feeling there's something more to it than her diet. As much as I know, the impact of soy on hormonal balance is still being studied and you would have to indulge in huge amounts of it, for it to have a potential impact on the thyroid gland.
I know a few ladies which have thyroid gland issues, I myself had some shorter hormonal balance and ai remember how awful I looked and felt. Skin goes crazy, your hair is thinning, you're bloated. Fleur doesn't look to me as someone who has a serious hormonal issue, so she is either probably handling it with medication successfully, or there is a deeper issue to the story.