Soy binds with thyroid hormones, making them unusable to your cells. If you have thyroid problems, its a good idea to avoid soy. Fermented soy products, like tempeh, soy sauce, and miso are ok, because the fermentation process destroys the enzymes that interfer with the thyroid. Cooking does NOT destroy it, by the way. There are some good articles on soy and thyroid on about.com's thyroid pages.