Found this interesting article on a thyroid site
"If you have a diagnosed or undiagnosed thyroid problem, or a history of autoimmune disease, overconsumption of soy isoflavones can potentially trigger a thyroid condition. Soy foods can worsen an existing diagnosed thyroid problem in many people. In both cases the symptoms such as fatigue, weight gain, and depression or moodiness are often overlooked and hard to diagnose."
http://thyroid.about.com/cs/soyinfo/a/soy.htm