<<Does anyone know exactly how soy interferes with thyroid function?>> Grain and legume proteins (etc. lectins...) can inflame an autoimmune response because these proteins are similar in structure to some body organs and glands. Your immune system "reads" the soy protein as a foreign invader and develops antibodies to "attack" the invader. Unfortunately, antibodies can confuse the thyroid for a soy protein and attack it too. Anybody with an autoimmune condition should be suspicious of leaky gut and avoid the worst grains and legumes. Rob