Having too much iodine will make an overactive thyroid worse and will lead the gland to producing too much thyroid hormone. A person with hyperthyroidism should avoid having large amounts of iodine-rich foods: dairy products, egg yolks, blackstrap molasses, baked good with iodate dough conditioners, food products that contain red dye, iodized salt, fish and shellfish, and seaweed or kelp. Caffeine, which includes coffee, black tea, chocolate, soda, and energy drinks, can also worsen symptoms of an overactive thyroid and should be avoided. For those with an underactive thyroid, or hypothyroidism, the suggested foods to avoid include foods with soy including miso, tofu and edamame, cruciferous vegetables like broccoli and cauliflower, gluten in bread, pasta and rice, fatty foods, sugary foods, processed foods, excess beans from beans and vegetables. The foods outlined may interfere with the production of thyroid hormone, and actual medications to treat the disease.