Transgressor #1 - Soy Products
Despite clear evidence many hypothyroidism consultants continue to endorse diets that are high in soy. DO NOT listen to this uninformed advice and I will tell you why.
Research has concluded that soy is estrogenic in nature. This means that human body reacts to it like its own supply of estrogen. Your body will try to adjust for elevated levels and in the process cause hormonal imbalances. More strictly, in terms of thyroid impact, estrogen and substances that mimic it block the thyroid gland from secreting thyroid hormone. Avoid soy containing products including: soy milk, edamame, tofu, soy burgers, soy cheese and all other soy. Avoid these and you'll avoid the negative thyroid consequences associated with their consumption.
Culprit #2 - Polyunsaturated Fats
Media exposure has been given to the body of research that shows such great benefits polyunsaturated fats can create by incorporating into your diet. This research ignores or is ignorant of just as much evidence demonstrating the catastrophic effects polyunsaturated fats can have on sufferers of hypothyroidism.
Polyunsaturated fats are responsible for blocking the ability of the thyroid gland to secrete thyroid hormone. These fats block proper circulation in order to distribute thyroid hormone throughout the body. In addition they also block the cells ability to utilize the thyroid hormone if and when it is finally received.
Polyunsaturated fats isolated by themselves are liquid at room temperature so besides some sources that contain amounts of them, many of the following AVOID list are oils. Avoid corn oil, safflower oil and soybean oil. Avoid mayonnaise (I'm sorry sandwich fans). Try to not consume walnuts, sunflower seeds and flax seeds (or flax seed oil). My final recommendation is to take a look at any salad dressing you plan on consuming because most of them are very high in polyunsaturated fats... many are comprised of soybean oil.