Just like gointrin and thiocyanates, soy flavonoids also reduce the formation of thyroid hormones by inhibiting the binding of iodine and tyrosine. Unfortunately, cooking soy products has no positive benefit.
Soy is often used as a protein alternative in patients who prefer not having meat or infants unable to have dairy-based formula. Infants who are fed soy formula are at a higher risk for hypothyroidism and for the later development of autoimmune disease.
One study examined 60 patients with borderline hypothyroidism. One group was given the 2mg or the equivalent amount of soy isoflavones in a typical diet while the other was given 60mg, which is the dosage found in the typical vegetarian diet. By the end of the study, three times more people in the high dose soy group went from having borderline hypothyroidism into overt clinical hypothyroidism.
Soy is mass-produced in North America, so you can find it throughout our food supply. Most products that contain vegetable oil will contain soy, as a large percentage of vegetable oil is derived from soy. Soy milk, soy chips, soy yogurt, soy protein, tofu, miso and countless other products also contain soy.