Can soy cause hypothyroidism?
Isoflavones inhibits enzymes (specifically thyroid peroxidase and 5′-deiodinase) involved in thyroid hormone biosynthesis. This results in decreased amounts of thyroid hormones, which then triggers increased secretion of thyroid stimulating hormone, resulting in enlargement of the thyroid gland (goiter). However, and this should be emphasized, this only occurs if the diet is very high in soy isoflavones and/or when the diet is low in iodine. When iodine is added to the diet, or the soy isoflavone consumption is stopped, the thyroid gland should return to normal size. In elderly women who are often at risk for hypothyroidism, screening for thyroid dysfunction is recommended by the American Thyroid Association, particularly when the women are consuming soy for post-menopausal symptoms. People who are taking medication for hypothyroidism should be checked regularly by their physicians who may recommend avoiding soy foods that contain isoflavones. There is a substantial research literature
