If you have any sort of thyroid disorder, you should pay attention to how much soy you are getting in your diet. Research shows that people are consuming more soy than ever in the U.S. It’s cheap and is often considered healthier than meat, although that is not the case for people with thyroid problems!
Most people are aware of the obvious sources of soy, such as tofu, edamame, soy milk and veggie burgers. Less apparent are the protein bars, high-protein cereals, protein powders and meat “extenders” or “analogs” used to make fake everything from chicken nuggets to cheese and bacon bits. Vegans, in particular, may be getting more soy than they suspect. Soy can be labeled in processed foods as textured vegetable protein (TVP), hydrolyzed plant protein (HPP) or soy protein isolate.
Although soy does have some health benefits, it has a big negative when it comes to your thyroid. Two components of soy, both isoflavones, genistein and equol, a metabolite of daidzen, inhibit an enzyme, thyroid peroxidase, used in the thyroid gland to make both T4 and T3. Diets high in soy can produce goiter (thyroid enlargement) and autoimmune thyroiditis such as Hashimoto’s. Being low in iodine, a nutrient essential for proper thyroid function, multiplies soy’s negative effects on the thyroid. Soy can also interfere with you body’s ability to use thyroid replacement hormones.
Research suggests that the thyroid-toxic effects of soy are most often seen at levels above 30 mg of soy isoflavones per day. There are estimates suggesting that Asians consume some 10 to 30 milligrams of isoflavones from soy a day at most — and it’s soy in traditional food form that is not processed or genetically modified. In the U.S., however, some people are getting as much as 80 to 100 milligrams of soy isoflavones a day, by consuming soy milk, soy nuts, soy protein shakes, soy candy bars, soy cereal, and foods enriched with soy, as well as soy supplements.
I’d suggest you avoid soy if you have had problems with your thyroid. If you must have some, limit to two to three servings a week. Do not eat soy foods within three to four hours of taking your thyroid hormone replacement medication, to avoid any interference with your thyroid medication. Do not use soy or isoflavone supplements, which are marketed over-the-counter for hot flashes and other symptoms of menopause.
Divi RL, Chang HC, Doerge DR. “Anti-thyroid isoflavones from soybean: isolation, characterization, and mechanisms of action.” Biochem Pharmacol. 1997 Nov 15;54(10):1087-96.
Doerge DR, Sheehan DM. “Goitrogenic and estrogenic activity of soy isoflavones.” Environ Health Perspect. 2002 Jun;110 Suppl 3:349-53.