Does Soy Actually Harm the Thyroid? 4 Key Points you Need to Know
Updated: Jan 30, 2019
When I was first diagnosed with hypothyroidism I loved soy. Not that I ate tofu often, but I definitely loved a nice soy latte or soy milk protein shake. Not to mention the tasty protein bars I would eat in between classes... turns out these were made with soy crisps too.
Once I was told I had hypothyroidism, I made it my mission to do everything in my power to improve my condition, this included researching the best diet to help my symptoms, and ditching any foods that could be harmful to my thyroid.
Throughout this research, soy kept popping up in my searches as a potential harmful agent to the thyroid. This confused me because for the past few years I had heard about all the health benefits of soy beans, and how it was a great source of protein for vegetarians and non-vegetarians alike!
Now I find there is a great deal of “soy confusion” amongst my patients- does it help or harm the health?
These are the 4 key points you need to know about soy when it comes to thyroid health:
#1 Genetic Modification
Many of the world’s soy bean crops have been genetically modified, resulting in the use of increased herbicide or pesticide use on these crops. As many of you may already know- these chemicals can be very toxic to the thyroid and are best avoided.
It is important to note that many packaged foods contain soy products (soy crisps, soy oil, soy protein). If the package does not specifically note GMO free soy then you can assume it has extra chemicals that you will want to avoid.
Goitrogens are substances found in soy (and some other food groups) that may block the synthesis of thyroid hormones and interfere with iodine metabolism. However, studies show conflicting information concerning the impact of soy on the thyroid. Although some molecules in soy do inhibit an enzyme involved in thyroid hormone synthesis, it has not translated into poor thyroid function in otherwise healthy individuals with adequate iodine intake.
#3 Phytic Acid
Another constituent of soy is phytic acid. This can bind to metal ions, inhibiting the absorption of certain minerals such as calcium, magnesium, and zinc. Some of these minerals, especially zinc, is very important for optimal thyroid function. Zinc helps to increase the conversion of the T4 hormone into the more active T3 hormone.
#4 Health Benefits
Although too much soy may not be beneficial for the thyroid, we can’t forget about the fact that it does have some health benefits. Soy has been shown to be beneficial in reducing inflammation and supporting cardiovascular function. Molecules in soy have also been shown to be beneficial in reducing menopausal symptoms including hot flashes. Soy also contains a high level of protein, which is helpful for thyroid function PLUS cooked soy beans (edamame) do contain some iron. The higher protein content with some added iron makes soy a good choice for vegans and vegetarians.
What’s the bottom line?
If you like soy, but you have thyroid disease, reducing your dose may be helpful given the findings above. Remember that studies have shown that the goitrogens found in soy do not impact thyroid function but only as long as your iodine levels are adequate. If you love soy- get your urine iodine levels tested! Additionally, as soy may interfere with the absorption of zinc, ensuring you are eating a diet rich in minerals or supplementing accordingly may be smart. If you are going to eat soy, ALWAYS choose non-GMO soy products. Last but not least, fermented soy products such as miso and tempeh, are thought to be easier for the body to digest and may confer additional health benefits.
If the thought of changing your diet is making your head spin - you should take a look at my Thyroid Truths Grocery List... It's my top non-negotiable food items for truly eating for thyroid health! Download it here :)
What are your thoughts on soy? I would love to hear from you!
Divi RL, Doerge DR. Inhibition of thyroid peroxidase by dietary flavonoids. Chem Res Toxicol. 1996 Jan-Feb;9(1):16-23.
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.
Chang HC, Doerge DR. Dietary genistein inactivates rat thyroid peroxidase in vivo without an apparent hypothyroid effect. Toxicol Appl Pharmacol. 2000 Nov 1;168(3):244-52.
Dillingham BL, McVeigh BL, Lampe JW, Duncan AM. Soy protein isolates of varied isoflavone content do not influence serum thyroid hormones in healthy young men. Thyroid. 2007 Feb;17(2):131-7.
Sirtori CR, Lovati MR. Soy proteins and cardiovascular disease. Curr Atheroscler Rep. 2001 Jan;3(1):47-53.
Tham DM, Gardner CD, Haskell WL. Clinical review 97: Potential health benefits of dietary phytoestrogens: a review of the clinical, epidemiological, and mechanistic evidence. J Clin Endocrinol Metab. 1998 Jul;83(7):2223-35.
Sandström B, Kivistö B, Cederblad A. Absorption of zinc from soy protein meals in humans. J Nutr. 1987 Feb;117(2):321-7.