When people turn away from meat and cow’s milk, often they replace these foods with soy products. Indeed, soy is central to the diets of many North American vegetarians. But do soy foods carry unanticipated side effects?
What Is the Impact of Soy Products on Thyroid?
Q. I started eating lots of soy products when I was a young woman. I developed low-level hypothyroidism around age 40.
My nurse practitioner put me on levothyroxine and I have been taking that. I still consume soy products just about every day even though I’ve read that it is bad for my thyroid.
Dairy products give me sinus problems, so I only use butter and a small amount of cheese. I drink soy milk every day. I’ve tried to cut back on soy but I have eaten it for so long that it is hard to change!
My TSH levels are tested every year and are normal. Do I really need to worry about soy and thyroid?
How Do Soy Foods Affect Thyroid?
A. You might need to find some ways to consume fewer soy products, but since your TSH is normal and you feel good, it is not an immediate problem. Soy foods might reduce the absorption of your thyroid pill, but presumably the dose can be adjusted (Messina & Redmond, Thyroid, March 2006).
You will want to make sure you are getting adequate iodine; you can do this simply by using iodized salt in your cooking and at the table.
The idea that soy products might interfere with levothyroxine absorption is not the only concern. Some research suggests that eating a diet containing nitrate, thiocyanate or soy isoflavones might trigger an autoimmune reaction to the thyroid hormone T3 (Colucci et al, Archives of Environmental Contamination and Toxicology, Aug. 2015).
Cruciferous vegetables such as broccoli, Brussels sprouts, cauliflower, cabbage and turnips are sources of thiocyanate. Processed meats such as bacon or salami are rich in nitrate. Vegetables such as beets, spinach and collard greens also contain nitrates. Obviously, all these vegetables belong in a healthful diet, just as soy products do. The key is to consume traditionally prepared soy foods in moderation (Zaheer & Humayoun Akhtar, Critical Reviews in Food Science and Nutrition, Apr. 13, 2017).
You may want to listen to our interview with thyroid patient advocate par excellence Mary Shomon and thyroid expert Antonio Bianco, MD. It was Show 1015: Thyroid Mysteries, Controversies and the Latest Research. There is also more information in our Guide to Thyroid Hormones.