This is part 2 in a series about soy.  There is a lot of misinformation and “soy bashing”  out there so I though it right to set the record straight with information from the literature to address these concerns so that people can eat soy without worrying about these consequences. Last time we dealt with soy and female hormone balance including the effect on hot flushes, bone health, and blood lipid levels.

This time we will deal with the issue of soy purportedly being a thyroid hormone blocker.  Here is the contention:  

Does soy cause thyroid suppression?    A 1991 study found that eating only 2 TBL/day of roasted and pickled soybeans for 3 months to healthy adults who were receiving adequate iodine in their diet caused thyroid suppression with symptoms of malaise, constipation, sleepiness, and goiters (Nippon Naibunpi Gakkai Zasshi 1991, 767: 622-629)!  

What does “adequate iodine” really mean?

Does it mean the person who eats iodized salt or the person who takes iodine supplements. This is excessively vague and meaningless.  Almost everyone whose iodine level I have tested is iodine deficient although they believed they were taking in “adequate iodine”.

Soy has been known to be a “thyroid blocker” in that it suppresses iodine uptake by the thyroid gland but so do cruciferous vegetables (broccoli, cauliflower, kale, and Brussel’s sprouts) and nobody is recommending that you stop eating these foods. This problem is easily overcome by eating more iodine rich foods or iodine supplements.

The RDA for Iodine at 150 mcg/day is much too low to get the thyroid and breast health effects of iodine.  I give people a 12 mg tablet of iodine/iodide onceweekly. That is important for prevention of breast cancer and thyroid cancer.  Iodine cures fibrocystic breast disease, which is a precursor of breast cancer, by normalizing the proliferation of ductal tissue.  [1]  I would say that if you want to eat soy and cruciferous vegetables, you should definitely consider taking an iodine supplement or eating iodine rich foods.    In Asia, where soy is eaten liberally, they have a high iodine diet of sea vegetables and sea food. 

[1] Dr. David M. Derry, MD, PhD  Breast Cancer and Iodine”  P. 83

Share this article