Just do a cursory search on the internet, also at the Weston A Price Foundation website. Same with aspartame. Both seem to be implicated in thyroid issues. Of course, that may or may not be true in my case - after all I ate other things too. the only thing is, before the low carb phase, I had no health problems, after about 2 years of the soy & artificial sweetners, I have developed an auto-immune disorder. So I am going to err on the side of caution. I used to eat a soy protein based bar for breakfast, veggie burger with soy for lunch, tofu & vegetables for dinner. That is whole lot of soy! Also aspartame in my tea, coffee & desserts.
Again, just as not everyone who smokes gets cancer, not everyone eating these things may develop issues. But I'm done with them; I also like to get the word out & atleast make people aware of it.