Soy is a well-known goitrogen - thyroid inhibitor.
Soybeans were first grown as fodder for hogs. They were cheap, and they fattened them up better than most other things precisely because they shut down the animals' thyroid glands. When consumers started wanting leaner cuts of meat, pork producers could no longer feed soybeans to the hogs, and the bottom fell out of the soybean market.
Voilá! ... to keep bean farmers from going broke, the campaign to sell soybeans as "health food" for people
was born! Now that's some slick marketing...