Hey Lulu, I’ve read a couple of books on this subject (both written by medical doctors) and this seems to be one mineral that we simply cannot get enough of in our diet, alone. They recommend around 13 mg for most people, which is about the same amount that Japanese people would get in their daily diet (from seaweed, etc) and they have low rates of breast, ovarian, and uterine cancers compared to us.
According to the books, our thyroid alone requires about 3 mg a day for maximum function, which is MUCH more than the RDA, and there’s almost no way to even get that amount (let alone 13 mg) without supplementation. They do talk about some of autoimmune issues and supplementation in the books, but I didn’t read that part since it doesn’t apply to me.
Anyway, I hope that helps.