Chemicals called glucosinolates found in cruciferous vegetables can reduce the uptake of iodine by thyroid cells, which may lead to decreased production of thyroid hormones.

Even though Brussels sprouts have chemicals that could theoretically mess with thyroid function, eating a lot of cooked Brussels sprouts for a month didn’t change thyroid hormone levels in people. So, the idea that they suppress thyroid function doesn’t seem to hold up in real life.