The thing with most goitrogenic foods is that cooking them tends to neutralize the affect on the thyroid. I do eat broccoli and brussel sprouts, but cooked for sure, and they're not my MAIN veggies.
I eat a lot of sweet potatoes as well since Im' trying to keep my carbs between 50-80g and without sweet potatoes and/or a banana it's pretty impossible with my veggies/meats. I tend to only need 1/2 of a sweet potato though to get me where I want to be, so take that into consideration. I took a look and it seems like most places do say that they have a mild goitrogenic effect, but in the raw state. They should be fine cooked.
Some lists don't even include sweet potatoes at all:
So my thought is not to worry about it too much, cook them well and load them up with buttah!
But if anybody has more info that'd rock, too!