Hey. It's funny that you say you were low in iodine but high in mercury and docs wanted to treat the mercury... why - because it's been shown that improving iodine status can actually help detoxify against excess mercury!
So they could have treated the low iodine and hit two birds with one stone.

For more on this, check out the book Iodine: Why You Need It, Why You Can't Live Without by- Dr. Brownstein. As for where you can get it, seaweed is the main source - kelp etc. However, it's hard to get enough through kelp. Generally if you are deficient (and post people are these days due to lower levels of iodine in our food) you have to supplement for a while to restore adequate levels. Even iodised salt is not enough (it seems) to keep iodine levels up high enough. For more, check out that book by Dr Brownstien. Its a good read.