I just looked up iodine on WebMD. It says that animal foods, eggs and liver in particular, are the best sources of iodine.
Although I’m not sure how trustworthy the article is, because there is a line in it, stating that because the body cannot manufacture iodine, it is therefore an essential micronutrient. Since the human body is not a nuclear reactor, that is equally true of every other element the body needs.
I wonder, since our body can’t manufacture arsenic, either, does that make arsenic an essential micronutrient, too? Also, so far as I know, the body can’t manufacture sucrose, either, which must be why people seem to feel they need to supplement with such large quantities.