Sea or otherwise?
Most iodized salt has some sort of anti-caking agent, often a corn derivative. Check out your salt- I bet you it has more than just "salt" in the ingredients! ;)
I have sea salt, not sure how it compares to celtic but I will get celtic. Looks like if you eat 1/4 tsp of Celtic Brand grey sea salt (I eat at least that much) per day you get the rda for iodine (and I realized the us rda is probably low).