How important is iodine in your diet?

Essential. It is essential, but luckily only a small amount is required. It is in certain vitamins and also present in iodized salt and bread made by batch processing in factories. It's also present in leafy green vegetables.
Very. Iodine is an essential nutrient. You need it primarily for proper thyroid health which is involved in the regulation of your metabolism, among other things. Most salt in the U.S. Is iodized - that is, it contains iodine. However, unless you have a thyroid disorder, you only need iodine a few times in your life - your body stores it and re-uses it.