It’s actually added to table salt iirc, but I don’t remember the exact function it serves. I did have a boss once tell me she was getting depressed and thought she was dying… Went to her doctor and found out her substituting sea salt (which doesn’t usually have iodine) was causing a lot of issues with her mental health. I can’t recall the physical issues she had developed, if any at that point. I think she’d only made the substitution a few months or so prior to getting it figured out