Been there done that. I have a quart of Lugol's sitting in my kitchen cabinet. Used it for many months. While I can't say that Iodine was completely useless, it did not produce any benefit for my fungus overgrowth that I could tell.
Nystatin has been 100x more effective in less than one week than Iodine was in a year.
As for my immune system, I know the cause. For some reason caffeine in general, and coffee in particular, had a devastating effect on my immune system. If I drink a large cup of coffee every morning for a few months I will have a full blown candida overgrowth guaranteed. I know this for sure now because I've experienced it twice (including once after I basically cured candida the first time).