Our SOIL for growing food has been iodine-deficient since at least the 1920's! Unless it's put back, (it's not), the food we grow cannot absorb Iodine from the soil.
I know, too, that sunshine plays a part in all this...at least to the point where we must NOT deny ourselves sunshine. That's the extent of my knowledge on that subject, however it might be enough for you to "carry that ball" further.