I don't think anyone knows for sure... the effects vary greatly from person to person. When we take most vitamin & mineral supplements, we don't feel any effects from those... so it would make sense that Iodine would be the same.
When I take iodine, sometimes I notice that I can go outside at night in 60 degree weather with shorts and a tshirt and I don't shiver. normally I'm sensitive to cold. My vision and senses increase too. That one is hard to explain.
When I don't feel that way after I take Iodine because I've taken a certain antioxidant that usually causes me not to feel anything I normally feel on iodine, I question whether or not it's working. I guess all that matters is that it's being consumed and going to the organs that need it. The effects are just an added bonus.
I would maybe take not feeling anything as a good sign. It's better than feeling the bad/detox!