I have used niacin and Iodine together, with no adverse effects, as a matter of fact, try googling those two words together and see what you come up with. If I were you, I would start with one Iodoral, and not slam the system...
Even with juice fast, I strongly recommend that you take plenty of vitamin C daily, along with 200 mcg selenium, and Sea Salt and some magnesium, too - all to keep the toxins moving out of the system - this is very important and makes the difference between feeling good and feeling like poo-poo.