What the extremes are, are undefined to me at this point.
It wasn't that long ago that the recommended RDA of Iodine (set in 1980 by a vetrinarian) was stated as 150 mcg daily.
Yet, PIMA syrup has been given over half a century as an expectorant, with a typical daily dose of potassium Iodide on the order of 900 - 1200 milligrams.
It could be that the RDA is extremely low and that Iodine doses of 500 mg daily are middle of the road. The few studies that have been done, all seem to indicate that levels of Iodine that are 60-6000 times the RDA are well-tolerated, as many MD's have pointed out.