They recommend that a person's urine Iodine excretion is measured, then the person takes a prescribed amount of Iodine every day for a while, then they re-run the urine test, and the person takes more iodine, etc. until the person reaches a point where the person is excreting something like 90% of what is administered, then they state or imply that at that point that the person is iodine-sufficient.

I disagree. I'd call that point "near saturation", not sufficiency.

One could do the same thing with many substances, but the body's ability to retain something, like Iodine for instance, is not necessarily a direct measure of sufficiency.

Thus, it is insufficient that the iodine loading test measures iodine-sufficiency. It only measures the saturation point.

"Sufficiency", as a word by itself, is meaningless. There must be a direct object, like "sufficient to ......" They never state what they mean by sufficiency.

My guess is that the body has sufficient iodine to do all of what it wants and needs at iodine content levels which are vastly lower than the serum levels present when a person is "iodine sufficient" according to that loading test.

One can always conjecture that soils had more iodine in ancient times, "before the fall", prior to the "great flood" or during the miocene era, but it will always be conjecture. Where did all that iodine go , if it were present ? Perhaps it escaped into outer space and we're still losing it. Nobody knows.

Could run a chloride sufficiency test by giving people more chloride, testing their urine, etc. and then proudly proclaim at some point that they are "chloride sufficient". Sufficient to do what exactly ? Certainly, most americans, by this definition, are chloride deficient since I can give them more chloride, check their urine and determine they haven't reached 90% saturation yet. Could do the same with calcium, or potassium. But in the case of potassium a point might be reached where the person has achieved amount "sufficient to cause cardiac arrest"

I think a lot of people are iodine deficient, and have excess bromide in them, and a lot of other issues. But I don't think that "iodine sufficiency", as understood by some MD's, is an accurate portrayal of what the body needs. Most likely, iodine sufficiency at those levels represents a significant excess. It may not be harmful, or it may be, I don't know and I don't intend to make myself a guinea pig. Iodine supplementation does have its place, but as with most other things health-related, moderation is in order. I take very little iodine now and have taken very little for quite some time. I think of all the really old people I know, who never even heard the word "Lugol" in their life and they seem to be doing just fine. In my case though, I do believe it helped me to piss out some funky stuff, and I don't discount the many testimonials of women relative to their FBD going away, as well as the double-blind studies which also proved efficacy in this regard.

Iodine sufficiency remains a nebulous concept.