Good question, and I don't know the answer. Trapper or Ginagirl can answer that better than I can.
Trapper always says that more Iodine is better. It was his advice that got me started on large doses of SSKI (800mg) that surprisingly cured my asthma, and as a result of that, I learned about Pima being used in large doses by doctors.
The Potassium Iodide in 50mg of iodoral was just not enough for my lungs, but did wonders with my prostate.
I do know the body takes whatever it wants before the rest goes out in urine. I suspect there is a lot about Iodine that we don't know.