A couple of years ago I started applying tincture of iodine onto a 2 or 3 inch square patch of skin somewhere on my body twice a day (usually on alternating thighs). When I started, the discoloration disappeared within an hour or two, but, it started taking longer to disappear and now the discoloration lasts for over 12 hours.
I heard over an alternate radio "health" show about this, and, although it seemed hoaky to me, I decided to give it a try since it is easy to do. The radio guest said that the time it takes for the discoloration to disappear is a measure of iodine deficiency or not. It if takes more than 12 hours to disappear, it means you are not deficient and the iodine on your skin does not get absorbed. If you are deficient, then the iodine on your skin will be quickly absorbed.
I was surprised when I saw how it worked. I can't say I noticed any difference in my health, except maybe some swelling issues I was having abated somewhat.
Tincture of iodine is sold in Walgreen's drugstores, but, take care not to use the "de-colorized" kind, as you need to be able to see when the brownish color goes away from wherever you apply it.