So it's possible that the bulk of Iodine reacted with fatty acid layer in the skin - is this Iodine bioavailable and slowly picked on, or is it shed off and not used. It's possible that forcing (reacting) Iodine with the outlayer of the skin may not allow for proper usage.
We don't know the amount of iodine that may have actually passed through transdermally and allowed to be picked on by vascular system.
It may come down to rate of asborption transdermally versus affinity between iodine and fatty acids on the skin.
It may be also hard to quantify iodine absorption via testing color change as everytime color is tested we are diluting, and the skin may be shedding or reacting when cleaning the skin.
Checking for symptomatic changes is the best bet probably
Mike