I have and though the jury is still out, I'm convinced iodine makes a difference for me at least. Be sure you don't have an iodine allergy if you're considering trying this.
Also, though iodine can be deficient and that can cause a number of problems, iodine alone is unlikely to be a total solution to multiple issues. I take ammonium iodide daily because it converts to free iodine immediately without which removes a step in the iodine conversion process, but even iodine supplementation alone were sufficient for everyone else, I can say with certainty I would be the exception where it is not. I wish!
That said, 1) after eliminating the possibility of an iodine allergy, 2) because skin only absorbs what is needed and 3) only a small amount is needed to begin with, it's safe for most people to paint it across the base of the throat where the thyroid is and see if it's absorbed quickly. Some say the alcohol content in OTC 2% iodine tincture causes it to disappear quickly, however I get the same results using non-alcohol potassium iodide or ammonium iodide. I'm sure there's a factor in how absorbable a person's skin is as well, e.g., dry and dehydrated as mine wants to be, versus hydrated skin and consideration should also be given to whether or not an oil or lotion barrier is on skin that could prevent absorption. I've noticed that iodine absorbs equally well in the same places on me if it's applied before or after progesterone in a Vitamin E carrier. So some things might be a carrier while others are barriers.
I've also experimented with painting the entire bottoms of my feet with 2% iodine or 10% povidone and watched it absorb almost immediately in various places. Obviously that's a given where skin is thinner or thicker in spots, but I'm specifically referring to thin skinned areas under the big toe areas above, below and where they bend. The skin is quite soft there and iodine is almost completely absorbed within minutes. Though it may only be conincidence, interestingly that is exactly the points where the thyroid gland and other glands are represented on reflexology charts
http://www.ofesite.com/health/reflex/chart
Until now I hadn't thought to try it on my hands and compare that to a hand reflexology chart, but I will tomorrow.
Here is some interesting reading about iodine immediately available online that may spur other helpful investigation:
http://albumen.stanford.edu/library/monographs/sunbeam/chap10.html
http://www.tpcsdirect.com/iodinestory.html
http://www.townsendletter.com/April2006/iodine0406.htm
http://www.arxc.com/townsend/sids.htm
http://www.thehair.com/vitamins/iodine.htm
"Cautions: Potassium iodide supplements should be used with caution in cases of dehydration, acne, heat cramps, adrenal insufficiency, and cardiac disease. Prolonged use during pregnancy is not advisable."