Most of the evidence is from studies on the effects of Iodine on cancerous tissues.
From The Extrathyronine Actions of Iodine as Antioxidant, Apoptotic, and Differentiation Factor in Various Tissues https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3752513/
From the section on Apoptosis, a section about two pages long.
"Various groups have described effects of Iodine in several cancer cell lines and have proposed different mechanisms and pathways that mediate the antiproliferative effect of I2, including a direct action, where the oxidized iodine dissipates the mitochondrial membrane potential, thereby triggering mitochondrion-mediated apoptosis, or an indirect effect through iodolipid formation and the activation of peroxisome proliferator–activated receptors type gamma (PPARγ), which, in turn, trigger the BAX-caspase apoptotic pathway (3,59,60)."
In relation to apoptosis, iodine is important in dell differentiation, meaning new cell growth and DNA error-checking:
" In relation to differentiation induction, cancer studies have shown that sufficient iodine prevents the transformation from differentiated to anaplastic thyroid cancer, the most aggressive type of thyroid cancer with a median survival of 4–12 months from the time of diagnosis"
Also available is
Molecular iodine induces caspase-independent apoptosis in human breast carcinoma cells involving the mitochondria-mediated pathway.
Which discusses the same effect in a briefer summary https://www.ncbi.nlm.nih.gov/pubmed/16679319
More readable for the layman is an excerpt from David Derry's book,
"Breast Cancer and Iodine"
The "test" of putting iodine on the skin to watch how fast it disappears is not an indicator of anything. The iodine disappearance rate is unrelated to thyroid disease or even iodine content of the body.(1-2) Meticulous research by Nyiri and Jannitti in 1932 showed clearly when iodine is applied to the skin in almost any form, 50% evaporates into the air within 2 hours and between 75 and 80 percent evaporates into the air within 24 hours. (1) A total of 88 percent evaporates within 3 days and it is at this point that the evaporation stops. The remaining 12 percent that is absorbed into the skin has several fates. Only 1-4% of the total iodine applied to the skin is absorbed into the blood stream within the first few hours. The rest of the iodine within the skin (8-11%) is slowly released from the skin into the blood stream.
However Nyiri and Jannitti's findings that "The percentage of iodine penetration through the skin is the same, irrespective of whether the cells have a high or low vitality, or are dead, and irrespective of the direction of penetration" have important implications for iodine functions in the body, skin, pregnancy and fetus and also for the therapeutic use of topical and oral iodine. Iodine would be the only compound of significance during early pregnancy which can pass rapidly through all tissues of the fetus without the aid of any blood vessel or lymphatic transport. It could be that not only does iodine control natural cell death (apoptosis) (5) in the fetus but may also influences stem cell development.
Nyiri and Jannetti also mentioned iodine applied on the skin can be used to shrink swelling of the thyroid gland (goiters). So even small amounts of iodine absorbed through the skin can have good effects on body organs. From my own clinical experience, repeated application of iodine (Lugol's) to the skin appears to cause regeneration of the skin from the bottom up (Quiescent, stable or stem cell) (3) eventually sloughing the old version of the skin off like a snake molting. If there was a pre-cancerous lesion on the old skin, it is replaced with new skin minus the lesion. There does not seem to be any skin lesions which are not helped or cured by this procedure. In some cases I found clinically obvious low thyroid conditions also needed to be treated to be more effective. Minor lacerations and healing of surgical wounds respond well. If skin regeneration is from the bottom, then there is little or no scar formation.
My parents' generation tended to put tincture of iodine onto a fresh wound to prevent infection. This turns out to be helpful but not the best way to use it. Besides it stung badly when applied. It is much more effective (and doesn't hurt) to apply iodine repeatedly after a scab has been formed. The iodine put onto the scab helps to organize total repair of the tissue. It is implied a similar approach could be taken to burns of all depths but at the same time the physiology of burns suggest there is an acute lack of thyroid hormone.
All pre-malignant lesions and many other oddities of the skin appear to respond to this regeneration process triggered by topical iodine. I have mentioned previously a patient with a biopsy-proven Breast Cancer lesion (she refused surgery because of previous cancer treatment) that was strongly fixed to the skin responding well to topical iodine and ended up being a dimple on the breast three year later.
It is my belief a water solution of iodine (like Lugol's) is an important therapeutic agent for skin. Because of its effectiveness and the results, perhaps many skin diseases are related to local tissue areas of relative iodine deficiency. Perhaps the most graphic lesions are the "keloid" (worm) incision scars formed after surgical procedures. If the iodine intake and tissue levels are adequate, such as in Japan, keloid formation doesn't happen (7). In addition, iodine's ability to trigger natural cell death (apoptosis) (5) makes it effective against all pre-cancerous skin lesions and likely many cancerous lesions. The local site is replaced with normal skin. However, even lesser doses of topical iodine seem to reverse the ominous appearance of skin lesions. Because my older brother died of metastatic melanoma, my chances of getting a malignant melanoma are increased by 400 times. (8-9) Having grown up in Venezuela near the equator my sun exposure at a young age was far above normal. So all suspicious lesions I notice are returned to normal with topical Lugol's.
One of the most famous of documentations of iodine skin therapy to a famous person occurred in the American Civil War.
"On September 29, 1862, Colonel John B. Gordon held the center of General Lee's army at the battle of Antietam, or Sharpsburg. The first volley from the northern lines sent a ball through the calf of Gordon's right leg; soon after, another went through the muscles of his thigh; a third pierced his left arm, tearing asunder the tendons and mangling the flesh; a forth ripped through his shoulder leaving a wad of clothing embedded in its track. Still, no bones were broken; but, while Gordon lingered in the firing line, "with", as he says himself, "but little of my usual strength", a fifth ball struck him squarely in the face.
Dr. Weatherly of the 6th Alabama Regiment, in charge of medical arrangements, had the Colonel removed to a base hospital, and prescribed tincture of iodine to be painted on the wounds three or four times a day. The case was unpromising. Gordon's eyelids were greatly swollen; one eye was completely closed, the other almost so; his jaw was immovably clenched, and, to make matters worse, erysipelas (staphylococcus infection of skin) had set in on the left arm.
Mrs. Gordon, his wife, who nursed him - her name was Fanny, and she was then a beautiful girl of 25 - put a liberal interpretation on her instructions and painted the wounds, not three or four times a day, but, as Gordon himself says: "I think three to four hundred time a day." Fanny's diligence and devotion were rewarded. Her husband survived, outlived the war, became the Governor of Georgia, a General, and Commander-in-Chief of the United Confederate Veterans. He died in 1904."
I think Fanny knew iodine applied often was a more effective way to deliver iodine to the body especially applied on multiple sites. Likely Gordon's iodine blood levels and tissue levels rose throughout his body. As iodine is by far the best antibiotic, antiviral and antiseptic of all time, the colonel's body defenses were able to gain control of the infections. At the same time local repeated application to the wound sites caused the skin to grow up from its bottom where all the cells (Stem cells) that make normal skin are residing. Likely this minimized or eliminated scar formation. Thus Nyiri's and Jannetti's studies confirm why Fanny's method of treatment worked so brilliantly. Some of our recent serious skin diseases such a "flesh-eating disease" should respond quickly to topical iodine treatment practiced by Fanny. Or, for another example, iodine put onto a mosquito bite would kill all bacteria and viruses at the site of the bite within 10-30 seconds making it impossible for any virus or bacteria to multiply and get started --such as in West Nile Fever. Naturally, this applies to tic bites (Lyme's disease) as well. For many decades in the 1800s people carried around little bottles of iodine around their necks to use on all occasions. People in mosquito and tic infested areas should think of doing this again. (8) In addition, it has been forgotten vaporized iodine rapidly kills air borne viruses such as polio and SARS viruses.. Used extensively in the forties and fifties it may be of use to explore this approach again.
There has been no significant clinical research on iodine therapy or use for 40 years. (11-15) I feel it is important for research to be directed at this potentially significant area of medical treatment. It is worth noting the greatest part of significant research with iodine was done before the Medline search facilities were available. Of course, since large doses of iodine are tolerated intravenously without side effects, it has yet to be explored what help this may have for many cancer patients or even other diseases.
( Quote originally found at http://mueller_ranges.tripod.com/links/compendium/iodine.html )