I just stumbled across this Q&A on Intelihealth, and thought it might be useful.
Q: What is in weight-loss patches? Do the patches work?
The majority of the weight-loss patches I was able to identify contain a form of sea kelp with the unappealing common name of bladderwrack. Sea kelp contains a large amount of iodine, which, when taken orally, theoretically causes an increase in thyroid activity and presumably weight loss.
While it sounds appealing, there is actually no validity to the claim. While trace amounts of iodine are needed to produce thyroid hormone, there is no evidence that excessive amounts actually increase levels of thyroid hormone to higher-than-normal amounts. Excessive amounts of iodine, however, can produce enlarged thyroid glands. The folks who make the patches claim the sea kelp is absorbed through the skin, and some even provide a summary of a medical study purporting to support the effectiveness of their product. I am unable to find any studies that show that the active ingredients in bladderwrack are absorbed or that the use of a patch aids in weight loss.