What is an Iodine Tincture, and how do I do one?

My sister says an inexpensive way to test if i have an iodine deficiency is to do an iodine tincture, and that i can get one from a pharmacy. She says that the faster it absorbs, the more deficient my iodine level is or whatever. What is an iodine tincture, and how do i do it? do i do that thing where i poke myself with a needle? or just put it on my skin? thanks to everyone. <3