Thyroxine is a hormone produced by the thyroid gland. In humans, this hormone controls metabolism, shedding, and other boring functions, but in amphibians (not just axolotls) it stimulates metamorphosis. Metamorphosis can be induced by giving the salamander thyroxine, but a more convenient alternative is to add iodine to the water.
The thyroid gland produces two similar hormones derived from the amino acid tyrosine: triiodothyronine (T3), and tetraiodothyronine (T4), or thyroxine. Both have similar effects, but T4 (the L version, I think) is used more by amphibians. Around 95 percent of the body's iodine intake goes to the thyroid to be incorporated into these hormones. Each molecule of T3 contains 3 atoms of iodine (I); each molecule of T4 contains 4 iodine atoms. Atleast in larval/neotenic salamanders, increased iodine intake causes increased thyroxine production. I don't know whether this is true of other animals or of adult salamanders. But putting iodine into your axolotl's swimming/drinking water will cause it to metamorphose.
It is true that enormous amounts of iodine in your axolotl's water will cause it to morph. This is not recommended. It's very stressful for the salamander and is not to be undertaken for your entertainment. If you really want or need to do it, talk to a veterinarian who can provide the T4 itself.