What happens to iodine-131 in the body after it has been used to treat thyroid cancer?

Same as plan iodine: You ingest iodine every day - in salt, salty foods, seafood, many breads, & red food coloring. The thyroid absorbs what it needs; the rest is excreted into the urine (+ a little into sweat, saliva, and other secretions). The i-131 atoms that decay in the thyroid cells (and give off beta and gamma energy that kills thyroid cells) become nonradioactive xenon-131 atoms, and are breathed out.

Passes out: Looses its radioactivity and is mostly passed out in the urine and saliva.