See my reply to your other post. It's a diagnostic tool to see where the body has "normal" thyroid tissue (that produces hormone). It's used pre-surgery to diagnose nodules and determine if they are thyroid tissue or something else. And it's used after surgery to determine if there is any remaining thyroid tissue in the body.
EndocrineWeb explains it well:
<A HREF="http://www.endocrineweb.com/tests.html" TARGET=_blank>http://www.endocrineweb.com/tests.html</A>
[Scroll Down to the sections called "Iodine Uptake Scan" and "Thyroid Scan"