Found this:
Bodybuilders who use thyroid drugs while attempting to carb load during the final week of their contest preparations may be wasting their time. The purpose of carb loading is to provide a fuller, more dense appearance to muscles. But taking thyroid drugs will inhibit glycogen synthesis. If a bodybuilder reduced his carb intake before the loading phase - as is the common practice - he may wind up looking "flat" onstage because the carbs simply won't kick in as expected due to the concomitant thyroid usage.
Bodybuilders who use thyroid drugs while attempting to carb load during the final week of their contest preparations may be wasting their time. The purpose of carb loading is to provide a fuller, more dense appearance to muscles. But taking thyroid drugs will inhibit glycogen synthesis. If a bodybuilder reduced his carb intake before the loading phase - as is the common practice - he may wind up looking "flat" onstage because the carbs simply won't kick in as expected due to the concomitant thyroid usage.