--but not nearly as high as those of people with progressive muscle diseases, who often have CPK levels in the thousands or even tens of thousands.

It is possible that this is a lab error; it's also true, though, that having thyroid conditions can raise CPK levels somewhat (decrease in T3, in particular, leads to rises in CPK) as can vigorous exercise, especially if muscle is damaged. Caffeine, interestingly enough, also leads to some elevation at times (one doctor I know says he always asks people to abstain from coffee for 24 hours before getting this test done).

It might not be a bad idea to get additional testing for the specific CPK types just to see if something involving muscle injury shows up.