It's not supposed to work on any international itineraries, but people have reported success when the international flight is a codeshare on a partner carrier and perhaps some other cases as well. Furthermore, you can ask for a reprint of your boarding pass after entering the US, and there have been many reports of success of Precheck when all of your remaining segments are domestic.

Also, if you fall in any of the categories above don't give the TDC any indication international segments were involved. It's also only supposedly open to US citizens (or Canadians via NEXUS), but any permanent resident or citizen of select countries can apply for GE and thus get a PASSID number. There have also been cases of frequent flyers of any immigration status getting invited to join Precheck and receiving it during domestic travel in the US. A friend of mine wasn't allowed to use the Precheck lane once because he showed a non-US passport even though his boarding pass scanned LLL.