Originally Posted by
craigthemif
TK would sell one of their direct flights to the US, with a final domestic leg on UA if needed.
AFAIK TK don't codeshare on other non-Turkey TATL routes (happy to be corrected if that isn't true) and a TK-->LH connection on a single ticket is not going to be cheap (again I don't think LH codeshares with TK). Perhaps using miles or separate tickets, but I would still wager that a newbie OP jumped to Turkey rather than the actual airline. Won't know until the OP updates us...
I've connected TK-UA/LH/AC/LX/OS many times - the A++ JV often allows intra-Europe segments to be operated by TK/A3/OU/JP/LO/SK on their fares frequently.
UA is responsible for fare filing for US point of sale, and they are very liberal with intra-region interline options. Similar to how they permit HKG-BKK, for example, to be operated by CX on even some of their cheapest fares.