Can I convert flat bar to drop bars on a hybrid bike I'm fairly certain that the bars should change out quite easily; I'm more concerned about the shifters and brakes. Currently I have a hybrid 18 speed with cable disc brakes and Shimano Deore XT derailer. I know that these are typically mountain bike parts. I'm eyeing up the Shimano 105 STI dual control shifter/brake. Is this even going to work? 

My main reason for wanting the change is after riding the road for quite a few years with flat bars my wrist are starting to take a bit of a beating. I put some bar end on the flat bars to change my grip position, which helped my wrists, but now I feel my grip is too wide. 

If pictures would help i can try to upload some tonight, any advice would help.
The 9-speed road and mountain shifters and derailleurs from shimano should be compatible (at least with 105 / Tiagra). Make sure you get 9-speed shifters, however some people have luck using alternate cable routing (attach the shifter cable to the oppisite of the clamp on the rear derallieur) to use 9-speed drivetrain with 10-speed rear shifter.  Keep in mind, You can't mix and match 10-speed road gear with any of the mountain gear labeled 'Dyna-Sys' as they have modified the cable pull ratios for those components. If you need to adapt different speeds and brands, you may have luck using a compatibility solution like Jtek Shift Mate.

Your brakes may have different cable pull ratios as well.  You likely need to swap your disc brakes for ones with compatibility with the shorter pull that is provided by road STI shifters.  Avid makes BB5 and BB7 road models which would probably work well.  You probably don't need to swap your rotors, just the calipers. 

If you want cheaper alternatives to STI levers, you can buy long pull brake levers that will work with your mountain disc brakes and use a different type of shifter like Retroshift (which offers a long pull brake lever option) or 9-speed Shimano Bar End Shifters.