Q: No brightness slider for Thunderbolt display

I recently bought a Thunderbolt Display off of Craigslist and it passed the dead pixel tests but I've noticed that I cannot change the brightness -- the brightness slider is not visible for the Thunderbolt display.

I have tried resetting NVRAM, changing the primary display and changing the order of the screens and have tried this on my MBPRetina (OSX 10.11.1) and my MBP15" (OSX10.10) and was not able to see the brightness slider on either.

My computer is completely up-to-date.

Do I have any options or does it sound like I'm going to need to take this to the Apple Repair Store?

Thanks!