I’ve been having the same problem and just managed to fix it.
brew install libtiff
brew install imagemagick --with-libtiff
I tried a number of times combinations of uninstalling and reinstalling imagemagick and libtiff, but had the same results each time. I was hoping that brew would magically pick up on libtiff and link it in.
I eventually found the with-libtiff option by looking at the output of the command
brew info imagemagick
Before January 30th, 2013, the option was called --use-tiff.