The T-mobile APN defaults to fast.t-mobile.com on my nexus 4 sim card. This works great normally. The default APN for t-mobile using an APN protocol of IPv6.
I noticed that when using wifi tethering, sites that do not use the IPv6 protocol do not load. Sites which do (google, facebook, twitter, etc) all work fine.
Changing the APN settings to IPv4 for the protocol causes wifi tethering to work fine. This implies that t-mobile isn't doing something right with wifi tethering and IPv6.