Owners of a T-Mobile flavored HTC One M8 should be see an Android 4.4.3 styled update heading their way today. We've already seen a few tips that people are waking up and seeing the 582MB update on their devices this morning.
The OTA update to version 2.22.531.5 is pretty much the same exact update that people in Europe, India and beyond have been seeing roll out in recent weeks. Besides Android 4.4.3 the update also adds security fixes from Android 4.4.4, performance optimizations for Wifi, Bluetooth and cellular networks and a bunch of updates to the stock HTC applications and Sense experience.
If you've not seen it yet, don't worry, it sometimes takes a little while to get to everyone. But if you didn't check yet, now might be a good time to go look!