All Tundras made starting in 2007 use the J2807 engineering standards for payload and towing.
So those take into account the weight of the w/d hitch at 70#, and 150 for each front passenger. There other parameters including cooling capability on a long grade, launch and acceleration and trailer sway.
I have a 2008 Tundra CM and my TT is 5600 dry. I haven't weighed it loaded yet, maybe next trip at the end of this month. Towed about 1200 miles during July 4th week and was a good experience all the way. Except a little bouncy on Seattle area freeways.