US citizens do not need a passport to re-enter the country. However, to get on your plane in Thailand, you would want to have a valid passport, to avoid any hassles. So, make sure you have at least one day of validity when you show up for your flight home.
As far as six months, the rules are different for different countries. Many don't care, as long as it's valid. Others require certain lengths of validity to be left when you enter.