Brown beef, onion, carrot and celery in a deep 12-inch 5-quart saute pan over medium-high heat for 9 minutes, breaking up the meat and cooking to 160°F. Drain excess fat.
Add garlic for 30 seconds, then the 4 cups broth, salt, pepper and thyme. Bring to a boil.
Stir in frozen noodles, separate any clumps as they thaw and return to a gentle simmer. Cover loosely and cook for 22 to 28 minutes, stirring every 4 minutes until the thick centers are tender; follow a longer package time if needed.
Add reserved broth a little at a time if the pan tightens before the noodles are done. If excess broth remains after they are tender, simmer uncovered for 2 to 3 minutes until lightly saucy.
Remove from heat, stir in butter and rest for 2 minutes before serving.