Melt butter in a 5-quart soup pot over medium-high heat. Cook onion, carrots and celery for 5 minutes, stirring often.
Add broth, thyme, salt and pepper. Cover and bring to a boil, about 5 minutes, then uncover and simmer for 4 minutes until carrots are nearly tender.
Stir in fine egg noodles and simmer for 2 minutes, separating them with a spoon.
Add shredded cooked chicken and simmer for 3 to 4 minutes until noodles are tender and the chicken reaches 165°F.
Stir in lemon juice off the heat and divide immediately among 4 bowls.