In a large pot, heat the chicken broth over medium heat.
Add the diced carrots and celery; cook for about 5 minutes.
Stir in the diced potatoes and continue cooking for another 10 minutes.
Add the shredded chicken to the pot.
Pour in the lemon juice and zest, stirring well.
Season with salt, pepper, and thyme if using.
Let the soup simmer for another 10 minutes.
Stir in the chopped parsley just before serving.
Taste and adjust seasoning as needed.
Serve hot, garnished with additional parsley and lemon slices if desired.