Place the frozen chicken thighs, 8 cups of water, and 4 cups of chicken stock into the pot on high heat. Bring to a boil; this takes about 10 to 15 minutes depending on the type of pot and stove you have.
Once boiling, reduce heat to medium heat and cover. Cook for 25 minutes.
After 25 minutes, take off the cover of the pot. Skim off any scum that forms (the white foamy stuff floating on the top). Add the celery, carrots, onions, garlic or granulated garlic, dried Italian seasoning, ground black pepper, and the remaining 4 cups of chicken stock. Stir, raise heat back to high and bring to a boil (takes about 2 minutes).
Once boiling, reduce the heat to medium, cover, and cook for 20 minutes.
Take off the lid of the pot and remove the chicken and set aside on a large plate or bowl. Add the egg noodles and cook for the length of time recommended on the package instructions.
While the noodles are cooking, remove the chicken and skin off the bones. Discard the bones and skin and shred the chicken with 2 large forks.
Add the shredded chicken to the pot. Taste the soup and, if needed, add up to ½ tsp additional salt.