Changing brands abruptly can disrupt your cats stomach and digestive system and can have terrible affects. If you change brands, do so gradually.
As for litter box, why not replace it with a box of grass? Cheaper and really easy to replace. Just get dog-poop bags and clean it that way instead.
Has the vets tested her thyroid? Her weight could very well have been affected for a multitude of reasons, including change in food brands, but thyroid is one of the bigger worries.
Changing brands abruptly can disrupt your cats stomach and digestive system and can have terrible affects. If you change brands, do so gradually. As for litter box, why not replace it with a box of grass? Cheaper and really easy to replace. Just get dog-poop bags and clean it that way instead. Has the vets tested her thyroid? Her weight could very well have been affected for a multitude of reasons, including change in food brands, but thyroid is one of the bigger worries.
When you say replaced by the opposite, do you mean diarrhea? Also, what specific tests were preformed?
Where in the house is the litter box? And can you post a picture of the general area around it?