I agree that hyperthyroidism would be rare in a cat as young as she is, but it does sometimes happen. Usually cats with hyperthyroidism are losing weight but eating excessively. They usually have a high heart rate as well. Being more vocal can be a sign of hyperthyroidism. It can be diagnosed with a blood test.
If she has a dietary allergy or IBD, inflammatory bowel disease, she would respond to prednisone. 7.5 mg is a fairly high dose. You might consider trying a diet that helps to treat IBD and dietary allergies, like Royal Canin GI diet. I have a cat that I got from the shelter; no one wanted him because he always had diarrhea with blood and mucus. I tried lots of things and did lots of testing but as soon as I started him on this diet, he is better.
There is a drug called budesonide that might help her. We use it to treat IBD; it has the effects of the prednisone on the intestines, but is not absorbed into her whole body.
Her not using the litter box could be related to discomfort if she has IBD, or it could be a behavior problem. You should have as many litter boxes as cats, plus one. So, if she is an only cat, you should have two. Adding Cat Attract to the litter or even catnip to the litter may help.
Let me know what else I can help with or answer. Rebecca