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Inappetence is definitely a sign of illness, so is listlessness. It sounds like she might need something a bit more than the cranberry capsules - possibly an antibiotic as well. I would call your vet and let them know and they may be able to get you some medicine this weekend for her to start taking.
Let me know if you have more questions,
If they diagnosed her with a UTI that is a urinary tract infection. This is caused from bacteria. So I'm not sure I'm following their rationale . . .
Have they done any bloodwork on her?
I'm in TN so I probably wouldn't be much help as far as finding someone in your area. The thyroid test might be what they are suspicious of as far as low of energy - that happens in dogs with a low thyroid hormone. They probably need to do a urine culture if there is a urinary tract infection to see exactly what is causing it and which antibiotic will clear it up. If there is a urinary tract infection - antibiotics are a must to clear it up. Cranberry capsules help prevent recurrence but won't clear up the infection.