How long has she been on the levothyroxine? I don't know how it is with dogs, but for myself I can tell you it took right about six weeks for levo to kick in and enable me to start losing weight. I felt considerably better (less tired and less mental fog) within about ten days of starting medication, but it took a full six weeks for me to be able to start losing weight. FWIW, I lost weight slowly but rather easily on the same number of calories that I was gaining on before my hypothyroidism diagnosis. It was a reduced calorie diet, but before diagnosis I was unable to lose weight even while dieting.