Energy Production-Untreated/Inadequately Treated Diabetes.
Lethargy, a general low energy level and lack of response, can signal a serious illness in any cat or dog. In a diabetic, it can be a signal of dangerously high or low blood sugar, or of other health problems.
Lethargy is one of the symptoms of uncontrolled/poorly controlled canine diabetes. Untreated or not adequately treated, the diabetic's tissues are unable to use glucose, amino acids and fatty acids properly. [1] It normally resolves quickly after starting insulin therapy.
Unusual lethargy for unknown reasons might be a veterinary emergency. 
References[]
- ↑ Fleeman, Linda, Rand, Jacqueline (2000). Long-Term Management of the Diabetic Dog. University of Queensland.