A 12-year-old intact female Schnauzer was referred for the evaluation of poorly controlled diabetes mellitus: despite insulin therapy, blood glucose concentration was consistently high, indicating a decreased insulin sensitivity. Laboratory analyses revealed persistent hyperglycemia, glucosuria, and ketonuria. Diagnostic approaches were performed to identify concurrent disorders that can cause insulin resistance. The dog was found to have concurrent hyperadrenocorticism, hyperlipidemia, pancreatitis, and vaginal cytology indicating diestrus in the estrus cycle. Trilostane administration for hyperadrenocorticism improved the insulin response; however, the dog remained hyperglycemic. Eventually, the dog showed complete remission without insulin administration 1 week after the ovariohysterectomy. The dog remained in remission for approximately 4 months, but eventually relapsed and the condition was permanent. Diestrus in intact females and hyperadrenocorticism are known to be the two main causes of insulin resistance in dogs. After the management of these conditions, the dog achieved diabetes remission, which rarely achieves in dogs. In cases of insulin resistance, such as hormonal imbalances or inflammatory conditions, remission can be achieved by addressing the underlying cause. Hence, it is important to assess the presence of comorbidities associated with insulin resistance in dogs with poorly controlled diabetes mellitus and to treat each condition as soon as possible.