In addition to pancreatic diabetes, there is Acromegaly, which is a pituitary tumor. The output of the tumor is Insulin-like Growth Hormone (IGH-1). This displaces native insulin in the body, basically creating type-1. They call this symptom Secondary Diabetes. To overcome it, you add insulin to the cat to try and displace the IGH-1. It's kind of a losing battle as the tumor grows.
There was a study in Great Britain in the last 8 years. They found that over 25% of all diabetic cats had some level of Acro. The percent may be higher, I can't remember. A lot of documentation is in the Acromegaly subforum. And I documented Leo's treatment in his thread, which was radiation of his tumor in Sept 2016.