David Bruyette, DVM, DACVIM, said that hypoadrenocorticism and hyperaldosteronism are so common that if you look for them, you will probably diagnose either disorder in the next month. Addison’s disease, or hypoadrenocorticism, is seen in middle-aged cats, and there’s no sex predilection. In dogs, Addison’s is an autoimmune disease, but it is unknown if it is the same in cats.
Source: DVM 360, May 10, 2018. Link.
The main reason Dr. Bruyette thinks Addisonian cats get misdiagnosed is that a lot of them present with azotemia and are diagnosed as having kidney disease. He says that if you have a feline patient that presents with azotemia that resolves after 24 hours of fluid therapy, then kidney disease was not the cause of that azotemia. In these cases, he recommends adrenal testing to the owner to rule out Addison’s.