Ferret adrenal disease -- did my ferret heal himself?

My ferret was diagnosed with adrenal disease/adrenal tumors. He had lost almost all the hair on his trunk, had lost a lot of weight with a distended belly, and was slowing down a lot. The vet advised against treatment, as my ferret is over 8 years old and most likely is at the end of his life. We opted for no treatment. About 3 months after his diagnosis, his hair grew back almost completely, he started to gain weight again, and he has a lot more energy. Did he recover on his own? Can ferret adrenal disease just "go away"??