This is indeed one of those frustrating cases.
 
I would stop the pred all together and run fluids to help rehydrate Scoots
and to lower his glucose level.  As for how much insulin and what type of
insulin to use, I use the same guidelines that are used for cats.  I start
with Ultralente insulin (0.2 Units) per ferret then adjust the amount based
on the blood glucose levels.  Some use NPH insulin starting at 0.1 units
per ferret.  These cases are frustrating because it is hard to get the
glucose level back into the normal range.  One theory as to why these
insulinomas ferrets become diabetics is that the tumor is actually a
glucagonoma.  Again this is a theory that has not been proven, but it
does seem logical.  Fortunately these cases are rare, but they do happen
sometimes.  Also this is a case that brewers yeast may help.
 
Good Luck with this one,
Jerry Murray, DVM
 
[Sukie note: this is an off-chance that leads some vets to test both
glucose levels and insulin levels before some treatments or surgery.
One of our's does it.  If you check the cross-posts in the archives or
the FHL files I believe that an alternative hypothesis for this type of
unusual situation is also discussed by Bruce Williams.]
[Posted in FML issue 3448]