Cushings disease is a cancer of the adrenal glands, and one of the first
signs of it is hair loss.
I've had lots of ferrets with hair loss and one even with NO hair at all
and there are a number of things you should try before resorting to
surgery:
1. Put the ferret strictly on natural light.  Ferrets depend on light
cycles to time their shedding.  If the ferret is kept in the house with
the lights on after dark, she probably just doesn't know what time of
year it is, and the mechanism that controls shedding may have thrown up
its hands in despair and just quit.
2. Sometimes hair doesn't grow in the tail because the follicles get
plugged up with blackheads.  I've had good results washing tails with
Oxy-Clean pads and adding extra vitamin E to the diet.
3. Get a blood test to check for Cushings.  If it is there, you must
catch it early in order to save the ferret.  If it isn't... what have
you got to lose (excpet money...)
4. Diet is also very important.  Around Boulder, a town described by
one "quack buster" as a place where "people are so open-minded their
brains are falling out," I see a lot of hairless or very poorly coated
ferrets who were being fed cheap dog food or even a vegetarian diet.
Once put on cat food and natural light cycles they do just fine.
                                                                          
[Posted in FML 0152]