Both Sukie and Karin have brought up salient points, and the need for
current bloodwork to assess kidney function is very important.
 
Without a measurement of the kidney to relate, it is very difficult,
as both Sukie and Karin astutely pointed out , to tell whether we are
dealing with one enlarged kidney, or one shrunken one.
 
Enlargement is more common in female ferrets due to blockage of the
ureter and inability for urine to flow down from the kidney.  This
usually arises from untreated (often silent) urinary tract infections.
However, one would not expect kidney samples to be normal (although you
don't mention what was sampled or how it was sampled.)
 
You can have one very small kidney (usually nonfunctional).  In such
cases, the other kidney will actually get larger over time to take up
the extra work of filtering the blood and forming urine.  However, the
smaller kidney is often misshapen, discolored etc., and it is usually
not a difficult call.
 
As she is not doing well, and is becoming more lethargic, I think that
some blood tests to ascertain whether the kidneys are working well are
probably in order at this time.
 
With kindest regards,
Bruce Williams, DVM
[Posted in FML issue 4122]