I received a reply from Dr. Jerry Murray about my questions.
[Posted here with Dr. Murray's permission.  BIG]
 
"No, Melatonin does not lower or change glucose levels....I am not sure
how one would determine if a ferret has a "headache"; however, no signs
of head pain or grumpiness with melatonin treated ferrets.... Different
doses based on oral administration giving a pulse of melatonin after
being absorbed while the implant gives a constant concentration in the
bloodstream....I did talk with Dr Oliver at the University of Tennessee
about testing melatonin levels....He can run a melatonin level for
ferrets, but it would cost roughly $1,000 to set up the test....I would
suggest starting all 3 of your ferrets on melatonin.
 
No logical reason to wait till they are 3 years old.  By then they may
already have hyperplastic or neoplastic adrenal glands.
 
Hope that clears up some of the confusion."
[Posted in FML issue 4602]