--or dysregulation of sugar levels in the pre-diabetic range comes in somewhere between 30 and 40 percent of all incidences, but that still leaves the majority of neuropathy sufferers with other reasons for it, and yes, a good third or so remain stubbornly "idiopathic", though a lot of that designation depends on how extensive the testing for causes is. That tends to vary a lot based on where you go and who your physician is.

I like to refer people to start just to this Quest Diagnostics paper of laboratory blood tests for causes of neuropathy, just to give people a sense of the wide variety of things that can cause nerve problems (there are over 200 known separate neuropathy etiologies, and probably more that haven't been discovered yet):

Quest Diagnostics: Test Directory