Hi mate, my PN was caused through prediabetes, a few docs [gp's] told me I had no signs of diabetes at all, I had a overnight fasting test for diabetes, it come back as normal, a few in office tests with a glucose monitor which all showed normal.
It wasn't until i found a neuro that knew what he was doing and ordered a glucose tolerance test and a bunch of other stuff, results were I was prediabetic, which he said straight away is the cause of my nerve damage which turned out to be spot on.

MrsD says you can have an insulin test which will show the same

good luck
Brian