There is a thing called "impaired glucose tolerance".

It is not frank diabetes, but sets the stage metabolically for
damage. Often it consists of LOWS...which can starve neurons.
The swinging from low to high, only shows up on 4hr or longer glucose tolerance testing.

That is a LONG time to be on metformin. One common characteristic of this drug is to affect acid/base balance of the blood. Acidosis is a common side effect, and an acid environment can cause burning paresthesias.
Here is a link that suggests metformin can be a "cause" or provoke PN:
http://www.ncbi.nlm.nih.gov/pubmed/22100324

It also can affect your kidneys. I hope you get those tested regularly. When the kidneys start to malfunction, renal acidosis may develop.

You can test this yourself with a dose of bicarbonate to see if the burning changes. I use AlkaSeltzer orig. formula for this for my burning feet. I only use it occasionally because my burning is less now. This contains aspirin, and if you can't take aspirin for some reason, you could try the aspirin free version. But I think the aspirin helps with the blocking of PN burning as well.

Chronic pain is an acidic event at the tissue level. There have been studies about this.

A simple remedy like this:
http://www.gettingbacktohealthy.net/...ice-and-water/
Is often offered for chronic pain. The citrate in citrus fruits actually may reduce acid in the blood.

This is the list of side effects reported on metformin:
http://www.drugcite.com/?q=metformin
Some people cannot tolerate metformin. I am one of those. I had PCOS and now am insulin resistant as well. Metformin made me so sick, both times I've given it a go.

There is another way to approach PCOS...
http://www.chiralbalance.com/
This is a special form of inositol and is very easy to handle.
I took it for 2 yrs. d-chiro-inositol. Its only downside is that is pricey.
There are many papers today on d-chiro inositol...like this one:
http://www.ncbi.nlm.nih.gov/pubmed/21210850
It can have a dramatic effect on lowering androgens, which are elevated in PCOS patients. That is the reason I took it. I wish it had
been around 35 yrs ago when I was trying to conceive. Those were trying times, and PCOS was not understood then.

So if you were up for it, you might discontinue metformin for a short time, see if the burning lessens. Take the Chiral Balance instead.

Also it is important that you do not take other RX drugs that have PN potential. There is a thread on the subforum above about drugs that may cause PN.

Read up on d-chiro inositol and discuss with your doctor.

Often when people get a nerve pain disorder... one should look to lifestyle and diet and drugs being used. Often the trigger may be hidden there. As far as diet goes... gluten can be a culprit.
Also the nightshade family of veggies can cause considerable burning in some people. I found this for myself a few years ago, and once I eliminated peppers and potatoes (and some concentrated tomato products), my burning feet and legs improved dramatically. I found the burning from nightshades to be delayed up to a day or 3... it is often not immediate in effect and hard to pin down. So an elimination diet is needed to see if they are culprits.

The body burning can show up from a viral infection. Some members here have that. Also a vaccine reaction. In these cases, only time helps if healing is to occur. The virus somehow damages the dorsal roots and in some healing can happen, but others lack the nerve growth factors to accomplish that. New research is looking into stem cell therapy for those patients who cannot heal properly.

Carpal tunnel does come from low thyroid. It is due to the mucin deposited in tissues, which compress nerves in wrists and ankles.
I had terrible CT when I finally got pregnant (after 6 yrs of trying).
It diminished after delivery but I still have some symptoms now and then. I would wonder if your birth control is doing it, as the doctor who gave me the EMG during my pregnancy said hormones can make it much worse. I use wrist braces made for CT at night when sleeping if I have issues which for me come from overuse..gardening or other strains on the wrist from activity. I am also hypothyroid, but I am negative for Hashimoto's.