Welcome to NeuroTalk:

Well, I'd start with the gluten issue. People with this intolerance typically are poor absorbers of nutrients from the diet.

So I'd get a B12 test and consider your D test so far as low.
You will want your numbers for the B12 since lab ranges in US are old and inaccurate for today's standards.

Raising the D to 50ng to 80 may help. But other nutrients may also be in short supply. Start with a B-50 complex, some Vit C daily (250mg -500mg), zinc, magnesium (not Oxide form) and see what happens.

The normal range for serotonin is this:
The normal range is 101-283 nanograms per milliliter (ng/mL).

There others:
Epinephrine: 0-900 picograms/milliliter (pg/ml)

Norepinephrine: 0-600 pg/ml

I don't really understand why the neurotransmitters were measured. Did your doctor explain that? I also don't understand the reported result since no concentration was included.

People with early onset of PN like your husband, who was around 30, can have endocrine problems. This would be low thyroid, or insulin resistance leading to prediabetes, etc.
Knowing his A1C would be helpful and I think he should have a thorough thyroid panel done. Gluten sensitive people often have autoimmune problems with the thyroid and diabetes.

My PN was thyroid induced and it started around age 30 for me.