Yellow toenail on big toes. Taking synthoid early morning & metformin after dinner. Started high potency iron, vitamin E rotating days in evening?

Fungus. Sounds more like fungal toenails. Everything else you mentioned does not cause nail discoloration. Go see a podiatrist and get a fungal culture.
Fungal infection. Since you have diabetes, it sounds like a fungal infection of the nails. See your doctor or a podiatrist. Don't believe the junk you read on the internet about nail fungus treatments.