What causes discoloration and thickness to toe nails?

Fungal infection. While a fungal infection is a likely cause of thickening and discoloration of toenails, there are several other possible causes. These include: psoriasis, lichen planus, poor circulation, inherited or acquired nail dystophies, and others.
Many possibilities. Some causes of nail discoloration include trauma due to tight shoes, infection, and medication side effects. You should try wearing shoes with a higher wider toe box. Get the feet inspected by a professional to determine the cause of the problem, as treatment varies depending on the diagnosis.
Depends. I have seen many patients with nail thickening that do not have fungus... however most people with fungus do have thicker nails! Check with a specialist for the best diagnosis and treatment!