Why are my toenails dark and thick?

Fungus: Trauma and various dermatologic problems can cause thickening and darkening of nails, but the most common cause is toenail fungus. Dermatophyte fungi infect the nail, essentially eating it and discarding the debris underneath. This causes the nail to thicken and discolor. The most effective treatment is oral antifungal medication. Lasers have become popular, but are expensive and not permanent.

Sounds like fungus: Keep in mind... The fda has recently come out with a blanket statement that toenail fungus can not be cured. It is "managed" by a number of modalities, but it can always come back in the future.

Nail fungus/Bruising: Nail fungus (onychomycosis) and bruising of the nail are common reasons for dark discoloration of the nail and/or nail bed. See a podiatrist or dermatologist to make the diagnosis.