Thickening nail on the foot?

Fungus or trauma: Thickening of the nails is either from a fungus, repetitive trauma to the nail, or both (i.e. Repetitive trauma creates opportunity for fungus to get under the nail). Go see a podiatrist, who can take a sample of your nail and send it as a biopsy to find out if it has a fungus. Then you can start appropriate treatment.

Many possibilities: Some causes of abnormal nail growth include trauma, infection, side effects of medication, and possible systemic diseases such as psoriasis. Have them inspected by a professional to determine the cause of the problem, as treatment varies depending on the diagnosis.