What causes thick and brittle toe nails?

Fungus: Fungus or trauma are two of the major causes. See a podiatrist and if needed a nail biopsy could make the definite diagnosis.

Have it evaluated: 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.