Thick and discolored toenails are often caused by a fungus that invades the nail. It is the same type of fungus that causes athlete’s foot. Sometimes this problem can be caused from trauma to the nails. Treatment for fungus to toenails often involves an oral medicine, but sometimes can be treated with topical medications. Sometimes thick nails are not fungus at all but are due to trauma – either from something dropping onto the toenail or shoes that are too tight, especially athletic shoes. Athletic shoes should be fit so that there is a full finger’s width in front of the longest toe while standing.