Nails can get discolored and thick from repetitive trauma in tight fitting shoes or from certain sporting activities that cause the toe nail to rub on the shoe repeatedly.   The great toe and little toe are mostly affected because of their location in the shoe, the 2nd toe can also be involved if it is the longest toe on the foot.  People active in sports may notice their nails thickening and separating from the nail bed and eventually falling off only to be replaced by a new nail.
 
Making sure the shoes are long and wide enough can prevent this from happening. 

Nail discoloration and thickening are also signs of a fungal infection of the nail plate, medically known as Onychomycosis.  May also be referred to as "ringworm of the nail' and "Tinea unguium".  People living along the Gulf Coast, where it is hot and humid for a good part of the year, are more likely to get it because it thrives in warm, moist areas.  As a fungus, the mature form produces spores that blow around in the breeze and when they find a warm, moist, dark place, a toe nail in a shoe, it will start to grow.  When we are young are nails grow fast and our immune systems are in good enough shape to fight off any fungal nail infection, but as we age our immune system and ability to fight off infections decrease and the fungus can take over the nail plate.  The nail will become brittle, crumbly, curve in at the end of the toe and can cause an ingrown nail infection.  The color of the nail plate can begin as an oily appearance on the ends of the nail and progress to a white or yellow discoloration and get thicker as we age.  Usually nail fungus doesn't cause any problems other than cosmetic, unless you have poor circulation, chronic ingrowing nails, pain from the nails, diabetes, or peripheral neuropathy or other conditions that can make you prone to infection. 

Other causes of nail discoloration and thickening are yeasts and molds.  Having a nail biopsy along with your medical and social history can help your podiatrist answer what may be the cause of the problem,  how to treat and prevent it from getting worse. 

It is always a good idea to keep your feet clean and dry as possible, remember, the fungus needs three things to grow: warmth, darkness, and moisture.  We can't stop the warmth our body produces or the darkness our shoes provide, but we can manage keeping our feet drier with moisture wicking socks and medicated foot powders.   There is no permanent cure for onychomycosis, but there are ways to manage it well.