The most common cause of yellowed, thick and/or deformed toenails is a fungal infection of the toenail. The fungus that infects the nail, most commonly, is the same fungus that causes athletes foot. It tends to be slowly progressive, damaging the nail to a greater and greater degree over time. The infection usually starts at the tip of the nail and works its way back. It usually is not painful and often not noticed until it has gotten well established.