There are various reasons toenails become yellow, but the most likely explanation is a case of nail fungus (onychomycosis). Fungal infections in the toenails are, unfortunately, fairly common. The responsible fungi are microscopic—so you cannot see them and know to avoid the tiny spores—and typically reside in warm, damp areas, including places where people normally walk barefoot (such as indoor pool decks, gym locker room floors, and communal showering areas). 
Toenail fungus is a stubborn infection, one that usually requires professional treatment. For optimal results, it is best to come in and see us at the earliest opportunity (when the infection is most easily treated). Left unaddressed, this problem will only continue to worsen.
Beyond a fungal infection, other potential causes of yellow toenails include diabetes, tuberculosis, bronchiectasis, jaundice, psoriasis, and thyroid problems. In some cases, toenails become yellowed simply due to natural aging processes. There is also an extremely rare genetic disorder known as “yellow nail syndrome,” which can present in conjunction with lymphedema.
