Nail psoriasis and fungal infections are the most common causes of yellow fingernails, according to the NHS Choices website. Other potential causes include nail polish stains, lymphedema, sinus inflammation, tuberculosis, certain medications, bronchiectasis, thyroid inflammation, jaundice or nail fold infections.
About half of all people with psoriasis experience nail symptoms. In addition to yellowing, pitting, thickening, deformation and separation from the nail bed can occur. Nail fungus most often affects the toenails, but fungus can affect the fingernails too. A yellowing of the nail is common, as well as the nails becoming ragged, crumbly and dull. When the nail pulls away from the nail bed, it can also have a foul odor.