The yellowing and thickening of the nails may be due to grey nails. The thickening of the nails is firstly considered as grey nails, also known as onychomycosis, which is caused by fungal infection to the nails and toenails. It must be noted that there are no nerves or blood vessels in the nails, so the usual topical foot remedies are not effective for grey nails. Clinical treatment of gray nails is mostly done with oral antifungal medications and topical sealants. Internal diseases may also affect yellowing and thickening of the nails, mainly pulmonary disorders that affect blood circulation, as the nails are the farthest part from the heart and therefore must have a normal blood supply. If the distal blood supply is inadequate when pulmonary heart disease is present, thickening of the nails may occur. Side effects of medications can also cause thickened and yellow nails, and some lipid-lowering medications have such side effects. Therefore, when you go to the doctor with thickened nails, if there is no powder or hollowing under the fingers, the doctor will usually rule out gray nails and ask if there are any other diseases or what medications you are taking to prevent the side effects caused by the medications.