Yellowing and thickening of toenails is usually caused by a nail fungus, known as gray nails. Gray nails are nail lesions caused by a fungal infection of the skin. After the fungus invades the nail plate as well as the skin, it can stimulate the skin to proliferate and destroy the cuticle, resulting in thickening and discoloration of the nail plate. Gray nails are relatively long and contagious and cannot heal on their own without systematic treatment. They can also lead to cloudy, separated, atrophied, peeling, buckling, uneven surface, hooked nails, and even nail infections that cause pain and bleeding under the nail. If the toenail becomes yellow and thick, the patient needs to go to the hospital to get a clear diagnosis through fungal smear microscopy and fungal culture. Once the diagnosis of gray nail is confirmed, the doctor will usually choose the appropriate treatment plan according to the condition and the different causative organisms. Currently, oral antifungal treatment with terbinafine and itraconazole is usually used, and topical treatment with terbinafine ointment and sertaconazole ointment is also available. If necessary, nail debridement or removal of the diseased nail should be performed as prescribed by the doctor. During treatment, patients should try to keep the affected area dry and follow up with the doctor after treatment.