The most likely diagnosis of thickened, yellowed, and hollowed out toenails is onychomycosis, or nail fungus, which is one of the more difficult fungal diseases to treat. You can take oral antifungal drugs such as terbinafine, itraconazole, fluconazole, and luliconazole, etc. The oral cycle is longer, about three months. Topical application of glacial acetic acid or amorolfine application, urea ointment can be used topically. Of course, you can also choose Chinese herbal therapy for nail fungus, you can apply Chinese herbal soup to the diseased nail externally, the medication cycle will also be longer. At the same time, patients should pay attention to personal hygiene, especially the cleanliness of the feet.