Thickened and hollow toenails are considered to have onychomycosis, also known as gray nails. Onychomycosis is a disease caused by fungal invasion of the nail plate or under the nail, when the toenail is infected by fungi, it can form cracks, become brittle, thickened, brown or black in color, and hollow and so on, you can go to the hospital to perform a microscopic examination of the fungus as well as fungal culture to determine the diagnosis. If it is determined that the fungal infection, you can use some topical antifungal drugs, such as fluconazole ointment, terbinafine hydrochloride ointment, etc., if the effect of topical drugs is not good, you can also increase the oral antifungal drugs. In addition, trauma can also cause thickened and hollow toenails. Trauma usually has obvious triggers, such as a heavy object hitting the toes or the toes bumping into hard objects and kicking a soccer ball, and so on.