Thick and hard toenails may be caused by congenital chilblains, onychomycosis and other causes. 1. congenital thick nail disease: congenital thick nail disease is a hereditary skin disease, manifested as the nail or toenail appear hypertrophy, hardening, loss of luster, easy to brittle crack and other symptoms, accompanied by palm and plantar hyperkeratosis, excessive sweating and other manifestations. 2. Tinea pedis: gray nails is due to the skin fungus invasion of the nail plate or nail bed caused by nail lesions, manifested as finger or toe nails appear cloudy, thickening, hardening, buckling, atrophy, shedding and other symptoms, the patient’s self-consciousness symptoms are mild, there may be pain and under the nails bleeding phenomenon. Thick and hard toenails may also have other causes, it is recommended to go to the hospital in a timely manner to improve the examination, to clarify the cause of the disease and then give targeted treatment.