Thick and hard nails are considered to be ageing, onychomycosis, thick nail syndrome and so on. 1. Ageing: With age, the body’s metabolism will slow down, and some keratin will accumulate on top of the nails, so the nails will become thick and hard. 2. Onychomycosis: Onychomycosis is a kind of fungal infection, mainly caused by dermatophytes invading the nail plate and nail bed, which is also called gray nail, the main manifestations of which are cloudy nails, thickening, separating, discoloration, atrophy, shedding, unevenness and so on. 3. Thick Nail Disorder: Thick Nail Disorder is a rare hereditary skin disease of nail hypertrophy, which is a chromosomal dominant inheritance, and gene mutation has a certain relationship, the main manifestations of significant nail hypertrophy, palm and plantar hyperkeratosis and so on. It is recommended that patients go to the dermatology department of the hospital in time for a detailed examination to find out the cause of the disease, and under the guidance of the doctor for symptomatic treatment.