Thickening and yellowing of nails may be caused by nail psoriasis, onychomycosis, lichen planus and other reasons. 1. Nail psoriasis: the cause of psoriasis is not clear, and it is considered to be caused by heredity and immune factors, etc. Nail damage can appear alone or accompanied by skin rash. Patients will not only have thickened and yellowed nails, but also have nail peeling and pus accumulation under the nails, erythema or small pustules in the lesion area. 2. ringworm of the nails: ringworm of the nails is mostly considered to be caused by dermatophyte infection, the patient will have yellowing and thickening of the nails, nail atrophy, nails show unevenness, inguinitis and other symptoms. 3. Lichen planus: the cause of lichen planus is still unclear, may be genetic, viral infection, immune factors, mental factors and so on. Patients will have thickened nails, yellow nails, red flat papules on the skin and so on. Therefore, patients with lichen planus will have thickened and yellowed nails. It is recommended that patients with thickening and yellowing of the nails need to consult a doctor in time, and the doctor will choose the appropriate measures for treatment after clarifying the cause of the disease.