Nails with horizontal grooves may be caused by gray nails, onychomycosis, hyperthyroidism and other diseases. 1. Gray nail: This is due to the nail being infected by fungus, resulting in the appearance of horizontal grooves and horizontal lines on the nails, as well as symptoms such as thickening of the nails and yellowing and darkening of the color. 2. Onychomycosis: Inflammatory infections lead to damage to the nail matrix cells of the nails, cellular dysfunction, nail plate synthesis stops, resulting in the appearance of horizontal grooves in the nails. 3. Hyperthyroidism: due to the excessive secretion of thyroid hormone, the synthesis of keratin in the nail cap will be blocked, which will lead to the nail plate not being able to grow, and the nails will appear to have horizontal grooves. There may also be other reasons for the appearance of horizontal grooves in the nails, such as nail psoriasis, nail lichen planus, etc. It is recommended to go to the dermatology department of the hospital to have the cause clarified by the doctor, and to carry out standardized treatment under the guidance of the doctor.