Hollow toenails may be related to onychomycosis, trauma, lack of nutrients and so on. 1. Onychomycosis: this disease is caused by fungal infections, such as dermatophytes and other causes, usually manifested in the deformation of the nail plate as well as the destruction of the hollow toenail. 2. Trauma: when the toenail is squeezed or contused, it will lead to atrophy of the nail bed under the toenail, and the toenail cap will become hollow. Generally with the growth of the toenail, the nail bed gradually recovers. 3. Lack of nutritional elements: mainly lack of vitamin D and calcium, which are essential elements for the healthy growth of toenails, and should be supplemented when necessary to improve the symptoms. Hollow toenails can also be seen in other causes, it is recommended to consult a doctor in a timely manner, in accordance with the doctor’s instructions for the relevant examination, with the help of the doctor to clarify the cause, and targeted treatment or treatment.