Lumps on the left side of the neck may be caused by enlarged lymph nodes and thyroid diseases. 1. Enlarged lymph nodes: there are abundant lymph nodes in the neck, which are mainly responsible for the lymphatic return of the ear, nose and throat, oral cavity, etc. When these parts of the above mentioned areas have lesions, it can lead to enlarged lymph nodes on the left side of the neck and lumps, such as gingivitis, sinusitis, etc.; when certain malignant tumors have metastasis, it can also appear as enlarged lymph nodes on the left side of the neck, such as nasopharyngeal carcinoma, gastric carcinoma, etc. 2. Thyroid disease: Normal thyroid tissue is located in the neck below thyroid cartilage, on both sides of trachea, when thyroid tissue is diseased, it may also lead to the appearance of left neck lump, such as left thyroid cyst, left thyroid hyperfunctioning adenoma, etc. When thyroid tumor occurs, it may also lead to the appearance of left neck lump. When tumor occurs in the thyroid gland, swelling of the adjacent lymph nodes may also occur, leading to the appearance of lumps, such as medullary thyroid cancer, papillary thyroid cancer, etc. There may be other reasons for the lump on the left side of the neck, so it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and then give targeted treatment or therapy under the guidance of the doctor.