Consider hyperthyroidism, trauma, tumor, sternocleidomastoid tendonitis and other causes. 1. Hyperthyroidism: diffuse enlargement of the thyroid gland can occur, accompanied by increased heart rate, hyperphagia, emaciation, and thyroid function tests can make a clear diagnosis. 2. Trauma: local soft tissue swelling after trauma to the neck can cause pain and restriction of turning the head to one side. 3. Tumor: thyroid tumor, stomach tumor or lung tumor metastasis to the lymph nodes in the neck, there can be swelling on one side of the neck with enlarged lymph nodes, pathological biopsy can confirm the diagnosis. 4. Sternocleidomastoid tendonitis: due to chronic strain injury, spasm of sternocleidomastoid tendon, localized swelling and pressure pain. There are many possible causes of swelling on one side of the neck, and you should consult a doctor in time for examination and treatment under the doctor’s guidance.