A 42-year-old woman inadvertently discovered a neck mass, which caused pain. Initially, she was treated with antibiotics at a local clinic; however, this treatment did not alleviate the symptoms. She visited the authors' outpatient clinic for further treatment and underwent thyroid ultrasonography, which revealed a mixed echo nodule. On day 4 after admission, surgery was performed to remove the diseased thyroid tissue and levofloxacin (0.4 g/day) was infused. Bacterial culture confirmed infection with Clostridium perfringens. Subsequently, the treatment was switched to ceftriaxone sodium (2 g/day) according to the results of the drug sensitivity test. Following treatment, the patient recovered fully and was discharged. She was then followed up with after discharge. Ultrasonography, laboratory testing and clinical manifestations did not indicate obvious abnormalities.