Acute suppurative thyroiditis with gas formation is very rare. A 70-year-old woman was admitted with dysphagia and fever. The clinical diagnosis of acute thyroiditis was supported by fine needle aspiration biopsy. X-ray examination showed gas collection within the soft tissue of the anterior neck and ultrasonography of the thyroid gland showed a cavity filled with thick liquid. The patient was treated with antibiotic therapy followed by thyroidectomy. Escherichia coli was the causative micro-organism cultured from the pus specimen.