A 57-year-old woman with an adenomatous goiter of the thyroid gland for 3.5 years, suffered rapid enlargement of the thyroid gland. Aspiration cytology showed only a few tumor cells having bizarre nuclei and abundant cytoplasm. Thyroidectomy and subsequent chemotherapy were ineffective, and the patient died within 4 months after the thyroidectomy. Thyroid gland histology showed an undifferentiated carcinoma composed of pleomorphic tumor cells with a few osteoclast-like giant cells. The present case shows that a few tumor cells with bizarre nuclei in cytology during the course of an adenomatous goiter of the thyroid gland indicates development of an undifferentiated carcinoma.
