Patient: female, 34 years old, consulted 3 years ago for “hand tremor and sweating”, diagnosed as primary hyperthyroidism, treated with methimazole and other drugs with poor results; admitted to the hospital 2 months ago for surgery, with typical symptoms and signs of hyperthyroidism; blood FT3 and FT4 elevated, TSH decreased; ultrasound showed The ultrasound showed “fire sea sign” and ultrasound changes consistent with toxic diffuse goiter, and no enlarged lymph nodes in both necks. He was admitted for subtotal bilateral thyroidectomy and discharged on the 5th day postoperatively; pathology showed: (left and right) thyroid considered as undifferentiated carcinoma and the rest as toxic nodular goiter; he was admitted for a second time on the 12th day postoperatively for total resection of the residual thyroid and lymph node dissection in the neck; pathology showed: (left and right) thyroid without cancer cells; (both neck lymph nodes) metastatic undifferentiated thyroid carcinoma. Microscopic examination showed that the heterogeneity of the cancerous tissue was obvious, with bundle-shaped arrangement of cancer cells and the coexistence of proliferating follicular epithelial cells; the cancer cells were clearly heterogeneous, with many nuclear divisions, round or ovoid, and bundle-shaped arrangement. The patient is in good general condition at the present follow-up. Summary: Primary hyperthyroidism combined with thyroid cancer is rare clinically, mainly papillary or follicular thyroid cancer, while undifferentiated carcinoma is rare. The median survival time is 2-12 months and the 5-year survival rate is only 5%. The pathogenesis of undifferentiated thyroid cancer may be closely related to TSH, as the feedback of anti-hyperthyroid drugs stimulates the increase of TSH, which in turn activates the genes related to cell proliferation and differentiation and induces malignant tumor development. According to the sixth revised TNM staging criteria of the International Union Against Cancer (UICC), all cases of undifferentiated carcinoma, once diagnosed, are stage IV. Most scholars prefer to perform total bilateral thyroidectomy and/or lymph node dissection in the neck, followed by radiotherapy, chemotherapy or gene therapy. In our case, total thyroidectomy and cervical lymph node dissection were performed in the second operation, and levothyroxine was given after the operation to reduce the burden of postoperative radiotherapy and chemotherapy, to slow down the growth of tumor, and to prolong the patient’s survival. If the thyroid gland suddenly enlarges recently and is accompanied by enlarged lymph nodes, it should be alerted to the possibility of combined thyroid cancer and promptly undergo surgery.