惡性骨肉瘤(osteosarcoma)罕見原發於甲狀腺的部位，目前在期刊發表只有十多個案例。病人起初以腹瀉與體重減輕為表現，接受抗甲狀腺藥物(antithyroid agents)治療甲狀腺功能亢進。甲狀腺腫瘤經穿刺細胞學檢查(fine needle aspiration cytology)，並無發現惡性細胞(negative for malignancy cells)。後來呼吸困難與咳嗽痰中帶血，發現甲狀腺腫瘤中有鈣化斑塊，壓迫氣管造成呼吸衰竭，甲亢治療穩定後進行手術，病理報告是原發性甲狀腺惡性骨肉瘤，術後接受化療與放射線治療。原發性甲狀腺惡性骨肉瘤(primary thyroid osteosarcoma)，是分化良好的甲狀腺腫瘤，容易與治療預後不佳的甲狀腺不分化癌(anaplastic thyroid carcinoma)混淆，有大片鈣化斑塊是特徵之一。甲狀腺惡性骨肉瘤成長速度極快，且腫瘤位於致命的呼吸道位置，需外科緊急介入手術，是影響預後的關鍵因素。

Primary thyroid osteosarcoma is a very rare primary malignancy of the thyroid gland. Only a dozen of cases were reported in published journals. We reported a case of a 83-year-old male who originally presented with diarrhea and weight loss. He had a history of hyperthyroidism and had been taking methimazole before thyroid ultrasound found a 3.5cm left nodular goiter with intrathoracic extension. Fine needle aspiration cytology reported negative for malignant cells. Three years later, the patient was admitted due to dyspnea and blood-tinged sputum. Chest X ray and CT scan found enlargement of the left thyroid lobe with calcification and thoracic extension, nodules in both lungs and trachea. He was immediately intubated and underwent surgical removal of intrathoracic goiter and right lung nodule. The pathology reported left intrathoracic thyroid osteosarcoma with lung metastasis. Removal of the tracheal nodule and tracheal stent implantation were performed via bronchoscopy. Palliative chemotherapy was prescribed and radiotherapy was planned. However, due to rapid progression of the residual tumor causing compression and obstruction of airway, he died of respiratory failure. The primary thyroid osteosarcoma is well-differentiated but with special characteristics of rapid tumor growth and coarse calcification. Urgent surgical intervention needs to be considered due to rapid progression, which determines the prognosis of thyroid osteosarcoma.