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.  (J Intern Med Taiwan 2021; 32: 56-62)