Primary thyroid leiomyosarcoma (LMS) is a very rare tumour with less than 40 published cases yet. Direct metastatic extension into the great cervical veins and caval veins is extremely uncommon. Extension to the right heart has not yet been reported.

A 62-year-old man was admitted for sudden onset of left neck pain and dyspnoea. Computed tomography and ultrasonography found an extensive mass of the left great cervical veins, superior vena cava, and a nodule in the left thyroid lobe. Transesophageal echocardiography visualized large protrusion of this mass into the right atrium. Cytology of a thyroid nodule diagnosed a benign hyperplastic nodule. The mass was considered to be likely an extensive thrombus. The patient was started on anticoagulant therapy. The next course was complicated by pulmonary embolism and later by enterorrhagia. Despite clinical stabilization, the patient died suddenly. Autopsy finding differed from the clinical conclusion. Microscopic investigation revealed that the mass seen in the cervical veins down to the right atrium was a spindle cell tumour with a primary site in the left thyroid lobe. Immunohistochemistry was consistent with the final diagnosis of primary thyroid LMS.

Differential diagnosis of the masses of great cervical veins and right atrium can be challenging. Pure venous thrombus and tumour thrombus must be distinguished. Thyroid LMS should also be considered in patients with masses in the right atrium and thyroid nodules.

Although papillary thyroid microcarcinoma (PTMC) is not considered a threatening tumor, in some cases, it can be aggressive. Metastatic thrombosis of papillary thyroid carcinoma, follicular thyroid carcinoma, Hürthle cell carcinoma, poorly differentiated thyroid carcinoma and anaplastic thyroid carcinoma have been reported in the literature, but there have been no reports about PTMC.

A 45-year-old woman presented with a thyroid mass and thrombosis in a middle thyroid vein during a physical examination. She had no symptoms, and the physical examination showed no positive signs. Subsequent ultrasonography-guided fine-needle aspiration biopsy results indicated an atypical lesion of ambiguous significance, with some actively growing cells (TBSRTC III) and the BRAF^V600E mutation not present. This patient underwent left thyroidectomy, isthmus lobectomy, prophylactic central lymph node dissection and thromboembolectomy. Postoperative pathology showed papillary microcarcinoma of the left thyroid, and the thrombus in the middle thyroid vein was a tumor thrombus.

Middle thyroid vein tumor thrombus is an extremely rare condition in PTMC, but it does exist. Lobectomy and thromboembolectomy may be an option for patients with thrombi in the middle vein of the thyroid, and we strongly suggest close follow-up of these patients.

• Thyroid neoplasms
• Papillary carcinoma
• Thyroid vein
• Venous thrombosis
• Surgery
• Case report

• Internal jugular vein
• Thrombosis
• Thrombus
• Thyroid cancer