Poorly differentiated thyroid carcinoma (PDTC) is a rare, aggressive malignancy. Cardiac metastasis from PDTC is exceedingly uncommon. We report early cardiac metastasis occurring shortly after radical thyroidectomy to highlight atypical distant spread and management challenges.

A 62-year-old woman presented four months after thyroidectomy with progressive exertional dyspnoea, fatigue, productive cough, facial oedema, lip cyanosis, and dizziness. Postoperative pathology showed poorly differentiated thyroid carcinoma of the right lobe and isthmus (pT2N1bM0) with capsular and recurrent laryngeal nerve invasion. Transthoracic echocardiography revealed a right atrial mass that enlarged to 5.3 × 4.0 cm and extended toward the tricuspid orifice; imaging also demonstrated continuous tumor thrombus involving the right internal jugular vein, brachiocephalic vein, and superior vena cava. Cytology from internal jugular vein puncture showed malignant cells, supporting metastatic intraluminal tumor thrombus with intracardiac extension. After surgery, levothyroxine was initiated as thyroid hormone replacement and for gradual thyroid-stimulating hormone (TSH) suppression as tolerated. During cardiology admission, the patient received antimicrobial therapy, anticoagulation, thrombolysis, and cardiac workload reduction, with symptomatic improvement. She was discharged against medical advice before definitive oncologic treatment could be initiated and was subsequently lost to follow-up.

In aggressive thyroid cancers such as PDTC, clinicians should remain vigilant for atypical distant metastases—including intracardiac extension via venous tumor thrombus—soon after surgery. Early multimodality imaging and coordinated multidisciplinary care are essential for individualized management.