Pulmonary Metastatic Follicular Thyroid Carcinoma Without Intrathyroidal Primary Thyroid Cancer

Background/Objective: Follicular thyroid cancer without an intrathyroidal primary cancer is rare. We present a patient with multifocal pulmonary metastatic follicular thyroid cancer without apparent cancer within her thyroid. Case Report A 44-year-old woman is referred to the thyroid cancer clinic via telemedicine for evaluation of intrapulmonary thyroid tissue. Past medical history included Roux-en-Y gastric bypass, hysterectomy with bilateral oophorectomy. Six months prior abdominal CT showed incidental bilateral lung nodules. Chest CT demonstrated 4 solid left and 1 solid right lung nodules. Lung nodule core biopsy revealed benign thyroid tissue. Thyroid ultrasound showed bilateral subcentimeter anechoic nodules. Chest CT 6 months after initial CT demonstrated stable lung nodules. TSH was 1.63 mIU/L (0.3-5.5), serum thyroglobulin 40.9 ng/mL (0-35), and thyroglobulin antibody < 1 IU/mL (< 4). PET/CT scan showed FDG-avid lung lesions 1.5 cm, 1.1 cm, 2.2 cm and others subcentimeter. Repeat lung core biopsy showed thyroid tissue with microfollicular architecture, favoring metastatic follicular carcinoma with NRAS mutation. Total thyroidectomy performed showed multinodular hyperplasia without thyroid cancer. Post-operative radioiodine scan demonstrated bilateral iodine avid pulmonary nodules, serum thyroglobulin 179.8 ng/mL, TSH 151.3 mIU/L and undetectable serum thyroglobulin antibody. She received 261 mCi of radioactive iodine. Fourteen months later chest CT revealed decreased lung nodules and serum thyroglobulin 0.7 ng/ml. Discussion About two cases of multifocal pulmonary follicular thyroid cancer without a primary source and no other site of metastasis have been reported. Conclusion Pulmonary follicular thyroid cancer without a primary source and no other site of metastasis is extremely rare.