FRI504 Ectopic Hyperactive Mediastinal Thyroid Tissue: A Case Report

Disclosure: C. Dimech: None. G. Jaiswal: None. A. Bhargava: None. Ectopic thyroid tissue is usually located along the normal path of the caudal migration of thyroid tissue during embryonic life. Substernal goiters are thought to occur by either the enlargement of ectopic mediastinal thyroid tissue (primary mediastinal goiter) or abnormal extension of thyroid issue into the superior mediastinum (secondary mediastinal goiter). Typically these goiters remain asymptomatic, and rarely result in hyperthyroidism or local compression of the surrounding area. We report a case of an 89-year-old Caucasian female with history of a multinodular goiter status post right hemithyroidectomy, who initially presented for subclinical hyperthyroidism. Further evaluation revealed negative thyroid stimulating antibodies. A thyroid ultrasound demonstrated a surgically absent right lobe, with a small heterogenous left lobe. A nuclear medicine radioactive iodine uptake and scan showed hypermetabolic residual thyroid tissue with a large area of increased radiotracer uptake projecting over the right anterior chest, concerning for ectopic thyroid tissue. A subsequent chest CT without contrast revealed an approximately 5 centimeter partially calcified anterior mediastinal/retrosternal mass. Given the patient’s age and co-morbidities, a decision was made to treat the hyperthyroidism with methimazole and monitor the ectopic tissue for worsening symptoms of compression. In cases with severe compressive symptoms, or difficult to treat hyperthyroidism, a surgical option must be considered. Presentation: Friday, June 16, 2023