Undescended Ectopic Parathyroid Adenoma Within Ectopic Cervical Thymus in the Carotid Sheath

Background: Due to their shared embryologic origin, the association between ectopic parathyroid glands and the thymus is well recognized. Abnormal implantation of thymic tissue during embryological migration to the mediastinum can lead to ectopic cervical thymus. Methods: A 33-year-old woman with persistent primary hyperparathyroidism (89 pg/mL) was referred following failed parathyroidectomy. Preoperative 4D computed topography identified soft tissue in the right carotid sheath corresponding to an area of increased uptake on sestamibi imaging. Revision parathyroidectomy was performed. Results: Intraoperatively, thymic tissue and an associated parathyroid adenoma were found in the right carotid sheath extending superiorly toward the hyoid bone. Post-excision parathyroid hormone levels fell within normal limits (24.2 pg/mL). Conclusions: We describe a novel location for an ectopic parathyroid adenoma within ectopic cervical thymic tissue outside of the thyroid bed and mediastinum. This highlights embryologic relationships that may aid in diagnostic and therapeutic management, particularly in revision or non-localizing cases.