Ectopic thymic remnant incidentally discovered in the parathyroid region: a case report

Ectopic thymic tissue in the cervical region is a rare developmental anomaly stemming from the shared embryological migration of the thymus and inferior parathyroid glands from the third pharyngeal pouch. Ectopic thymic remnants can macroscopically mimic parathyroid tissue, posing diagnostic and surgical challenges. We report an unusual case of an ectopic thymic remnant incidentally discovered during surgery for secondary hyperparathyroidism in a patient with end-stage renal disease (ESRD). A 29-year-old woman with ESRD on chronic hemodialysis presented with severe secondary hyperparathyroidism (parathyroid hormone [PTH]: 1691 pg/ml). SPECT/CT identified a single hyperfunctioning right posteroinferior gland. Intraoperatively, four glands were identified; three macroscopically enlarged glands (right inferior, left superior, and suspected left inferior) were excised, while the smaller right superior gland was preserved in situ to achieve a subtotal parathyroidectomy. Histopathology confirmed parathyroid hyperplasia in two specimens and revealed that the left inferior specimen consisted entirely of ectopic thymic tissue with Hassall’s corpuscles, verified as thymic by immunohistochemistry (TdT+, CK5+, p63+) and lacking parathyroid parenchyma. Consequently, the true left inferior parathyroid gland remained unexcised, leaving two glands intact in situ. Institutional constraints precluded intraoperative PTH monitoring and frozen section analysis, which would have prompted immediate further exploration. Nevertheless, the excision of the two hyperplastic glands achieved biochemical control, with postoperative PTH falling to 106 pg/ml (a reduction greater than 50% from baseline). Lifelong surveillance was initiated, with any future reintervention guided by long-term PTH kinetics targeting a nephrologist-defined threshold of ~ 500 pg/ml. Ectopic thymic tissue can macroscopically mimic parathyroid hyperplasia in ESRD, creating a pitfall for incomplete parathyroidectomy. This case underscores the necessity of systematic four-gland exploration, highlights the critical value of frozen section analysis and intraoperative PTH monitoring for real-time guidance, and reaffirms the indispensability of definitive histopathological and immunohistochemical examination.