Endocrine Abstracts

Introduction: The thymus is a primary lymphoid organ and the initial site for development of T cell immunological function. It originates high in the neck in early fetal life and reaches its definitive location in the mediastinum after a process of descent, generally leaving no trace behind. Occasionally, during this descent, remnants of thymic tissue can be implanted along the cervical pathway, later appearing as a cervical mass.

Case Report: This is a case of a 39-woman undergoing surgery for a left thyroid nodule. During surgery, a yellowish mass located behind the lower pole of the left thyroid lobe was discovered. The mass was not removed. A biopsy was performed. On histologic examination, it corresponded to a thymic tissue.

Conclusion: The third and fourth pouches are responsible for the development of the parathyroid glands, the thymus, and a portion of the thyroid gland. Ectopic parathyroid tissue can be found anywhere along their pathway of descent. However ectopic thymus is rarer and is usually found at the mediastinum. The diagnosis of ectopic thymus is difficult and can be misleading, it should be suspected in case of yellowish tissue located posteriorly to the thyroid gland. If ectopic thymus is suspected, frozen section biopsy should be performed. If the diagnosis is confirmed, the mass should not be excised unless it is causing dyspnea or dysphagia.