Retropharyngeal Ectopic Thyroid Found Histologically in Late-Term Fetuses: A Case Report With an Embryological Study

OBJECTIVE: To identify a pathogenesis of retropharyngeal ectopic thyroid (ET) independent of the thyroglossal duct-derived usual ET. METHODS: We incidentally found a unilateral retropharyngeal ET in histological sections of a fetus at 28 weeks of gestational age (GA). The other histological sections from 21 embryos and fetuses (GA 5-8 weeks and 14-18 weeks) were examined to consider the pathogenesis. RESULTS: The retropharyngeal ET existed with the usual thyroid and it contained loosely distributed clusters of follicles without capsule. In the other 21 specimens, we found 3 candidates of the pre-laryngeal ET and they contained tightly packed follicles with a thick capsule. Thus, the retropharyngeal ET was most likely different from the pre-laryngeal ET in structure. At 6 to 7 weeks of GA, the developing thyroid was composed of abundant trabeculae. Notably, at their posterior ends, some trabeculae were adjacent to the hypoglossal nerve as well as the nodosa ganglion of the vagus nerve. CONCLUSION: During ascent of the hypoglossal nerve and nodosa ganglion, the trabecular thyroid seemed to descend along the anterior aspect of these nerve elements. Therefore, the nerve(s) might swipe and cut a trabecula to separate the fragment from the main lobes, resulting in the retropharyngeal ET.