Abstract

Midline ectopic thyroid, as the only functioning thyroid, is very rare and used to be thought that these thyroid remnants do not undergo malignant transformation. We experienced a case of carcinoma arising in midline ectopic thyroid (representing the total thyroid mass) including thyroglossal duct remnant in a 20 year old man. The cervical mass was explored and removed under the impression of thyroglossal duct cyst. The mass was located in thyroid cartilage area and was connected to the hyoid bone by a fibrous tract. The tumor was ovoid and well encapsulated. It measured 3.5×4×4 cm in maximum dimension. Microscopically, the vast majority of the mass consisted of hyperplastic and involutional thyroid follicles admixed with dense collagen fibrosis. There were foci of infiltrative solid nests of small follicles within fascia and skeletal muscle. The follicular lining cells showed moderate cellular atypism. A well encapsulated thyroid nodule adjacent to hyoid bone revealed disorganised follicles with atypical, vesicular lining cells. Several ductal structures lined by stratified squamous epithelium were also encountered in the hyoid bone level, representing thyroglossal duct remnants. The histological features of the carcinoma seen in this case suggested the follicular carcinoma rather than papillary carcinoma.