Answer: Ectopic thyroid tissue (i.e. lingual thyroid)

Histology: A tumor nodule occupies the musculature of the tongue. Although the tumor is relatively circumscibed, in some areas it displays an irregular interface with the surrounding tissues. In these areas, the tumor shows extension among skeletal muscle fibers. The tumor is comprised of variably sized follicles filled with pink colloid-like material. Papillary formations are not present. The cells lining the follicles are round and uniform. They do not demonstrate appreciable nuclear crowding, nuclear grooves or intranuclear inclusions.

Discussion: Reflecting its embryologic path of descent, ectopic thyroid tissue is consistently noted along the midline of the neck anywhere from the base of the tongue (lingual thyroid) to the region of the hyoid bone (component of thyroglossal duct cyst), to the anterior mediastinum. Migration failure may be partial or complete. For complete migration failure, removal of the lingual thyroid necessitates thyroid hormone replacement therapy.

The presence of thyroid tissue beyond the thyroid gland raises the possibility of thyroid ectopia (e.g. lingual thyroid), parasitic thyroid nodules in the setting of multinodular hyperplasia, and extrathyroid spread of thyroid carcinoma either by direct local extension or metastatic implantation. Morphologically, lingual thyroid is comprised of thyroid follicles that are lined by normal appearing follicular epithelial cells. It lacks the papillary growth and the nuclear atypia that characterize papillary thyroid carcinoma. Further, ectopic thyroid tissue lacks the encapsulation with vascular and transcapsular invasion that characterize follicular thyroid carcinoma. Ectopic thyroid does, however, entrap skeletal muscle and adipose tissues in the midline of the neck. This finding is quite common and should not be construed as invasive tumor growth. One should always be alert to the possibility of malignant transformation in ectopic thyroid tissue, but this is rather infrequent for lingual thyroid. Only a handful of well-documented cases have been reported in the literature.