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•The thyroid originates as an invagination of the epithelium covering the base of the tongue.

•This invagination extends caudally as the thyroglossal duct.

•On reaching the area caudal to the larynx, the thyroglossal duct expands to give rise to the bilobed structure of the thyroid.

•The thyroglossal duct may persist in adulthood, extending rostrally in the median line from the isthmus linking the 2 lobes of the thyroid.

•These may be found anywhere between the isthmus of the thyroid and the tongue as cysts or aberrant thyroid tissue.

•Grossly, the thyroid is a deeply orange/brown structure, faintly lobulated in appearance.

•It weighs approximately 30 grams in the adult, and is somewhat larger in females in whom it can become significantly larger during pregnancy and lactation.

•On cut surface, the organ is a glassy, deep orange/brown, with some evidence of lobulation.

•Microscopically, the thyroid is composed of innumerable spherical acini or follicles.

•The follicles of the thyroid are lined by cuboidal epithelium in a normal resting thyroid gland.

•The cells are uniform in appearance.

•In a very active thyroid, the cuboidal epithelium can appear more columnar.

•Lying within the follicles is material that stains deeply eosinophilic in standard H&E preparations.

•In a resting thyroid, this deeply eosinophilic material, called colloid, is in contact with the lining epithelial cells around the circumference.

•In an active thyroid, however, the rim of the colloid appears scalloped.

•This appearance is indicative of hyperactivity of the thyroid gland.

•In addition to the cells lining the thyroid, additional cells called C cells are distributed both among and between the lining cells.

•These cells, also known as the parafollicular cells, secrete calcitonin.

• Current literature from PubMed at National Library of Medicine