Development of the thyroid gland begins as a ventral endodermal diverticulum in the floor of the developing oral cavity. The origin of the thyroid is marked by the presence of the foramen cecum on the surface of the tongue.
Initially, the thyroid gland is connected to the oral cavity by way of a thyroglossal duct, which eventually disappears. The thyroid gland migrates inferiorly through the anterior neck to become located over the anterior surface of the upper tracheal rings. As a result, ectopic thyroid tissue may be located anywhere along the migratory path. A common location for these masses of thyroid tissue is in the tongue.

Thyroid follicular cells, that produce active thyroid hormone, thyroxine, are developed form the endodermal cells.

Neural crest cells that interact with the ultimobranchial body form a second type of endocrine cell within the thyroid gland. These are the parafollicular cells that secrete calcitonin, and calcium regulating hormone.