The thyroid gland develops in the third week in utero as a diverticulum from the foramen caecum at the junction of the oral cavity and the oropharynx. Abnormalities of descent can lead to lingual thyroids and/or thyroglossal duct cysts. As the gland migrates, it incorporates cells from the neural crest. Some of these cells become C cells, which will eventually secrete calcitonin; others become the parathyroid glands.
In the presence of a lingual thyroid, these cells (the cells from the neural crest) migrate to their eventual positions in the neck in utero. Their embryological development is therefore unaffected by the failure of the thyroid gland to descend.