Thyroglossal duct cyst is a remnant of the thyroglossal duct formed during embryonic life. The thyroid gland migrates from the posterior part of the tongue into the neck via this duct. If the duct persists, it gives rise to a swelling which can be located anywhere along the path of migration of thyroid.
As the cyst is attached to the tongue, protrusion of the tongue results in superior movement of the cyst. This is a characteristic clinical sign for this condition.