cyst, thyroglossal

cyst, thyroglossal A fluid-filled sac that is present at birth and located in the midline of the neck. A thyroglossal cyst is a congenital malformation (a birth defect). It results from incomplete closure of a segment of the thyroglossal duct, a tube-like structure that normally closes as the embryo develops. Also called a thyroglossal duct cyst or a thyrolingual cyst.