D2I2.
congenital

Thyroglossal cyst

A thyroglossal cyst or thyroglossal duct cyst is a fibrous cyst that forms from a persistent thyroglossal duct. Thyroglossal cysts can be defined as an irregular neck mass or a lump which develops from cells and left over after the formation of the gland during developmental stages.

Underlined words are explained — tap any of them.

Plain-language summary adapted from Wikipedia. Not medical advice.