**Core Concept**
A thyroglossal cyst is a congenital anomaly resulting from the failure of the thyroglossal duct to obliterate during embryonic development. This duct is responsible for the migration of the thyroid gland from its origin near the foramen cecum of the tongue to its final position in the neck.

**Why the Correct Answer is Right**
Thyroglossal cysts typically present as midline neck masses that move upward with tongue protrusion, a characteristic feature that helps distinguish them from other types of neck masses. The cysts contain squamous epithelium and thyroid tissue remnants, which are remnants of the thyroglossal duct. Surgical excision, often via the Sistrunk procedure, is the treatment of choice for thyroglossal cysts to prevent recurrence.

**Why Each Wrong Option is Incorrect**
* **Option A:** This statement is not accurate. Thyroglossal duct cysts indeed contain thyroid tissue remnants but not functioning thyroid gland tissue.
* **Option B:** This statement is false. The movement of the cyst with tongue protrusion is a classic clinical sign of thyroglossal duct cysts.
* **Option D:** This statement is not true. Thyroglossal cysts are benign and do not metastasize.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that thyroglossal duct cysts typically present in the midline of the neck and demonstrate upward movement with tongue protrusion, a characteristic feature that aids in their diagnosis.

**Correct Answer:** D.