Thyroglossal duct cyst

• Painless, midline anterior neck mass in a child that elevates with tongue protrusion
• Thin-walled cystic lesion on ultrasound with posterior acoustic enhancement and splaying of strap muscles
• Cyst located anywhere along the thyroglossal duct course (suprahyoid ~20-25%, at hyoid ~30%, infrahyoid ~45%) with most (~70%) in true midline

• Ultrasound: hypo- or anechoic thin-walled cyst with posterior acoustic enhancement, splaying of strap muscles, and usually no internal vascularity unless infected; may show internal complexity with debris, septa, or proteinaceous material even without infection, particularly in adults.
• CT: thin-walled, smooth, well-defined homogeneous lesion with variable density (20-80 HU) in anterior midline or paramedian location; demonstrates thin rim (capsular) enhancement; infrahyoid cysts typically embedded within strap muscles.
• MRI T1: variable signal intensity—low signal if low protein or uncomplicated, but most commonly high signal due to previous hemorrhage, infection, or high proteinaceous content.
• MRI T2: typically high signal consistent with cystic/fluid content.
• MRI T1 with gadolinium: no enhancement in uncomplicated cysts, or thin peripheral rim enhancement may be seen.
• Ectopic thyroid tissue occurs in approximately 40% of cases as a small (~0.5 cm) focus within the cyst wall or adjacent soft tissues and may enhance.
• Infected cysts show thickened or irregular walls with increased blood flow and surrounding inflammatory change on ultrasound; any internal enhancement or solid component raises concern for rare malignancy (almost always papillary thyroid carcinoma).
• Location distribution: ~70% are truly midline, ~30% are off-midline (typically adjacent to thyroid cartilage), and virtually all are within 2 cm of the midline.

• Branchial cleft cyst: three times less common and usually located well away from the midline, whereas thyroglossal duct cysts are characteristically midline.
• Epidermoid cyst: located superficial to the strap muscles, whereas thyroglossal duct cysts splay and embed within the strap muscles.
• Thyroid cyst or thyroid neoplasm: originates from the thyroid gland itself rather than the midline anterior neck along the course of the duct.
• Delphian adenopathy: lymph node enlargement in the midline anterior neck that enhances and does not show simple cystic morphology.
• Laryngocele: air-filled lesion typically lateral to the midline with characteristic air density/signal.
• Ectopic thyroid: may be associated with thyroglossal duct cyst as a soft tissue lesion, but presents as enhancing thyroid tissue rather than a fluid-filled cyst.

Describe the cyst by its location relative to the hyoid bone (suprahyoid, at hyoid, or infrahyoid) and note whether it is truly midline or off-midline; document any solid components, internal enhancement, or ectopic thyroid tissue, and comment on whether the cyst is uncomplicated or shows signs of infection; if infected, describe wall thickening and surrounding inflammatory change.

• Mistaking a thyroglossal duct cyst for a branchial cleft cyst: remember that thyroglossal duct cysts are characteristically midline (~70%), whereas branchial cleft cysts are typically well away from the midline.
• Overlooking internal complexity and proteinaceous debris in adult patients: even uncomplicated cysts may show heterogeneous internal echoes or intermediate signal intensity on MRI, particularly in older patients, and this does not necessarily indicate infection or malignancy.
• Failing to recognize ectopic thyroid tissue (~40% of cases) as a soft tissue lesion associated with the cyst, which may be confused with suspicious solid component or malignancy.
• Missing signs of malignancy: solid enhancing components, internal calcification, or marked enhancement should raise suspicion for papillary thyroid carcinoma, which occurs in ~2% of cases and is often discovered incidentally after surgical resection rather than preoperatively.