Abstract

Abstract Disclosure: C.M. Godar: None. A.J. Spiro: None. K.F. Brown: None. M.K. Shakir: None. T.D. Hoang: None. Introduction: Thyroglossal duct cysts are the most common congenital cyst in the head and neck, found during childhood as a midline neck mass. We present a rare case of a large intrathyroidal thyroglossal duct cyst with left lateral extension in an adult patient. Case Report: A 50-year-old man with family history of familial adenomatous polyposis presented for thyroid nodule evaluation. Thyroid ultrasound for screening of thyroid cancer showed a 4.8 x 4.5 x 2.5cm mixed cystic/solid hyperechoic thyroid nodule with smooth margins in the left lower thyroid lobe. He reported no compressive symptoms. FNA with standard 27G needle was insufficient for evaluation with Bethesda I result. Attempted syringe aspiration did not produce cystic fluid for analysis. The patient returned to clinic 8 weeks later due to visible enlargement of his anterior neck and new neck pressure with swallowing. Physical exam was significant for a new, firm palpable mass over the left thyroid isthmus. Bedside ultrasound noted expansion of the previously visualized mass now extending into the thyroid isthmus. FNA was reattempted with a 25G needle, and cytologic material was again inadequate. He elected to undergo left hemithyroidectomy. Histologic sections demonstrated a 3.4cm simple squamous, non-keratinizing cyst lining overlying a thickened fibrotic wall with mixed acute and chronic inflammation surrounded by benign thyroid tissue, consistent with an intrathyroidal thyroglossal duct cyst. Discussion: The thyroglossal duct derives from epithelial tissue during embryogenesis and atrophies during weeks 8-10 of gestation. A persistent duct may lead to cyst formation in the presence of remnant epithelial tissue secretions and local neck inflammation. A thyroglossal duct cyst is usually located midline in the anterior neck. This stands in comparison to a branchial cleft cyst, which arises due to failure of atrophy of the second branchial groove during embryogenesis and forms a cyst that is usually located in the upper lateral neck compartment. Other common neck masses include lipomas, dermoid cysts, or ectopic thyroid tissue. This case highlights the rarity of intrathyroidal thyroglossal duct cyst in an adult patient and the importance of definitive diagnosis and appropriate management. Presentation: 6/2/2024

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