Ectopic Thyroid Tissue Mimicking a Thyroglossal Duct Cyst in a Child: A Diagnostic Pitfall Leading to Postoperative Hypothyroidism

Ectopic thyroid tissue is an uncommon developmental anomaly resulting from abnormal migration of the thyroid gland.It may occur anywhere along the embryological descent pathway and can clinically mimic a thyroglossal duct cyst (TGDC).Recognition before surgery is particularly important because ectopic thyroid tissue may represent the patient's only functioning thyroid tissue, and its inadvertent excision can result in hypothyroidism.A 10-year-old girl presented with a painless midline neck swelling that had been present since childhood and had increased slightly in size over the preceding year.The swelling moved with deglutition and tongue protrusion, and a clinical diagnosis of TGDC was made.Preoperative thyroid function testing showed mildly elevated thyroid-stimulating hormone (TSH) of 5.455 µIU/mL (normal range 0.4 to 4.0 µIU/mL).Fine-needle aspiration cytology was reported as Bethesda category II, consistent with a benign cystic thyroid nodule.Ultrasonography suggested a thyroid lesion, while contrast-enhanced computed tomography demonstrated a 29 × 32 × 36 mm solid-cystic lesion anterior and inferior to the hyoid bone, with the possibility of a TGDC containing ectopic thyroid tissue.Importantly, the normally located thyroid gland was not visualized.A separate 8 × 7 mm hyperdense lesion at the tongue base was considered radiologically suggestive of ectopic thyroid tissue.The patient underwent Sistrunk's procedure.Histopathological examination unexpectedly confirmed ectopic thyroid tissue.On postoperative day seven, TSH increased markedly to 99.65 µIU/mL, and levothyroxine therapy was initiated.Subsequent Tc-99m pertechnetate thyroid scintigraphy demonstrated no tracer uptake in the thyroid bed or at recognized ectopic sites.Ectopic thyroid tissue should be considered in the differential diagnosis of a midline neck swelling clinically suggestive of TGDC, particularly when the normally located thyroid gland is not visualized.Identification of the anatomical and functional status of thyroid tissue before surgery is essential to minimize the risk of postoperative hypothyroidism.This case emphasizes the importance of integrating clinical, radiological, cytological, and functional assessment before excision of presumed TGDCs in children.