Delayed Diagnosis of Ectopic Thyroid Due to Ignored Borderline Result of Newborn Screening for Congenital Hypothyroidism.

BACKGROUND Newborn screening for congenital hypothyroidism is a critical public health measure for early detection and intervention, but borderline results can contribute to diagnostic delays. This case describes a patient with lingual thyroid-related congenital hypothyroidism whose diagnosis was delayed despite a borderline newborn screening result. CASE REPORT A girl first evaluated at age 8 presented with a history of constipation and cold intolerance since age 5, along with recent poor concentration in school. She had no history of fatigue, hair loss, throat fullness, or difficulty swallowing. Physical examination revealed normal vital signs and growth, but no thyroid gland was present in her neck. A 3×5 cm red, well-vascularized mass was identified at the base of her tongue, which moved during swallowing. Thyroid function tests showed elevated thyroid-stimulating hormone (13.87 mIU/L, reference range: 0.35-4.94 mIU/L) and low free thyroxine (8.43 pmol/L, reference range: 9.00-19.00 pmol/L), confirming hypothyroidism. Review of her newborn screening results revealed a borderline thyroid-stimulating hormone level (43.647 mIU/L, borderline reference range: 30-60 mIU/L) and normal free thyroxine (13.7 pmol/L). An isotope thyroid scan confirmed a lingual thyroid. Her symptoms improved after initiation of levothyroxine therapy, and she has continued endocrine follow-up through age 15. CONCLUSIONS This case underscores the importance of a thorough oropharyngeal and neck examination in patients with a history of borderline newborn screening for congenital hypothyroidism. Early detection of an ectopic thyroid gland is essential for anticipating and preventing future thyroid decompensation, thereby ensuring timely treatment and improved long-term outcomes.