SPR Unknown # 60 - FINAL
Publication Date: 2011-10-18
A Tc-99m pertechnetate scan demonstrates a focal area of abnormal uptake in the region of the base of the tongue. No radiotracer activity is appreciated in the expected location of the thyroid bed. Subsequent ultrasound of the region of interest at the base of the tongue demonstrates a small homogenous echogenic soft tissue mass, corresponding to a lingual thyroid.
Generally, congenital hypothyroidism is suspected from the newborn screen. The most common cause of congenital hypothyroidism is thyroid dysgenesis (70-75%). Amongst thyroid dysgenesis, a subset is caused by ectopic thyroid tissue (up to 50%), with 90% located at the base of the tongue (foramen cecum). Absent thyroid hormone prevents normal growth and development. Rarely, central congenital hypothyroidism may be a cause, and can be differentiated from ectopic thyroid tissue based on whether serum TSH is elevated or depressed.
Normally, the thyroid gland migrates from the base of the tongue at foramen cecum, and descends caudally along the tract of the thyroglossal duct to the expected location within the neck. Occasionally, embryonic rests may remain along this tract. In the setting of hyperstimulation or autonomous functioning, airway obstruction is a potential complication as ectopic thyroid tissue will often enlarge like a traditional goiter. In such an instance, a mass may be palpated by the clinician.
Diagnostic imaging encompasses a multimodality approach. In Tc-99m pertechnetate thyroid scintigraphy, the newborn is injected with approximately 200-300 μCi with images obtained at 15-20 minutes. Preferably, anterior parallel-hole collimator images are acquired. A lingual thyroid will show absent uptake within the expected region of the thyroid bed with an abnormal ectopic focus of activity along the tract of the thyroglossal duct. Importantly, one must also use markers to delineate the thyroid bed.
Ultrasound is used as an adjunct to confirm the absence or dysplasia of thyroid tissue in the traditional thyroid bed. If the region of interest can be scanned, sonography would also demonstrate any potential echogenic lesion corresponding to an ectopic focus along the midline nec
Epidemiologically, females are twice as likely to be affected by congenital hypothyroidism. Newborns are generally asymptomatic with the only clue from the newborn screen demonstrating markedly elevated serum TSH, reflecting the absence of negative feedback. If an infant or young child presents older, they usually demonstrate findings of growth and mental retardation.
Early diagnosis and immediate thyroid hormone replacement is critical, as the prognosis is poor for normal mental development if therapy is delayed after one year of age. Conversely, 80% of affected patients treated within three months of birth respond with normal intellectual development. Additional treatment such as surgical excision can be performed if the ectopic focus becomes goiterous. Also, if a lingual thyroid is left in, a 3-5% incidence of papillary carcinoma remains.
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