Heterogeneous tissue in the thyroid fossa on ultrasound in infants with proven thyroid ectopia on isotope scan--a diagnostic trap.

BACKGROUND: Thyroid imaging is of proven help in establishing a diagnosis of congenital hypothyroidism in infants. US often shows tissue in the thyroid fossa when radionuclide scintigraphy reveals only ectopic uptake. OBJECTIVE: Our hypothesis was that the use of US alone could lead to the mistaken diagnosis of normal or dysplastic thyroid in cases of scintigraphy-proven thyroid ectopia. MATERIALS AND METHODS: We undertook a detailed retrospective review and analysis of imaging and concurrent biochemistry in infants with thyroid ectopia, confirmed by radionuclide scintigraphy. RESULTS: Eighteen infants had thyroid ectopia; ten of the original US reports had suggested that cervical thyroid tissue was present. Review showed bilateral tissue in the thyroid fossa in all that was non-thyroidal in nature since, apart from showing no radionuclide uptake, it exhibited some or all of the following typical features: hyperechogenicity, heterogeneity, small size, poor vascularity, and anechoic and/or hypoechoic cysts. Also, extension of the tissue both around and behind the large cervical blood vessels was a universal finding. CONCLUSION: Considerable experience is required to interpret neonatal thyroid US. We caution against diagnosing a dysplastic/hypoplastic thyroid gland in situ on the basis of US alone, particularly if the tissue exhibits any of the non-thyroidal features described.