Ectopic intrathyroidal thymus in children: a long-term follow-up study

Ectopic intrathyroidal thymus has recently been reported in children as a cause of surgery and/or invasive diagnostic procedures when mistaken for a thyroid nodule. Thymus has a unique appearance at ultrasound (US).

We report a follow-up study (mean 34 months, range 6-84) performed by US on 9 children (5 females) with a mean age of 6.3 ± 3.2 years with intrathyroidal thymic inclusions diagnosed by US as 'incidentalomas'. None has palpable nodules.

Intrathyroidal thymic inclusions appeared on US as a hypoechoic area, with regular linear or punctuate internal hyperechoic echoes. The 2 oldest patients (13 and 17 years) showed a regression in both size and hypoechogenicity of thymic inclusions over time--reflecting the normal thymic involution with advancing age.

Indeed, the lack of progression seen in our 9 patients over a mean time of 34 months confirmed the substantially benign and self-limited nature of this process. The increasing use of thyroid ultrasonography in children may result in an increased detection of intrathyroidal thymic inclusions--an embryologic anomaly that should be considered in the differential diagnosis of thyroid nodules in children and adolescents.