Age-related reduction of the prevalence of ectopic intrathyroidal thymus in children and its size reduction with age

Ectopic intrathyroidal thymus in children refers to the failure of the thymus to descend from the thyroid during intrauterine development, and although rare, it presents with characteristic features on thyroid ultrasonography. The prevalence of ectopic intrathyroidal thymus and the changes in the size of intrathyroidal thymus over time were analyzed among 4130 children who underwent thyroid ultrasonography at the Ito Hospital from January 2003 to September 2015 (896 boys and 3234 girls; median age: 6 years [range: 0–15]). Male children (25 cases, 2.79%) more prominently demonstrated ectopic intrathyroidal thymus than female children (26 cases, 0.80%). The median age of the children with this condition was 6 years (range: 1–14 years), and the median maximum diameter of the ectopic thymus was 7.2 mm (range: 4.2–17.0 mm). The ectopic thymus was located in the right thyroid lobe in 24 (47.1%) cases, in the left thyroid lobe in 23 (45.1%) cases, and was bilateral in 4 (7.8%) cases. The prevalence decreased with age. The thymus size decreased by ≥ 20% in 11 (40.7%) cases, increased by ≥ 20% in 1 (3.7%) case, and changed in size (decreased or increased) by < 20% in 15 (55.6%) cases among the 27 (52.9%) cases assessed for the changes in the sizes of ectopic intrathyroidal thymus glands over time. The ectopic intrathyroidal thymus decreases its prevalence with age. Additionally, the size of an ectopic intrathyroidal thymus frequently appears to decrease over time during follow-up of cases.