Prevalence of Ectopic Intrathyroidal Thymus in Japan: The Fukushima Health Management Survey

Background: Ectopic intrathyroidal thymus is thought to be a rare entity, often discovered incidentally, and is due to aberrant thymic migration during embryogenesis. Objective: The aim of this study was to determine the prevalence of ectopic intrathyroidal thymus in children using ultrasound screening. Methods: This study was cross-sectional and was conducted with the initial preliminary survey of the Fukushima Health Management Survey between October 9, 2011, and March 31, 2012, after the Fukushima Daiichi Nuclear Power Plant accident. A total of 37,816 children were examined in the survey. Results: Diagnostic criteria are based on the ultrasonographic appearance of ectopic intrathyroidal thymus, which were round, oval, or polygonal hypoechoic or hyperechoic areas, with multiple granular and punctate echogenic foci. A total of 375 (0.99%) cases (164 girls) with ectopic intrathyroidal thymus were observed. The mean age was 7.0 years (range 0-18 years). Ectopic intrathyroidal thymus was located in the right (n=180), left (n=178), or bilateral (n=17) thyroid lobes. The incidence of ectopic intrathyroidal thymus was inversely correlated with age and body mass index. Conclusion: The results reflect the prevalence of ectopic intrathyroidal thymus using ultrasonography in the general population. Further examination will be needed by way of longitudinal follow-up.