title: Prevalence of ectopic intrathyroidal thymus in Japan: the Fukushima health management survey.
abstract: 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: 1 Department of Thyroid and Endocrinology, Fukushima Medical University , Fukushima, Japan .
authors.forename: Toshihiko
authors.initials: T
authors.lastname: Fukushima
authors.forename: Satoru
authors.initials: S
authors.lastname: Suzuki
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Ohira
authors.forename: Hiroki
authors.initials: H
authors.lastname: Shimura
authors.forename: Sanae
authors.initials: S
authors.lastname: Midorikawa
authors.forename: Akira
authors.initials: A
authors.lastname: Ohtsuru
authors.forename: Akira
authors.initials: A
authors.lastname: Sakai
authors.forename: Masafumi
authors.initials: M
authors.lastname: Abe
authors.forename: Shunichi
authors.initials: S
authors.lastname: Yamashita
authors.forename: Shinichi
authors.initials: S
authors.lastname: Suzuki
mesh_terms: D000293:Adolescent; D002648:Child; D002675:Child, Preschool; D002828:Choristoma / Q000000981:diagnostic imaging / Q000453:epidemiology*; D003430:Cross-Sectional Studies; D005260:Female; D062406:Fukushima Nuclear Accident; D006801:Humans; D007223:Infant; D007564:Japan / Q000453:epidemiology; D008297:Male; D015995:Prevalence; D013950:Thymus Gland; D013959:Thyroid Diseases / Q000000981:diagnostic imaging / Q000453:epidemiology*; D013961:Thyroid Gland / Q000000981:diagnostic imaging*; D014463:Ultrasonography