Abstract
The ectopic intrathyroidal thymus (EIT) is located anywhere in the thyroid gland along the developmental pathway of thymic descent due to thymic migration during embryogenesis. Ultrasonographic findings of papillary thyroid carcinoma (PTC) resemble those of EIT, which is frequently found in children. We comprehensively evaluated the clinical factors associated with EIT to understand its physiological implications and to explore helpful information for clinical discrimination between EIT and PTC. Approximately 320,000 datasets of thyroid ultrasound examinations conducted in the Fukushima Health Management Survey were systematically analyzed. Trend analyses were performed following stratification into groups of the age-sex-adjusted standard deviation score (SDS) for body mass index (BMI-SDS) and body surface area-sex-adjusted SDS for both the width and thickness of the area (BWTAR) as an indicator of thyroid volume (BWTAR-SDS). The prevalence of EIT was 3.2% in the study. Compared with negative EIT, the age- and sex-adjusted odds ratios (95% confidence intervals) for the BMI-SDS, BWTAR-SDS, and presence of diffuse goiter, cysts, and nodules were 0.919 (0.901-0.939), 0.976 (0.957-0.996), 0.821 (0.602-1.121), 0.892 (0.853-0.932), and 1.602 (1.302-1.972), respectively. EIT was not associated with the presence of diffuse goiter but was independently associated with male sex, young age, small thyroid volume, low BMI, absence of cysts, and presence of nodules. The series of clinical factors related to EIT shown in the study might provide complementary information in addition to ultrasonographic findings in cases with asymptomatic thyroid nodules resembling PTC found in young patients.