Most cases (80% to 90%) of ectopic thymus present as an asymptomatic mass in the neck (84%), typically on the left side (68%), and may be are quite often associated with congenital heart disease.¹ Heterotopic thymic tissue is more frequent in males and usually occurs between the age of 1 and 13 years.^2,3 Ectopic retropharyngeal and intrathyroidal thymus is a rare occurrence.^1,4,5 The occurrence of aberrant thymus in the posterior mediastinum is very uncommon. It is usually positioned on the right side.³

Ectopic thymus is generally asymptomatic,⁶ and rarely results in a palpable mass.⁴

The prevalence of ectopic parathyroid glands is about 2% to 43% in anatomical series and 14% to 16% in patients with primary and secondary hyperparathyroidism.⁷ Ectopic inferior parathyroids are most frequently found in the anterior mediastinum, in association with the thymus (38%) or the thyroid gland (18%), while the most common position for ectopic superior parathyroids is the tracheoesophageal groove and retroesophageal region (31%).^7,8 Uncommon mediastinal sites include the aortic arch and tracheal bifurcation.⁹

Most of the cases with ectopic mediastinal thyroid are known to develop in the anterosuperior mediastinum, the so-called cervicomediastinal region.¹⁰ The posterior mediastinum is an atypical localization for the occurrence of ectopic thyroid.¹¹ The incidence of mediastinal goiter varies from 0.16% to 3.3%.¹¹