Objective: The authors introduce thoracic intrathymic thyroid as a clinical entity. Summary Background Data: Although accessory aberrant thyroid has not been found in other tissues in the mediastinum, a thoracic intrathymic location has not been described previously. It is believed that mediastinal thyroid tissue represents accessory ectopic tissue from the median thyroid anlage. Moreover, the close association of the thymus and thyroid supports the theory that mediastinal ectopic thyroid tissue develops from abnormal descent of these structures during embryogenesis. Methods: Benign thoracic intrathymic thyroid lesions are described in patients with mediastinal masses. Conclusion: Thoracic intrathymic thyroid is a distinct entity. Its occurrence is supported both clinically and embryologically.
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