Purpose or Case Report: Ultrasound is the most commonly used modality in evaluating the thyroid gland and in classifying thyroid nodules. We present a three-case series of intrathyroidal ectopic thymus. Two of these cases were initially classified as high grade (TI-RADS 4) thyroid nodules and subsequently underwent fine needle aspiration (FNA). The third was recognized initially as intrathyroidal ectopic thymus, avoiding the need for FNA and followed conservatively. Ectopic thymic tissue can occur anywhere along the embryologic path of descent from the 3rd branchial pouch to the anterior mediastinum. Embryology of the thymus and common locations of ectopic thymic tissue will be reviewed. We will also review and illustrate ultrasound imaging characteristics of normal thymic tissue in the mediastinum as well as in ectopic locations, focusing primarily on the thyroid gland. We hope that the radiologist will consider the diagnosis of intrathyroidal ectopic thymus when evaluating thyroid nodules by ultrasound, differentiating them from other thyroid lesions and avoiding the need for FNA.
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