Ectopic cervical thymus (ECT) is an essentially benign condition that is identified during the development of the thymus, and is rare among the adult population. Thymic hyperplasia is another thymus-associated condition, which is known to occur secondary to hyperthyroidism. A 39-year-old woman with hyperthyroidism presented with an enlarging submandibular mass, which was diagnosed as a benign hyperplasia of the ECT. Computed tomography and magnetic resonance imaging largely contributed to its

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... agnosis, and an unnecessary surgical resection was avoided. To the best of our knowledge, this is the first report that describes clinically significant hyperthyroidism-induced ECT hyperplasia during adulthood.