Background Ectopic thyroid tissue is a rare embryological aberration described by the occurrence of thyroid tissue at a site other than in its normal pretracheal location. Depending on the time of the disruption during embryogenesis, ectopic thyroid may occur at several positions from the base of the tongue to the thyroglossal duct. Ectopic mediastinal thyroid tissue is normally asymptomatic, but particularly after orthotopic thyroidectomy, it might turn out to be symptomatic. Symptoms are normally due to compression of adjacent structures. Case presentation We present a case of a 66-year-old male submitted to a total thyroidectomy 3 years ago, due to multinodular goiter (pathological results revealed nodular hyperplasia and no evidence of malignancy), under thyroid replacement therapy. Over the last year, he developed hoarseness, choking sensation in the chest, and shortness of breath. Thyroid markers were unremarkable. He was submitted to neck and thoracic computed tomography, magnetic resonance imaging, and radionuclide thyroid scan. Imaging results identified an anterior mediastinum solid lesion. A radionuclide thyroid scan confirmed the diagnosis of ectopic thyroid tissue. The patient refused surgery. Conclusions Ectopic thyroid tissue can occur either as the only detectable thyroid gland tissue or in addition to a normotopic thyroid gland. After a total thyroidectomy, thyroid-stimulating hormone can promote a compensatory volume growth of previously asymptomatic ectopic tissue. This can be particularly diagnosis challenging since ectopic tissue can arise as an ambiguous space-occupying lesion.

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