The presence of ectopic thyroid tissue is a frequent diagnostic feedback related to a possible histogenetic abnormality or a result of post-surgical seeding. The important challenge is the diagnostic definition of its nature, which could lead to a different therapeutic approach. We describe a case with all the possible implications and differential diagnoses supported by the application of immunohistochemistry and BRAF-V600E molecular detection. A 36-year-old girl with a left thyroid nodule underwent surgery with a diagnosis of angio-invasive tall cell variant of papillary carcinoma without lymph-node metastases. The patient history revealed a previous right lobectomy in another institution ten years before. The right neck dissection was histologically characterized by the presence of thymic residual and evidence of microfollicular microfoci (less than 1mm) of thyroid tissue. Thyroid tumor cells were positive for thyroglobulin, TTF-1, HBME-1 and Galectin-3, while the residual ectopic thyroid tissue showed positivity only for TTF-1 and thyroglobulin. The molecular tests on the thyroid nodule revealed the presence of BRAF-V600E mutation, but wild type BRAF in the ectopic tissue. The histo-morphological definition of these remnants plays the most important role, but the application of immunohistochemistry and molecular biology may help to reach a conclusive diagnosis.
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