Medullary Thyroid Carcinoma in the Background of Non-neoplastic Toxic Nodular Goiter

Background/Objective: Medullary thyroid carcinoma (MTC) is an uncommon thyroid cancer (TC), rarely found in hyperfunctioning goiter. Case Report: We present a case of a woman treated for breast carcinoma (BCA) found to have a benign hyperfunctioning nodular goiter, its likely transformation to MTC, and its treatment. Family history revealed papillary thyroid cancer in her nephew. Discussion: Most TCs in hyperfunctioning nodules are differentiated carcinomas. Familial MTC or MTC in association with multiple endocrine neoplasia 2 is the expected genetic association in this case. Conclusion: The association of BCA and MTC may have been coincidental, given the high prevalence of BCA in females. It could have been the result of a common genetic precursor of both tumors and/or treatment modality such as external beam radiation therapy used to treat BCA. This case highlights the importance of considering MTC as a potential diagnosis even in cases of hyperfunctioning nodular goiter. We call for consideration of calcitonin level measurement in the workup of thyroid nodules in select cases. Close follow-up of thyroid nodules, particularly in patients with another primary malignancy, is important because of possible common genotype triggers.