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

Medullary thyroid carcinoma (MTC) is an uncommon thyroid cancer (TC), rarely found in hyperfunctioning goiter.

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.

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.

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.