Clinical challenges with calcitonin-negative medullary thyroid carcinoma: three case reports and a review of the literature.

Medullary thyroid carcinoma (MTC) is relatively rare, and has the main feature of calcitonin (Ct) secretion. However, a few cases of MTC with negative serum calcitonin have been reported in the literature, so the diagnosis and follow up of Ct-negative MTCs are still a challenge. Here we present three cases of Ct-negative MTCs, illustrating the rarity of the disease and challenges in managing it, together with a review of the literature of 39 MTCs with negative serum Ct.