Calcitonin-Secreting Neuroendocrine Tumor of the Lung: A Diagnostic Dilemma.

Calcitonin is primarily secreted by parafollicular C cells of the thyroid, with significantly elevated levels serving as the primary serum marker for medullary thyroid carcinoma. However, ectopic calcitonin secretion, though rare, has been reported in neuroendocrine tumors arising in extra-thyroidal sites such as the lung, breast, prostate, liver, and thymus. This can pose diagnostic challenges due to the immunohistochemical similarity between ectopic calcitonin-secreting neuroendocrine tumors and medullary thyroid carcinoma, particularly in patients with coexisting thyroid lesions. Here, we present a rare case of a calcitonin-secreting neuroendocrine tumor of the lung, aiming to enhance diagnostic awareness and highlight the potential for elevated serum calcitonin due to ectopic secretion from an extrathyroidal source.