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

## TL;DR

A rare lung tumor secretes calcitonin, causing confusion with thyroid cancer and highlighting the need for accurate diagnosis.

## Contribution

Presents a rare case of a lung tumor secreting calcitonin, emphasizing diagnostic challenges and awareness.

## Key findings

- Elevated calcitonin levels can originate from non-thyroid tumors, complicating diagnosis.
- Immunohistochemical similarity exists between lung tumors and medullary thyroid carcinoma.
- This case highlights the importance of considering extrathyroidal sources for calcitonin elevation.

## Abstract

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.