Calcitonin-negative medullary thyroid carcinoma in a young adult.

Medullary thyroid carcinoma (MTC) is typically identified by elevated serum calcitonin levels, as the tumour's C-cell origin generally produces this hormone. On rare occasions, however, a patient with MTC may present with a non-elevated calcitonin level, complicating early detection. We report such a case of a woman in her early 20s with a thyroid nodule and no initial biochemical markers suggestive of MTC. Despite normal laboratory findings, she was ultimately diagnosed with calcitonin-negative MTC following comprehensive evaluation, which included cytology and immunohistochemistry. We also discuss the current understanding of this rare condition's pathogenesis, which remains under investigation, as well as the current standard of care, both pharmacological and surgical management and finally the potential and recommended follow-up procedures for patients with this rare diagnosis in accordance with existing guidelines. This case underscores the necessity for clinicians to sometimes rely on more than standard tumour markers when evaluating thyroid nodules that appear ambiguous.