CALCA and medullary thyroid gland carcinoma: Among these, medullary thyroid carcinoma (MTC) is a rare but clinically significant cancer arising from parafollicular C cells of the thyroid, often secreting calcitonin and occasionally associated with paraneoplastic endocrine syndromes (e.g., via ectopic adrenocorticotropic hormone (ACTH) production), which would be a significant differential diagnosis in some CUP cases.