A Case of Micro-medullary Thyroid Carcinoma Presenting as Cancer of Unknown Primary.

Cancer of unknown primary (CUP) with neuroendocrine features is classified as a favorable subset, and treatment is recommended based on the grade of the neuroendocrine tumor (NET). Although rare, medullary thyroid carcinoma (MTC) can present as CUP with neuroendocrine features and should be carefully considered as a crucial differential diagnosis in such cases. A 65-year-old man with no relevant medical history presented with a gradually enlarging left cervical mass. Laboratory evaluation revealed hypokalemia due to adrenocorticotropic hormone (ACTH)-dependent Cushing's syndrome. Contrast-enhanced computed tomography (CT) demonstrated multiple lymphadenopathies, hepatic masses, and bilateral pulmonary ground-glass opacities. A biopsy of a cervical lymph node revealed a neuroendocrine tumor grade 2 (NET G2), but the primary tumor was undetectable on imaging. Elevated serum calcitonin and thyroid ultrasound identified a small left thyroid nodule, fine-needle aspiration, and calcitonin immunostaining on the lesion pathologically revealed MTC. Given the rapidly progressive disease, vandetanib was initiated while awaiting molecular testing for rearranged during transfection (RET)-alteration. Following the detection of the RET M918T mutation, treatment was switched to selpercatinib, and rapid tumor response and endocrine symptom resolution were observed. This case highlights the importance of evaluating serum calcitonin, specifically in patients with neuroendocrine carcinoma of unknown primary, where MTC should be considered as a critical differential diagnosis. Thyroid ultrasound should be performed to identify even small lesions of medullary thyroid carcinoma (micro-MTC). Identification of MTC not only leads to targeted therapies that may improve prognosis, but also allows for genetic risk assessment and early intervention in family members when multiple endocrine neoplasia type 2 (MEN2) is suspected.