Medullary thyroid carcinoma
Also called: MTC, MEN 2
Clinical reviewer: Jacob Moore, MD · Last checked September 24, 2026
What is medullary thyroid carcinoma (MTC)?
Medullary thyroid carcinoma is a cancer that develops in the thyroid's C cells, the cells that make the hormone calcitonin. It is also called MTC. A personal or family history of MTC, or of Multiple Endocrine Neoplasia syndrome type 2 (MEN 2), is a contraindication on semaglutide and tirzepatide labels.
Where does MTC start, and how is it linked to MEN 2?
Medullary thyroid carcinoma is a cancer that develops in the C cells of the thyroid, the cells that make the hormone calcitonin, which helps keep blood calcium at a healthy level [1]. MTC is the most common sign of Multiple Endocrine Neoplasia type 2 (MEN 2), a group of disorders that affect the body's hormone-producing glands [2].
Why is MTC on GLP-1 drug labels?
The concern comes from animal data: semaglutide caused thyroid C-cell tumors in rodents, and tirzepatide caused them in rats, though the labels say the human relevance of these findings "has not been determined" [3][4]. A personal or family history of MTC or MEN 2 is one of the contraindications listed for Wegovy and Zepbound, and the labels direct clinicians to counsel patients about the potential risk of MTC and symptoms of thyroid tumors [3][4]. Both labels add that routine calcitonin or ultrasound monitoring "is of uncertain value for early detection of MTC" [3][4].
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This entry is for education and is not medical advice. Talk with a licensed clinician about your own situation before starting, stopping, or changing any treatment.
