Medullary thyroid carcinoma (MTC)

Medullary thyroid carcinoma, or MTC, is a rare cancer of the thyroid’s C cells — the cells that make calcitonin. It accounts for a small minority of thyroid cancers, and a meaningful share of cases run in families, often as part of an inherited syndrome called Multiple Endocrine Neoplasia type 2 (MEN 2).

MTC is the subject of the boxed warning at the top of every injectable GLP-1 label, and of Foundayo’s tablet label too. The warning is precise about what is known and what is not. In rodents, semaglutide and tirzepatide cause dose-dependent and treatment-duration-dependent thyroid C-cell tumors at clinically relevant exposures. Whether the drugs cause such tumors in humans is unknown, because the human relevance of the rodent findings has not been determined.

What follows from that uncertainty is a hard contraindication rather than a caution. These medications are not to be used by anyone with a personal or family history of MTC, or with MEN 2. That is an absolute exclusion on the label, not a risk to weigh. The labels also tell clinicians to counsel patients on symptoms of a thyroid tumor: a lump in the neck, trouble swallowing, shortness of breath, or persistent hoarseness.

One detail is widely misunderstood. The labels state that routine monitoring of serum calcitonin or thyroid ultrasound is of uncertain value for early detection of MTC in patients on these drugs. So the right response to the warning is an accurate family history, not a standing lab order — and a clinician who reflexively schedules calcitonin surveillance is going beyond what the label supports.

Before a first prescription, ask whether anyone in your family has had thyroid cancer or an endocrine tumor syndrome, and tell your prescriber even if you are not certain. Our thyroid cancer evidence review and thyroid lab monitoring guide cover the rest.