Definition: A thyroid C-cell tumor starts in the C cells of the thyroid, which the NCI says make calcitonin, a hormone that helps control blood calcium. Medullary thyroid cancer (MTC) is the cancer of these cells. Semaglutide and tirzepatide labels carry a boxed warning because the drugs caused C-cell tumors in rodents.
| Label | Animal finding | In people |
|---|---|---|
| Ozempic and Wegovy (semaglutide) | C-cell tumors in mice and rats | Not known whether it causes them, including MTC |
| Mounjaro and Zepbound (tirzepatide) | C-cell tumors in rats | Not known whether it causes them, including MTC |
All four labels say the drug must not be used by anyone with a personal or family history of MTC or with Multiple Endocrine Neoplasia syndrome type 2 (MEN 2).
The NCI says that when blood calcium is too high, calcitonin lowers it. Because the tumors were seen in animals, the labels state the risk in people as unknown rather than giving a rate. The labels list a lump in the neck, trouble swallowing, shortness of breath and hoarseness that does not go away as possible signs of a thyroid tumor to report.
The warning turns on family history. Keeping a note of thyroid conditions in your family alongside your medicines means the answer is ready when a prescriber asks. Thyroid tests such as TSH measure thyroid function, not C cells.
Read more on Dose Track: Ozempic side effects · Wegovy side effects · Mounjaro side effects · Zepbound side effects