Medullary thyroid carcinoma (MTC) is a rare form of thyroid cancer that arises from the thyroid's C cells. It is the reason GLP-1 medication labels carry a boxed warning, based on findings that some of these drugs caused thyroid C-cell tumors in rodents; whether the same risk applies to humans has not been established.
A personal or family history of MTC, or of the inherited syndrome MEN 2, is listed as a contraindication to GLP-1 receptor agonists. For what the warning does and doesn't mean in practice, see our guide on the thyroid C-cell tumor warning on GLP-1 medications.