Evidence-graded answer
Can GLP-1 cause thyroid cancer?
The boxed warning is based on rodent studies showing medullary thyroid carcinoma (MTC) in mice and rats. No human cases have been attributed to GLP-1 use despite 10+ years post-approval surveillance.
Personal or family history of MTC or Multiple Endocrine Neoplasia type 2 (MEN-2) is a hard contraindication — never start a GLP-1.
For everyone else, the absolute risk in pharmacovigilance data is indistinguishable from baseline thyroid cancer rates.
Routine thyroid ultrasound screening before or during GLP-1 use is not recommended unless there is a personal or family MTC/MEN-2 history.
Source: Wegovy prescribing information, accessdata.fda.gov