Serum calcitonin (CT) is a well-accepted marker of C-cell proliferation, particularly in medullary thyroid carcinoma. Chronic glucagon-like peptide-1 (GLP-1) receptor agonist administration in rodents has been associated with increased serum CT levels and C-cell tumor formation. There are no longitudinal studies measuring CT in humans without medullary thyroid carcinoma or a family history of medullary thyroid carcinoma and no published studies on the effect of GLP-1 receptor agonists on human serum CT concentrations.