GLP-1s and Thyroid Cancer: Deconstructing the Boxed Warning

Last Updated: MARCH 2026

The prescribing information for GLP-1 medications like Ozempic, Wegovy, and Zepbound carries a prominent boxed warning regarding the risk of thyroid C-cell tumors. This warning specifically cautions against using these drugs in patients with a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia syndrome type 2 (MEN2). While alarming to see, MTC is a rare form of thyroid cancer, accounting for only 1-2% of all thyroid malignancies in the United States, while the most common types are papillary and follicular, making up over 90% of cases (American Cancer Society, 2024) [3]. Understanding the distinction between these cancer types and the origin of the warning is crucial for patients considering GLP-1 therapy.

The Origin of the Boxed Warning: Rodent Studies

The MTC boxed warning is rooted in animal studies, not human clinical trials. Specifically, in preclinical toxicology studies, GLP-1 receptor agonists were found to cause thyroid C-cell tumors (adenomas and carcinomas) in rodents. These tumors developed at drug exposures that were clinically relevant or even lower than those achieved in humans at therapeutic doses.

The prescribing information for Ozempic (semaglutide), for example, states: “In rodents, semaglutide caused dose-dependent and treatment-duration-dependent thyroid C-cell tumors (adenomas and carcinomas) at clinically relevant exposures. It is unknown whether Ozempic causes thyroid C-cell tumors, including MTC, in humans” [1]. This finding prompted the FDA to include the boxed warning as a precautionary measure, despite the lack of confirmed risk in humans.

Medullary Thyroid Carcinoma vs. Common Thyroid Cancers

It is critical to distinguish between medullary thyroid carcinoma (MTC) and the more prevalent forms of thyroid cancer.

• Medullary Thyroid Carcinoma (MTC): This is a neuroendocrine tumor that originates from the C-cells of the thyroid gland, which produce the hormone calcitonin. MTC is typically more aggressive than papillary or follicular thyroid cancers and often has a genetic component, particularly in cases linked to MEN2 syndrome.
• Papillary and Follicular Thyroid Carcinoma: These are differentiated thyroid cancers that arise from the follicular cells, which produce thyroid hormones. They account for the vast majority of thyroid cancer diagnoses, are generally slow-growing, and have a high cure rate when detected early.

The table below highlights key differences:

Human Data: No Clear Signal for Increased Risk

Despite the rodent findings, extensive human data, including long-term clinical trials and large observational studies, have not established a causal link between GLP-1 receptor agonist use and an increased risk of MTC or other thyroid cancers.

A comprehensive review published in Targeted Oncology in 2024, involving the Clayman Thyroid Center (one of the largest thyroid cancer centers globally), examined over 15 years of human data. The authors concluded: “There is no convincing evidence that GLP-1 medications cause common thyroid cancers, nor has a pattern linking GLP-1 use to medullary thyroid carcinoma been observed by surgeons at the Clayman Thyroid Center within their large clinical practice” [2].

This aligns with multiple other analyses and observational studies. For instance, a meta-analysis of GLP-1 receptor agonist trials published in JAMA Internal Medicine found no statistically significant increase in the risk of thyroid cancer with GLP-1 use compared to placebo. While some studies have reported a slight increase in thyroid adverse events, these have largely been attributed to increased detection during medical monitoring rather than a causal effect, and specifically, MTC has not shown an increased incidence.

Furthermore, post-marketing surveillance data, which tracks adverse events after a drug is widely available, has not revealed a disproportionate signal for thyroid cancer in the millions of individuals worldwide using GLP-1 medications. If there were a strong causal link, particularly to a rare cancer like MTC, a significant signal would likely emerge in these large datasets.

Who Should Exercise Caution?

The boxed warning dictates specific populations for whom GLP-1 medications are contraindicated:

• Patients with a personal history of medullary thyroid carcinoma (MTC).
• Patients with a family history of MTC.
• Patients with Multiple Endocrine Neoplasia syndrome type 2 (MEN2).

MEN2 is a genetic condition that significantly increases the risk of developing MTC, as well as tumors in other endocrine glands. For individuals with these risk factors, alternative weight management or diabetes treatments should be explored.

For all other patients, the current scientific consensus suggests that the theoretical risk highlighted by rodent studies has not translated into a demonstrable risk in humans. However, patients should be vigilant for symptoms of thyroid tumors, such as a lump or swelling in the neck, hoarseness, difficulty swallowing, or shortness of breath.

Conclusion

The FDA’s boxed warning for GLP-1 medications regarding thyroid C-cell tumors, specifically medullary thyroid carcinoma (MTC), is a critical piece of information for prescribers and patients. Its inclusion is a consequence of preclinical findings in rodents, not a reflection of established risk in humans. Current, extensive human data from clinical trials and real-world observational studies have not found a clear link between GLP-1 receptor agonist use and an increased incidence of MTC or the far more common papillary and follicular thyroid cancers. While caution is warranted for individuals with a personal or family history of MTC or MEN2, the available evidence does not support a widespread concern for thyroid cancer in the general population using these medications.

Sources

1. U.S. Food and Drug Administration. (2023). Ozempic (semaglutide) injection prescribing information. Retrieved from https://www.accessdata.fda.gov/drugsatfda_docs/label/2023/214472s009lbl.pdf
2. Clayman, G. L., et al. (2024). Nation’s Largest Thyroid Cancer Center Publishes White Paper Finding No Convincing Evidence That GLP-1 Medications Cause Common Thyroid Cancers. Targeted Oncology. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10328905/
3. American Cancer Society. (2024). What Is Thyroid Cancer? Retrieved from https://www.cancer.org/cancer/types/thyroid-cancer/about/what-is-thyroid-cancer.html