Semaglutide thyroid tumor warning, explained
The boxed thyroid tumor warning on semaglutide exists because semaglutide caused thyroid C-cell tumors in lifetime studies in mice and rats. It does not mean semaglutide has been shown to cause thyroid cancer in people.
That distinction matters.
The U.S. prescribing information for Ozempic and Wegovy states that semaglutide caused thyroid C-cell adenomas and carcinomas in rodents. It also states that the relevance of those findings to humans is unknown.
Human trials have monitored thyroid-related events. They have not established that semaglutide causes medullary thyroid carcinoma or other thyroid cancers in people. But clinical trials cannot settle every long-term cancer question, particularly when the outcome is rare and may take years to develop.
So the warning should be taken seriously, but read accurately. It is a regulatory warning based on an animal finding and unresolved human relevance, not proof that semaglutide causes thyroid cancer in humans.
What does the semaglutide boxed warning actually say?
The U.S. prescribing information for both Ozempic and Wegovy carries a boxed warning for risk of thyroid C-cell tumors.
C cells are specialized cells in the thyroid. Medullary thyroid carcinoma, usually shortened to MTC, is a cancer arising from these cells.
The Ozempic prescribing information reports that semaglutide produced a dose-dependent and treatment-duration-dependent increase in thyroid C-cell tumors, including adenomas and carcinomas, in mice and rats after lifetime exposure.
The label then makes the key limitation explicit: it is unknown whether Ozempic causes thyroid C-cell tumors, including MTC, in humans because the human relevance of the rodent findings has not been determined.
The warning also states that semaglutide is contraindicated in people with a personal or family history of MTC or in people with multiple endocrine neoplasia syndrome type 2, commonly called MEN 2. MEN 2 is an inherited syndrome associated with an increased risk of MTC.
That contraindication is precautionary. It does not change the underlying evidence question: whether semaglutide itself causes C-cell tumors in humans remains unresolved.
Where did the warning come from?
The warning traces directly to animal toxicology studies.
Lifetime exposure studies in mice and rats found thyroid C-cell adenomas and carcinomas after exposure to semaglutide. The U.S. label describes the tumors as dose-dependent and treatment-duration-dependent.
This is important because animal carcinogenicity studies are designed to identify potential hazards. A tumor signal in animals can justify warnings and additional precautions even when researchers do not yet know whether the same effect occurs in humans.
But an animal finding is not automatically a human finding.
Smits and Van Raalte note biological differences between rodent and human thyroid tissue, including that the GLP-1 receptor is only marginally expressed in the thyroids of non-human primates and humans. These differences are one reason researchers have questioned how directly the rodent findings translate to people.
The correct interpretation is therefore narrow: semaglutide caused C-cell tumors in rodents, and we do not know whether that finding predicts the same risk in humans.
What the warning does not mean
The boxed warning does not say that human trials proved semaglutide causes thyroid cancer.
They have not.
A review of semaglutide safety data from the SUSTAIN clinical program identified three adjudicated malignant thyroid neoplasms: two among 5,933 semaglutide-treated participants and one among 4,736 participants receiving comparator treatments. None was medullary thyroid carcinoma. The review also reported no notable difference in mean calcitonin levels between treatment groups. Calcitonin is a hormone produced by thyroid C cells and is sometimes monitored as a biomarker relevant to C-cell disease.
The same review reported the thyroid findings from the PIONEER program, which studied oral semaglutide. There were four thyroid malignancies among semaglutide-treated participants versus one in the comparator group. One medullary thyroid carcinoma case occurred in a patient with preexisting thyroid nodules and elevated baseline calcitonin, which the review attributed to preexisting disease rather than treatment.
The STEP weight-management program also monitored thyroid-related safety.
In STEP 3, one papillary thyroid cancer occurred in the semaglutide group. No medullary thyroid carcinoma cases were reported in that trial. Papillary thyroid cancer arises from a different type of thyroid cell and is not the tumor type flagged in the animal studies.
It is also important to understand who was in these trials. SUSTAIN and STEP generally excluded people with a personal or family history of medullary thyroid carcinoma or MEN 2. Separately, the U.S. labels contraindicate semaglutide in those groups as a precaution.
That exclusion matters when interpreting the evidence. Trial results from a population selected to exclude the highest-risk people do not answer every question about risk in every possible patient.
What did the longer SELECT trial find?
SELECT provides useful additional human safety data because it was a large cardiovascular outcomes trial in people with overweight or obesity and established cardiovascular disease, without diabetes at enrollment.
A dedicated SELECT safety analysis reported malignant neoplasms with primary location in the thyroid gland in six semaglutide-treated participants and eight placebo-treated participants, each fewer than 0.1% of its group. The paper reported no statistical comparison for this subgroup.
Three cases of medullary thyroid cancer were reported. All three occurred in the placebo group and were identified through baseline calcitonin assessments, indicating they predated treatment.
This does not prove that semaglutide cannot cause thyroid cancer. It does show why the boxed warning should not be read as evidence that human thyroid cancer cases were piling up in the semaglutide trials.
They have not.
Why does the contraindication still exist?
Regulators do not need proof of human harm before acting on a credible animal safety signal.
The U.S. labels therefore take a precautionary approach. Semaglutide is contraindicated in people with a personal or family history of medullary thyroid carcinoma and in people with MEN 2. People who may be at higher risk because of these histories are told not to use semaglutide, even though the human risk itself has not been established.
The distinction is straightforward:
Animal evidence: Semaglutide caused thyroid C-cell tumors in lifetime mouse and rat studies.
Human evidence: Clinical trials have not established that semaglutide causes thyroid C-cell tumors or thyroid cancer in people.
Regulatory response: Because the animal signal exists and its relevance to humans remains uncertain, U.S. labeling carries a boxed warning and a contraindication for the highest-risk groups.
Those three statements can all be true at the same time.
What do observational studies show?
Randomized trials are not the only human evidence. Several large population studies have examined thyroid cancer in people using GLP-1 receptor agonists, and their results are not all in the same direction.
A French nested case-control analysis (Bezin et al., Diabetes Care, 2023) reported an increased risk: one to three years of GLP-1 receptor agonist use was associated with an adjusted hazard ratio of 1.58 for all thyroid cancer and 1.78 for medullary thyroid cancer. Later commentary raised concerns about surveillance/detection bias and case definitions in that study (e.g. Thompson CA, Sturmer T, Diabetes Care 2023;46:249-251; published comments, Diabetes Care 2023;46(5)), so the finding is disputed rather than settled.
Larger cohorts that followed found no increased risk. A Scandinavian cohort study (BMJ, 2024) compared about 145,000 GLP-1 receptor agonist users with about 292,000 people starting DPP-4 inhibitors and reported a hazard ratio of 0.93 for thyroid cancer, with the confidence interval spanning no effect. An international multisite cohort (Thyroid, 2025) of about 98,000 GLP-1 receptor agonist users across six countries reported a hazard ratio of 0.81.
A 2026 pooled analysis of 93 liraglutide and semaglutide trials plus postmarketing surveillance (Vilsboll et al.) reported a hazard ratio of 1.70 for thyroid cancer with GLP-1 receptor agonists versus placebo, based on 45 versus 17 cases, but the difference was not statistically significant.
These studies do not settle the question either. Observational studies can be affected by detection bias and confounding, and even large pooled trial datasets remain too small to answer a rare-cancer question like medullary thyroid carcinoma. The human risk remains unresolved.
What remains unknown?
The biggest unresolved issue is long-term human risk.
Cancer safety questions are difficult to answer when the event is rare. A clinical trial can include thousands of participants and still be too small, or too short, to detect a rare cancer that develops over many years.
Trial duration creates another limitation. Lifetime rodent studies and human randomized trials operate on very different time scales. A two-year human trial does not answer the same question as a lifetime exposure study in rodents.
That is why an absence of a demonstrated increase in human trials should not be rewritten as proof of zero risk.
The evidence supports a narrower conclusion: the rodent signal is established, while its relevance to human thyroid cancer remains uncertain.
Longer follow-up and larger human datasets can narrow that uncertainty, but they cannot be replaced by confidence alone.
Bottom line
The thyroid tumor warning on semaglutide is real. But its meaning is often overstated.
Semaglutide caused thyroid C-cell adenomas and carcinomas in lifetime studies in mice and rats. Those findings are the direct basis of the boxed warning.
Human clinical trials have not established that semaglutide causes thyroid cancer. SUSTAIN and STEP monitored thyroid-related events and reported no medullary thyroid carcinoma signal in semaglutide-treated participants. SELECT's larger safety dataset showed no excess of thyroid malignancies with semaglutide.
That does not make the warning irrelevant. It explains what the warning actually represents: a precaution based on an established animal finding whose relevance to humans has not been determined.
The U.S. labels contraindicate semaglutide in people with a personal or family history of medullary thyroid carcinoma or MEN 2. Those contraindications are precautionary, not proof of human harm.
This is research information, not medical advice. Nothing here recommends using, stopping, or changing any prescribed treatment. Questions about personal thyroid risk belong with the prescribing clinician.
Sources
Ozempic U.S. Prescribing Information. Novo Nordisk. Boxed Warning and Section 5.1, Risk of Thyroid C-Cell Tumors.
Wegovy U.S. Prescribing Information. Novo Nordisk. Boxed Warning, Risk of Thyroid C-Cell Tumors.
Smits MM, Van Raalte DH. Safety of Semaglutide. Frontiers in Endocrinology. 2021;12:645563. PMID: 34305810. DOI: 10.3389/fendo.2021.645563. Discussion of SUSTAIN and PIONEER thyroid neoplasms, calcitonin monitoring, and preclinical thyroid findings.
Wadden TA, et al. STEP 3 randomized clinical trial. JAMA. 2021. Thyroid malignancy and adverse-event reporting.
Kushner RF, et al. Safety profile of semaglutide versus placebo in the SELECT study. Obesity. 2025. PMID: 39948761. DOI: 10.1002/oby.24222.
Bezin J, et al. GLP-1 Receptor Agonists and the Risk of Thyroid Cancer. Diabetes Care. 2023;46(2):384-390. PMID: 36356111.
Pasternak B, et al. Glucagon-like peptide 1 receptor agonist use and risk of thyroid cancer: Scandinavian cohort study. BMJ. 2024;385:e078225. DOI: 10.1136/bmj-2023-078225.
Baxter SM, et al. Glucagon-Like Peptide 1 Receptor Agonists and Risk of Thyroid Cancer: An International Multisite Cohort Study. Thyroid. 2025;35(1):69-78. DOI: 10.1089/thy.2024.0387.
Vilsboll T, et al. Assessment of thyroid cancer risk associated with glucagon-like peptide 1 receptor agonist use. Diabetes, Obesity and Metabolism. 2026. DOI: 10.1111/dom.70291.
Chao AM, et al. Clinical Insight on Semaglutide for Chronic Weight Management in Adults: Patient Selection and Special Considerations. Drug Des Devel Ther. 2022;16:4449-4461. (STEP trials excluded personal/familial MTC/MEN 2 risk and high baseline calcitonin.)
SUSTAIN 3 trial registry record, ClinicalTrials.gov: exclusion criterion "Personal or family history of medullary thyroid carcinoma (MTC) or multiple endocrine neoplasia syndrome type 2 (MEN 2)".