Why GLP-1 Drugs Cause Thyroid Tumors in Rats but Probably Not in Humans
GLP-1 receptor agonists cause thyroid C-cell growth and calcitonin release in rodents, but primate and human thyroid cells lack the receptor expression needed for this effect, suggesting the rodent finding may not apply to people.
Quick Facts
What This Study Found
GLP-1 receptor agonists including liraglutide caused thyroid C-cell hyperplasia and calcitonin release in rodents through a GLP-1 receptor-mediated mechanism. However, this effect appears species-specific: human and monkey thyroid C-cells had low GLP-1 receptor expression, GLP-1 agonists did not activate calcitonin release in primate cells, and 20 months of liraglutide treatment at >60x human exposure did not cause C-cell hyperplasia in monkeys. In human patients treated with liraglutide for 2 years, calcitonin levels remained at the lower end of normal with no meaningful increase above clinically relevant thresholds.
Key Numbers
>60x human exposure in monkeys for 20 months · No C-cell hyperplasia in primates · Human calcitonin remained in lower normal range · 2-year human exposure data · Cutoff: 20 pg/ml calcitonin
How They Did This
Multi-species study: GLP-1 receptor localization via immunohistochemistry in rodent, monkey, and human thyroid tissue. Calcitonin release and gene expression measured in rodent C-cells. Adenylate cyclase activation tested in primate cells. 20-month liraglutide treatment in cynomolgus monkeys at >60x human exposure. 2-year calcitonin monitoring data from human clinical trials.
Why This Research Matters
This is the key study that explained why GLP-1 drugs carry a thyroid cancer warning on their labels — rodent studies showed thyroid tumors — while providing the first strong evidence that this risk may not translate to humans. It delineated a species-specific difference in GLP-1 receptor biology that has been central to the safety debate around every GLP-1 RA since.
The Bigger Picture
This study has been one of the most cited papers in GLP-1 RA safety discussions. The thyroid cancer signal in rodents delayed drug approvals and required prominent label warnings. By demonstrating species-specific GLP-1 receptor expression in the thyroid, this work provided the mechanistic basis for cautious optimism that the rodent signal wouldn't translate to humans — a conclusion supported by subsequent years of pharmacovigilance data from millions of patients.
What This Study Doesn't Tell Us
The human data comes from clinical trial calcitonin monitoring, not direct thyroid tissue examination. Long-term consequences of sustained GLP-1 receptor activation in human thyroid remain unknown. The 2-year human follow-up may not be sufficient to detect very slow-growing thyroid cancers. Monkey study used one GLP-1 RA (liraglutide) and may not generalize to all agents.
Questions This Raises
- ?After 15+ years of GLP-1 RA use in millions of patients, has the human epidemiological data definitively resolved the thyroid cancer question?
- ?Do different GLP-1 receptor agonists vary in their potential to affect human thyroid C-cells, even at the low receptor expression levels found?
- ?Could very long-term GLP-1 RA use (decades) produce thyroid effects that 2-year clinical trials cannot detect?
Trust & Context
- Key Stat:
- >60x human dose, no thyroid effect in primates Monkeys given liraglutide at more than 60 times the human exposure level for 20 months showed no C-cell hyperplasia, in stark contrast to rodents
- Evidence Grade:
- This is a rigorous multi-species preclinical study combining rodent, primate, and human tissue data with clinical trial biomarker monitoring. Published in a top endocrinology journal by researchers from Novo Nordisk and leading academic collaborators. The evidence is moderate overall — strong for mechanism but the human safety question requires longer follow-up.
- Study Age:
- Published in 2010, this study was conducted during liraglutide's regulatory review. Its core finding — that the rodent thyroid signal is species-specific — has been reinforced by over a decade of subsequent human safety data, though the FDA boxed warning remains on all GLP-1 RA labels.
- Original Title:
- Glucagon-like Peptide-1 receptor agonists activate rodent thyroid C-cells causing calcitonin release and C-cell proliferation.
- Published In:
- Endocrinology, 151(4), 1473-86 (2010)
- Authors:
- Bjerre Knudsen, Lotte, Madsen, Lars Wichmann, Andersen, Søren, Almholt, Kasper, de Boer, Anne S, Drucker, Daniel J, Gotfredsen, Carsten, Egerod, Frederikke Lihme, Hegelund, Anne Charlotte, Jacobsen, Helene, Jacobsen, Søren Dyring, Moses, Alan C, Mølck, Anne-Marie, Nielsen, Henriette S, Nowak, Jette, Solberg, Helene, Thi, Tu D L, Zdravkovic, Milan, Moerch, Ulrik
- Database ID:
- RPEP-01587
Evidence Hierarchy
Frequently Asked Questions
Should I worry about thyroid cancer if I'm taking a GLP-1 drug like semaglutide or liraglutide?
The thyroid cancer warning on GLP-1 drugs comes from rodent studies where these drugs caused thyroid C-cell tumors. However, this study showed that human thyroid cells have very few GLP-1 receptors compared to rodents, and the effect does not appear to occur in primates. Over a decade of real-world use has not shown a clear increase in thyroid cancer in humans. Still, people with a personal or family history of medullary thyroid carcinoma should not use these drugs.
What are thyroid C-cells and what do they have to do with GLP-1 drugs?
C-cells are specialized cells in your thyroid gland that produce calcitonin, a hormone involved in calcium regulation. In rodents, these cells have many GLP-1 receptors, so GLP-1 drugs overstimulate them, causing abnormal growth. In humans, C-cells have far fewer GLP-1 receptors, which is why the drugs don't appear to have the same effect on human thyroids.
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Cite This Study
https://rethinkpeptides.com/research/RPEP-01587APA
Bjerre Knudsen, Lotte; Madsen, Lars Wichmann; Andersen, Søren; Almholt, Kasper; de Boer, Anne S; Drucker, Daniel J; Gotfredsen, Carsten; Egerod, Frederikke Lihme; Hegelund, Anne Charlotte; Jacobsen, Helene; Jacobsen, Søren Dyring; Moses, Alan C; Mølck, Anne-Marie; Nielsen, Henriette S; Nowak, Jette; Solberg, Helene; Thi, Tu D L; Zdravkovic, Milan; Moerch, Ulrik. (2010). Glucagon-like Peptide-1 receptor agonists activate rodent thyroid C-cells causing calcitonin release and C-cell proliferation.. Endocrinology, 151(4), 1473-86. https://doi.org/10.1210/en.2009-1272
MLA
Bjerre Knudsen, Lotte, et al. "Glucagon-like Peptide-1 receptor agonists activate rodent thyroid C-cells causing calcitonin release and C-cell proliferation.." Endocrinology, 2010. https://doi.org/10.1210/en.2009-1272
RethinkPeptides
RethinkPeptides Research Database. "Glucagon-like Peptide-1 receptor agonists activate rodent th..." RPEP-01587. Retrieved from https://rethinkpeptides.com/research/bjerre-2010-glucagonlike-peptide1-receptor-agonists
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkPeptides research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.