The Link Between GLP-1 Drugs and Thyroid Cancer Could Be Smoke and Mirrors

If you’ve ever thought about taking GLP-1 drugs like Ozempic, you might have been scared off by the possibility of thyroid cancer. Recent research, however, finds that this potential danger could actually be non-existent.

Doctors at the Clayman Thyroid Center in Florida examined the medical literature on GLP-1 use and thyroid cancer. They failed to find any concrete data, at least in humans, pointing to a causative link between the two. That said, people with a family history of a specific, rare type of thyroid cancer should still avoid GLP-1 drugs, the researchers say, echoing the recommendation from the Food and Drug Administration.

“The best available human evidence does not show that GLP-1 receptor agonists cause common thyroid cancers or even has an effect if you have developed a thyroid cancer,” they wrote in a white paper released last month.

GLP-1s and thyroid cancer

GLP-1s have been around for two decades, though it’s only in the last few years that newer drugs like semaglutide (Ozempic and Wegovy) and tirzepatide (Mounjaro and Zepbound) have become effective front-line treatments for obesity.

These drugs are known to commonly cause gastrointestinal side-effects like nausea, diarrhea and constipation. But they’ve also long carried a black box warning about the possible risk of thyroid cancer, specifically medullary thyroid cancer (MTC). The rationale for including this warning was largely based on rodent studies of GLP-1s, however. And that’s led scientists to since examine whether the same pattern can be found in people who take these drugs.

The Clayman researchers analyzed data from clinical trials, adverse event reports, and large studies of real-world patients. All in all, they found next to nothing suggesting that GLP-1 use is linked to a genuinely higher rate of thyroid cancer. Some studies did find that GLP-1 users are more often diagnosed with these cancers (or unusual nodules that could become cancerous), but this is probably due to something known as detection bias, the researchers say.

People taking GLP-1 drugs are more likely to see their doctors regularly and/or receive thyroid cancer screening, and this higher level of surveillance can then result in more diagnosed cancers, rather than any effect of the drug. When studies have tried to account for this bias, such as looking at diagnosis rates soon after starting GLP-1s (before the drug could realistically spark a new cancer), they’ve largely found no evidence of a thyroid cancer link.

“When diagnoses rise shortly after starting a medication, that pattern often reflects finding something that was already there,” said co-author Rashmi Roy, in a statement from the center. “That is very different from a drug actually causing a new cancer.”

When to stay away from GLP-1 drugs

The team’s findings should alleviate fears from patients and doctors about GLP-1s and thyroid cancer in general, the researchers say. And there’s even a decent chance that MTC isn’t a real risk to be concerned about.

The Clayman Thyroid Center handles about 2,000 cases of thyroid cancer annually, making it one of the largest specialized clinics in the world. And to date, they haven’t spotted any increase of MTC cases in particular since the popularity of these drugs has exploded.

“In a practice seeing more thyroid cancer than anywhere globally, we are not observing an MTC wave in GLP-1 users. This aligns with what the best research shows,” they wrote.

Still, the researchers caution their experiences are anecdotal. And because MTC is so rare, accounting for roughly 3% of thyroid cancers, it might take incredibly large population studies to truly detect or debunk an added risk from GLP-1 use. So for now at least, the researchers are still advising that people with a personal or family history of MTC (or a closely related, inherited condition called MEN2) not take these drugs.

GLP-1s, like any drug, have their risks. But thankfully, thyroid cancer doesn’t appear to be a major one.