New research, with the caveat of a relatively short follow-up, adds to encouraging data showing an overall low risk of the development of thyroid cancer associated with glucagon-like peptide 1 receptor agonist (GLP-1 RA) treatment.

However, a notably increased rate of thyroid cancer detections limited only to the first year after drug initiation — but not later, coupled with an increased rate of thyroid cancer screening during that period, raises the suggestion that a thyroid cancer concern among the millions of patients taking the immensely popular drugs could exacerbate the already problematic over-detection of incidental thyroid cancers.

“Our study confirms the increased likelihood of being diagnosed with what appears to be low-risk thyroid cancer after treatment with GLP-1 RAs — but not because patients treated with GLP-1 RAs are more likely to develop thyroid cancer but rather because they are more likely to be diagnosed with it because we, as clinicians, are more likely to look for it,” senior author Rozalina G. McCoy, MD, associate professor and associate division chief for clinical research in the Division of Endocrinology, Diabetes, and Nutrition, Department of Medicine at the University of Maryland School of Medicine, in Baltimore, told 

The findings were published in JAMA Otolaryngology-Head & Neck Surgery.

Thyroid cancer concerns surrounding GLP-1 RAs stem from preclinical studies showing a risk for medullary thyroid cancer in rodents, resulting in a boxed warning from the US Food and Drug Administration recommending that those with a family history of the condition avoid the drugs.

More recent studies showing GLP-1 receptors in human papillary thyroid cancer cells has further raised concerns of a risk beyond medullary thyroid cancer, however, recent studies evaluating the risk have been inconclusive.

To further investigate, McCoy and colleagues evaluated claims data on 351,913 adult enrollees in Medicare Advantage and fee-for-service plans in the United States who had type 2 diabetes, a moderate risk for cardiovascular disease, and no history of thyroid cancer.

Of the patients, who were about half women, 41,112 started treatments with GLP-1 RAs; 76,093, started with dipeptidyl peptidase-4 (DPP-4) inhibitors; 43,499, with sodium-glucose cotransporter 2 (SGLT2i) inhibitors; and 191,209 with sulfonylurea therapy between January 2014 and December 2021.

After inverse probability weighting, baseline characteristics in the drug groups were well-balanced.

Overall, with median follow-up times of 660 days (range, 254-1157 days) in the GLP-1 RA group; approximately 4 years in the sulfonylurea and DPP-4 inhibitor groups; and about 2.5 years in the SGLT2 inhibitor group, the absolute risk for thyroid cancer diagnoses in the treatment groups were low, with a rate of 0.17% in the GLP-1 RA group, 0.23% in the DPP-4 inhibitor group, 0.17% in the SGLT2 inhibitor group, and 0.20% in the sulfonylurea group.

The modified intention to treat analysis concluded no increase with GLP-1 RA use in thyroid cancer risk compared with the other three diabetes drugs (hazard ratio [HR], 1.24).

Higher Cancers Detected in First Year Likely From Higher Screening

Importantly, the risk for thyroid cancer was significantly higher within the first year after GLP-1 RA initiation (HR, 1.85) and was also higher among patients who stayed on the medication and didn’t add other medication (HR, 2.07).

With the latency period for the development of thyroid cancer from external exposures taking several years (such as radiation, taking approximately 2.5 years), some studies in fact exclude patients diagnosed with cancers within a year of starting GLP-1s, as it is implausible for a cancer to develop so soon.

Cancers detected that early therefore are considered to likely have been found due to stepped up screening resulting from an increased concern of thyroid cancer with GLP-1 use.

In support of that theory, the study showed that those in the GLP-1 group had a significantly higher rate of receiving thyroid ultrasounds than those in the non−GLP-1 group, at 1.2% at 6 months for GLP-1RA initiators vs 0.8% for non−GLP-1 RA initiators, and an estimated 2.1% at 12 months for GLP-1RA initiators vs 1.5% for non−GLP-1RA initiators (P < .01).

Though the percentages were small, in relative terms, the 6-month rate of screening in the GLP-1 RA group is a 50% increase, and the 12-month rate represents a 40% increase, McCoy pointed out. 

“If we think about 1.2% of people getting an ultrasound in the context of over 41,000 patients, that is a lot of ultrasounds,” she said. 

“To my knowledge, this is the first study to actually look at rates of thyroid ultrasounds with GLP-1 RA initiation, so it will definitely be important to look at the trends more broadly,” McCoy added.

McCoy noted that a key caveat of the study is the relatively short follow-up of only about 2 years.

“We have not been able to follow patients for decades to know if there is risk of new thyroid cancers developing after prolonged exposure,” she said. “But we generally don’t know this for any medication until it is used by many millions of people for many decades.”

Consistent Findings in Recent Research

In another recent study evaluating the risk for thyroid cancer among patients receiving GLP-1s, with a longer follow-up of 3.9 years, Bjorn Pasternak, MD, PhD, of the Division of Clinical Epidemiology, Department of Medicine, Karolinska Institutet, Stockholm, Sweden, and colleagues reported similar findings — the overall risk for thyroid cancer with GLP-1s was not increased (HR, 0.93), and the risk beginning 1 year after treatment initiation was lower, at 0.83. 

However, the risk in that study only looking at the first year alone was again higher, at HR 1.47. 

“Our analyses indicating a nominally increased risk restricted to the first year after starting treatment might be consistent with an increased detection of thyroid cancer among patients using GLP-1 RAs,” he told 

In addition to the short follow-up of the current study, Pasternak noted the caveat of the study’s inclusion mainly of patients who initiated the four diabetes drug groups, which is a limitation “because a high proportion of the patients with type 2 diabetes in routine care who start GLP-1 RAs have started it as their third, fourth, or even fifth diabetes drug, especially if they have had diabetes for some time.”

“To capture a larger proportion of the full population of GLP-1 RA users, it would have been preferable to create three different cohorts for the purpose of this study and investigate them separately, including GLP-1 vs DPP4i; GLP-1 vs SGLT2i; GLP-1 vs sulfonylurea,” he said.

Rise in Incidental Thyroid Cancer Detection, An Ongoing Concern

The advent of more advanced imaging and thyroid cancer testing in recent years has already led to a significant problem of over-detection and sometimes over-treatment of incidental, small thyroid cancers that will likely remain harmless in the majority of patients. 

With approximately 64 million prescriptions of GLP-1 RAs dispensed between 2000 and 2015 and the percentage increasing at a rate of about 10%-30% per year, increased thyroid testing among those patients could indeed add to the problem.

Meanwhile, thyroid testing is currently not recommended for patients on GLP-1s who do not have a thyroid cancer risk, and the results from the current and Pasternak’s studies, as well as several other recent studies, appear to support those recommendations.

“Collectively, these reassuring data could potentially help reduce [thyroid cancer] over-detection and screening,” Pasternak said. 

McCoy echoed that the evidence indicates that “for the vast majority of people there is no indication that GLP-1 RA causes thyroid cancer.”

She underscored that overdiagnosis can have important implications, sharing her own personal experience on the receiving end of a cancer diagnosis.

“Overdiagnosis of these low-risk thyroid cancers may ultimately lead to more harm for the patient than not making the diagnosis, since surgery for thyroid cancer carries real risks (including of death) and there are downstream consequences to having your thyroid removed and relying on lifelong hormone replacement,” McCoy said.

“As a cancer survivor myself, though of lymphoma, not thyroid cancer, I also know the profound impact that a cancer diagnosis has on the individual and their loved ones,” McCoy said.

“It is not something I would wish for anyone, which is why I always speak to my patients about searching for something we may not want to —