Do Semaglutide or Tirzepatide cause thyroid cancer?

There have been concerns about whether weight loss medications are linked to an increased risk of developing thyroid cancer.

What current evidence shows on thyroid cancer and weight loss drugs:

• In large human studies, neither semaglutide nor tirzepatide have been proven to increase the risk of thyroid cancer. In clinical trials involving thousands of people, there were very few thyroid cancers, and no clear pattern linking the medicines to these cases has been identified. A recent large European review (EMA, 2024) looked at all GLP-1 medications (including semaglutide and tirzepatide) and found no evidence of a link to thyroid cancer in people.
• One safety review in the US looked at reports up to early 2024 and found a small number of thyroid cancer cases in people taking these medicines. However, this doesn’t mean the medicine caused the cancer — it could simply be that people and doctors were more likely to report anything unusual while on a new treatment
• A few people had mild changes in a thyroid marker called calcitonin, but they didn’t develop cancer.
•  Some very rare cases of temporary thyroid inflammation (thyroiditis) have been reported with tirzepatide; these usually go away after stopping treatment
• These medicines did cause certain thyroid tumours in rats, which is why they carry a safety warning. Because of what happened in animal studies, the manufacturers and regulators include a warning about the theoretical risk of a rare thyroid cancer (called medullary thyroid cancer, or MTC), which is a rare type of cancer, and most people are not at risk.
• Human C cells have far fewer GLP 1 receptors, making this less likely to occur in humans
•  As a precaution, these medicines should not be used if you or someone in your family has had MTC or a genetic condition called MEN2 (Multiple Endocrine Neoplasia type 2).
• However, the way rat thyroid cells respond to these drugs seems different from human cells — so it doesn’t necessarily mean the same would happen in people.

If you’re taking semaglutide or tirzepatide, seek medical advice if you notice:

• A lump or swelling in your neck
• Hoarseness or voice changes
• Trouble swallowing
• Any unexplained neck pain or pressure

The bottom line based on studies:

• Human clinical and real-world data (up to ~3–4 years) show no confirmed increase in thyroid cancer risk with either medication.
• Animal studies prompted safety warnings and contraindications for rare thyroid conditions.
• Pharmacovigilance signals exist but are likely due to more frequent testing and screenings.
• Overall: strong reassurance for most patients, but extra precautions if you have a history of MTC or MEN2; continuous vigilance is required.

For more advice on thyroid cancer and weight loss drugs, arrange a consultation at our weight loss clinic.