A Balanced Review of the Current Evidence

With the rise in popularity of weight management and diabetes medications like Mounjaro® (tirzepatide), we have received numerous enquiries regarding potential impacts on thyroid health. As an ENT and Thyroid specialist practice, we believe it is vital to provide a balanced overview of what we know, what we don’t know, and the current safety advice.

Please note: This information is for educational purposes and does not replace a consultation with your prescribing physician or specialist.

What is Mounjaro?

Mounjaro® (active ingredient: tirzepatide) is a medication used for the treatment of Type 2 diabetes and is also used for weight management. It works as a dual receptor agonist (GIP and GLP-1), helping to improve insulin sensitivity, reduce appetite, and promote weight loss.

The Core Concern: Does Mounjaro Cause Thyroid Cancer?

The short answer is: We do not categorically know yet. There is currently no definitive evidence that Mounjaro causes thyroid cancer in humans, but safety monitoring is ongoing.

The concern stems from two main sources:

1. Animal Studies: In pre-clinical studies involving rats, tirzepatide was found to cause thyroid C-cell tumors, including Medullary Thyroid Carcinoma (MTC). However, it is currently unknown if this result applies to humans, as rodents have specific thyroid biology that differs from ours.
2. Database Signals: Recent reviews of the FDA Adverse Event Reporting System (FAERS) have noted a positive association between GLP-1 receptor agonists (like tirzepatide) and reported cases of thyroid cancer. However, these databases rely on voluntary reporting and cannot prove that the drug caused the cancer, only that the two were reported together.

What Does the Latest Human Research Say?

While animal studies raised flags, human data provides a more nuanced picture. Two major 2025 updates shed light on this:

• Comparison to Metformin: A 2025 study published in the Journal of Surgical Research compared patients taking GLP-1 analogues against those taking Metformin (a standard diabetes drug). While there was a slightly higher risk compared to patients taking no drugs, the study found no significant difference in thyroid cancer risk between patients on GLP-1s and those on Metformin. This suggests that the underlying conditions (such as obesity or diabetes) may be driving the risk, rather than the medication itself.
• Current Monitoring: Human studies with follow-up periods of 1.8 to 3 years have not found a significant increase in thyroid cancer risk. However, experts agree that data is still too limited to rule out long-term risks entirely, especially for rare tumor types.

The UK Government & MHRA Stance

In November 2024, the UK Government addressed these concerns in Parliament. The Medicines and Healthcare products Regulatory Agency (MHRA) stated:

"Thyroid cancer has not been causally linked with Mounjaro or any other medicines that belong to the same therapeutic class... and it is currently not listed as a side-effect associated with GLP-1RA medicines in the approved United Kingdom prescribing information."

The MHRA and the European Medicines Agency (EMA) reviewed the evidence and decided it was insufficient to establish a link. Consequently, unlike in the USA where the FDA requires a "boxed warning" based on animal data, UK labeling does not currently carry this warning because the human relevance of the rat studies is unproven.

Understanding Thyroid Cancer & Neuroendocrine Tumours

To understand the specific risks discussed in safety literature, it helps to understand the anatomy of the thyroid.

1. Medullary Thyroid Carcinoma (MTC)
This is the specific type of cancer seen in the rat studies. MTC is rare and accounts for a small percentage of thyroid cancers.

• It is a form of Neuroendocrine Tumour (NET).
• What is a NET? Neuroendocrine tumours develop from cells that release hormones into the blood in response to signals from the nervous system. In the thyroid, these are called "C-cells" (which produce calcitonin).

2. Papillary and Follicular Thyroid Cancer
These are the most common types of thyroid cancer. They develop from follicular cells (which produce T3 and T4 hormones), not C-cells. Interestingly, while the animal concern is about MTC, some database reports have noted cases of Papillary cancer. However, as noted above, obesity itself is a significant risk factor for these cancers.

Who Should Avoid Mounjaro?

Based on the manufacturer’s guidelines (Eli Lilly), you should not use Mounjaro if:

• You have a personal or family history of Medullary Thyroid Carcinoma (MTC).
• You have Multiple Endocrine Neoplasia syndrome type 2 (MEN 2). This is a rare genetic condition that almost always leads to MTC.

There are generally no specific reasons why people with benign (non-cancerous) thyroid nodules or hypothyroidism cannot use these medications, provided they do not have the family history mentioned above.

Recommendations for Patients

1. Be Alert to Symptoms
While on treatment, you should monitor for symptoms including:

• A lump or swelling in the neck.
• Persistent hoarseness.
• Trouble swallowing.
• Shortness of breath.
• If you experience these, contact your healthcare provider.

2. If You Take Levothyroxine
If you have an underactive thyroid (hypothyroidism), substantial weight loss can change your medication needs. It is recommended to check your thyroid function (TSH levels) approximately 2–3 months after starting weight loss treatment to see if your Levothyroxine dose requires adjustment.

3. Discuss with Your Team
If you have a history of other types of thyroid cancer or are concerned about nodules, discuss this with your specialist. Decisions should be made based on your individual medical history, weighing the proven benefits of weight loss and diabetes control against the theoretical risks.

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  References:
• Eli Lilly. Open Letter Regarding the Use of Mounjaro® (tirzepatide) and Zepbound® (tirzepatide). March 7, 2024.
• Daou, C. et al. "Exploring Connections Between Weight-Loss Medications and Thyroid Cancer: A Look at the FDA Adverse Event Reporting System Database." Endocrinology, Diabetes & Metabolism. 2025. Balachandra, S. et al. "Evaluating Thyroid Cancer Risk in Glucagon-like Peptide-1 Analog Users With Thyroid Nodules." Journal of Surgical Research. August 2025.
• UK Parliament. Question UIN 13677, answered by Karin Smyth (Department of Health and Social Care), 18 November 2024.
• Neuroendocrine Cancer UK. "Mounjaro® (Tirzepatide) & Neuroendocrine Cancers." July 14, 2025.
• The Thyroid Trust. "Can I use weight loss injections if I have a thyroid condition?" June 2025.