Thyroid safety is a topic that frequently arises when discussing GLP-1 based medications, including Mounjaro (tirzepatide). Understanding the evidence and precautions can help UK patients make informed decisions in consultation with their healthcare team.
The Thyroid Concern With GLP-1 Medications
In preclinical studies, GLP-1 receptor agonists (including tirzepatide) caused thyroid C-cell tumours in rodents at elevated doses. This led to a boxed warning in the United States and corresponding precautions in UK prescribing information. However, it is important to understand the context:
- Rodent thyroid C-cells have a much higher density of GLP-1 receptors than human C-cells
- No causal link between GLP-1 receptor agonists and thyroid cancer has been established in humans
- Large-scale epidemiological studies and post-marketing surveillance have not identified an increased risk
Who Should Not Take Mounjaro
The MHRA and the Mounjaro Summary of Product Characteristics advise that tirzepatide is contraindicated in patients with:
- A personal or family history of medullary thyroid carcinoma (MTC)
- Multiple Endocrine Neoplasia syndrome type 2 (MEN 2)
If you have either of these conditions, Mounjaro should not be prescribed and alternative treatments should be explored.
Hypothyroidism and Mounjaro
Many UK patients take levothyroxine for underactive thyroid (hypothyroidism). If you are in this group, be aware that Mounjaro's effect on gastric emptying may influence levothyroxine absorption. Practical steps include:
- Continue taking levothyroxine on an empty stomach, ideally 30-60 minutes before food
- Request thyroid function tests (TSH and free T4) 6-8 weeks after starting Mounjaro and after dose changes
- Your GP may need to adjust your levothyroxine dose based on results
Hyperthyroidism Considerations
Patients with active or uncontrolled hyperthyroidism should have their thyroid condition stabilised before starting Mounjaro. The increased metabolic rate associated with hyperthyroidism, combined with reduced caloric intake from tirzepatide, could lead to excessive weight loss and nutritional concerns.
Monitoring Recommendations
For all patients starting Mounjaro, being aware of thyroid-related symptoms is prudent. Contact your GP if you notice:
- A new lump or swelling in the neck
- Difficulty swallowing or persistent hoarseness
- Unexplained changes in energy levels, heart rate, or weight beyond expected treatment effects
Overall, the thyroid safety profile of Mounjaro is considered acceptable by UK regulators for the vast majority of patients. Discuss any thyroid concerns openly with your prescriber to ensure safe and effective treatment.