Mounjaro and Thyroid Medication: UK Safety Guidance

Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist licensed in the UK for type 2 diabetes mellitus. Many patients requiring Mounjaro also take thyroid medications, particularly levothyroxine for hypothyroidism, making potential interactions a common clinical concern. Mounjaro slows gastric emptying, which may theoretically affect absorption of oral thyroid hormones. Whilst no absolute contraindication exists for concurrent use, understanding the pharmacological considerations, monitoring requirements, and safety profile—including thyroid C-cell tumour findings in animal studies—is essential for safe prescribing. This article provides evidence-based guidance for UK patients and healthcare professionals on managing Mounjaro alongside thyroid medication.

Understanding Mounjaro (Tirzepatide) and Thyroid Medication Interactions

Mounjaro (tirzepatide) is a dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonist licensed in the UK for the treatment of type 2 diabetes mellitus. As with any medication, understanding potential interactions with other treatments is essential for safe and effective use.

Thyroid medications, particularly levothyroxine for hypothyroidism, are among the most commonly prescribed treatments in the UK. Many patients requiring Mounjaro for diabetes management also have concurrent thyroid disorders, making the question of drug interactions clinically relevant. The pharmacological mechanism of Mounjaro involves enhancing insulin secretion in a glucose-dependent manner and slowing gastric emptying, which can theoretically affect the absorption of oral medications. This effect on gastric emptying is most pronounced after initiation and each dose escalation.

Key considerations include:

• The potential impact of delayed gastric emptying on thyroid hormone absorption

• Potential changes in thyroid function test results during weight loss

• The theoretical risk of thyroid C-cell tumours observed in animal studies

• Timing of medication administration to optimise therapeutic efficacy

Understanding these interactions requires knowledge of both Mounjaro's mechanism of action and the pharmacokinetics of thyroid replacement therapy. Patients taking both medications should be aware that whilst no absolute contraindication exists, careful monitoring and potential dose adjustments may be necessary. Healthcare professionals must consider individual patient factors, including the stability of thyroid function, diabetes control, and overall treatment goals when prescribing Mounjaro to patients on thyroid medication.

Can You Take Mounjaro with Levothyroxine or Other Thyroid Hormones?

Yes, Mounjaro can generally be taken alongside levothyroxine and other thyroid hormone replacement therapies, but specific precautions should be observed. There is no established direct pharmacological interaction between tirzepatide and levothyroxine that would make concurrent use dangerous. However, the MHRA/EMC SmPC notes that Mounjaro's effect on gastric emptying may influence the absorption of oral medications, particularly those with a narrow therapeutic index like levothyroxine.

Levothyroxine is typically absorbed in the small intestine, with optimal absorption occurring when taken on an empty stomach. The delayed gastric emptying caused by Mounjaro could theoretically alter the rate and extent of levothyroxine absorption, though clinical evidence for significant interaction remains limited. To minimise any potential impact, patients should maintain consistent timing of their thyroid medication.

Practical guidance for concurrent use:

• Take levothyroxine first thing in the morning on an empty stomach (at least 30 minutes before food) or consistently at bedtime

• Maintain consistent timing of thyroid medication throughout Mounjaro treatment

• Avoid taking specific interacting medications or supplements (iron, calcium, antacids, cholestyramine) within 4 hours of levothyroxine as per NHS guidance

• Continue regular thyroid function monitoring as recommended by your GP

For patients taking liothyronine (T3) or combination thyroid preparations, similar principles apply. Patients should inform their healthcare provider about all medications they are taking, including over-the-counter preparations and supplements, to ensure comprehensive medication review and optimised treatment planning. If significant weight loss occurs or new symptoms develop, thyroid function tests may need to be checked to ensure appropriate dosing.

Thyroid Cancer Risk and Mounjaro: What UK Patients Need to Know

One of the safety considerations surrounding Mounjaro and related GLP-1 receptor agonists involves the potential risk of thyroid C-cell tumours, specifically medullary thyroid carcinoma (MTC). This concern arose from preclinical studies in rodents, where tirzepatide and similar medications caused dose-dependent and treatment-duration-dependent thyroid C-cell tumours, including medullary thyroid carcinomas.

However, it is crucial to understand that there is no established link between Mounjaro use and thyroid cancer in humans. The relevance of rodent C-cell findings to human risk remains uncertain, as rodents have a much higher density of GLP-1 receptors in thyroid C-cells compared to humans. Current data have not demonstrated an increased incidence of MTC in patients treated with GLP-1 receptor agonists, including tirzepatide, though surveillance continues.

The UK SmPC for Mounjaro includes information about these rodent findings and advises:

• Patients should be counselled about the symptoms of thyroid tumours

• The human relevance of rodent C-cell tumours is unknown

• Healthcare professionals should maintain clinical vigilance

Before initiating Mounjaro, healthcare professionals should take a thorough personal and family history regarding thyroid disorders. Patients should be informed about the symptoms of thyroid tumours, including a neck mass, dysphagia, dyspnoea, or persistent hoarseness. These symptoms may warrant urgent suspected cancer referral via the 2-week-wait pathway according to NICE NG12 guidance.

Patients are encouraged to report any suspected side effects to the MHRA Yellow Card Scheme at yellowcard.mhra.gov.uk or search for 'MHRA Yellow Card' in the Google Play or Apple App Store.

Managing Hypothyroidism and Hyperthyroidism While on Mounjaro

Patients with pre-existing thyroid disorders can safely use Mounjaro, but their thyroid condition should ideally be optimised before initiation and monitored throughout treatment. For those with hypothyroidism, stable thyroid hormone replacement is beneficial before starting Mounjaro, as uncontrolled hypothyroidism can affect metabolic parameters and complicate diabetes management.

Weight loss induced by Mounjaro may necessitate adjustments to levothyroxine dosing. As body weight decreases, thyroid hormone requirements often change, potentially leading to over-replacement if doses are not reviewed. Symptoms of excessive thyroid hormone (thyrotoxicosis) include palpitations, tremor, anxiety, heat intolerance, and unintentional weight loss beyond that expected from Mounjaro therapy. Conversely, inadequate replacement may manifest as fatigue, cold intolerance, and constipation—symptoms that can overlap with Mounjaro's gastrointestinal side effects.

For patients with hyperthyroidism or those on antithyroid medications (carbimazole or propylthiouracil), Mounjaro is not contraindicated but requires careful consideration. Hyperthyroidism itself can affect glucose metabolism and cardiovascular function, and the addition of Mounjaro necessitates close monitoring of both thyroid status and glycaemic control.

Management recommendations include:

• Optimise thyroid control before initiating Mounjaro where possible

• Monitor thyroid function tests following NICE NG145 principles (e.g., 6–8 weeks after any levothyroxine dose change) and adjust for clinical status/weight change

• Adjust thyroid medication doses based on biochemical results and clinical symptoms

• Be alert to symptoms suggesting thyroid dysfunction during weight loss

• Maintain coordinated care between GP and endocrinology teams where appropriate

NICE guidance emphasises individualised care for patients with multiple endocrine conditions, recognising that optimising one condition may impact others.

Monitoring Thyroid Function During Mounjaro Treatment

Monitoring of thyroid function during Mounjaro treatment should be individualised based on clinical circumstances, particularly for those with pre-existing thyroid disorders or those on thyroid hormone replacement therapy. Whilst Mounjaro does not directly alter thyroid hormone production, the metabolic changes associated with weight loss and improved glycaemic control can influence thyroid function test results and medication requirements.

For patients without known thyroid disease, baseline thyroid function testing (TSH at minimum) can be considered before initiating Mounjaro if there are symptoms suggestive of thyroid dysfunction or risk factors such as family history, autoimmune conditions, or previous thyroid surgery. Routine monitoring during treatment is not mandatory for those with normal baseline function, but clinicians should maintain a low threshold for testing if symptoms develop.

For patients on levothyroxine or other thyroid medications, monitoring should be clinician-guided:

• Baseline assessment: TSH and free T4 before starting Mounjaro, if not recently checked

• Consider testing: If clinically meaningful changes occur (significant weight loss, new symptoms, or after substantial dose escalation)

• Ongoing monitoring: Follow usual annual monitoring when stable, in line with NICE NG145 guidance

• Post-weight loss: Consider additional testing once target weight is achieved and stabilised

Significant weight loss can alter the volume of distribution and metabolism of thyroid hormones, potentially requiring dose adjustments. A decrease in TSH with elevated or high-normal free T4 may indicate over-replacement, whilst an elevated TSH suggests inadequate replacement. Clinical correlation is essential, as symptoms should guide management alongside biochemical results.

Documentation of weight changes, medication adherence, and timing of blood tests relative to medication administration helps ensure accurate interpretation of results. As per NHS guidance, patients should be advised to take their levothyroxine consistently and to inform the phlebotomy service if blood tests are scheduled, ideally before the morning dose of thyroid medication.

When to Seek Medical Advice: Thyroid Symptoms and Mounjaro Use

Patients taking Mounjaro alongside thyroid medication should be aware of symptoms that warrant prompt medical attention. Whilst most individuals tolerate the combination well, certain signs may indicate thyroid dysfunction, medication interaction issues, or rare but serious complications requiring clinical assessment.

Call 999 immediately if you experience:

• Severe chest pain

• Severe breathing difficulty

• Signs of severe thyrotoxicosis: extreme confusion, high fever, or very rapid heart rate

Seek urgent medical advice (contact GP or NHS 111) if you experience:

• A lump or swelling in the neck, particularly if rapidly enlarging

• Unexplained persistent hoarseness or voice changes (for example >3 weeks)

• Difficulty swallowing (dysphagia) or breathing (dyspnoea)

• Unexplained persistent neck pain

• Palpitations, irregular heartbeat, or non-severe chest pain

These symptoms may warrant an urgent suspected cancer referral via the 2-week-wait pathway according to NICE NG12 guidance.

Contact your GP for routine review if you notice:

• Symptoms of under-active thyroid: excessive fatigue, cold intolerance, unexplained weight gain (beyond expected), constipation, dry skin, or low mood

• Symptoms of over-active thyroid: heat intolerance, excessive sweating, tremor, anxiety, or weight loss exceeding that expected from Mounjaro

• Persistent gastrointestinal symptoms that interfere with medication adherence

• Concerns about medication timing or interactions

It is important to distinguish between expected side effects of Mounjaro (nausea, reduced appetite, gastrointestinal disturbance) and symptoms of thyroid dysfunction. Keeping a symptom diary can help identify patterns and facilitate discussion with healthcare professionals. Patients should never adjust thyroid medication doses without medical guidance, as both under- and over-treatment carry health risks. Regular medication reviews, ideally every 6–12 months or when circumstances change, ensure that both Mounjaro and thyroid treatments remain optimally dosed. If you are under the care of both an endocrinologist and GP, ensure both are aware of all medications prescribed, as coordinated care improves outcomes and reduces the risk of adverse events.

If you suspect an adverse reaction to Mounjaro, report it to the MHRA Yellow Card Scheme at yellowcard.mhra.gov.uk or search for 'MHRA Yellow Card' in the Google Play or Apple App Store.

Frequently Asked Questions