The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Mounjaro And Hyperthyroidism — What UK Patients Need To Know
If you have hyperthyroidism and are considering weight loss medication, it's important to understand how Mounjaro (tirzepatide) and Wegovy (semaglutide) may interact with thyroid conditions. Both are prescription-only GLP-1 based injections licensed for weight management in the UK, but they work differently and may carry different considerations for people with overactive thyroid. This guide explains the key differences, clinical evidence, and why individual assessment by your GP or prescriber is essential. Neither medication should be started without thorough clinical evaluation of your thyroid status and current treatment.
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What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
How Mounjaro And Wegovy Work — And Why Mechanism Matters For Thyroid Health
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — meaning it activates two separate hormone pathways. This dual mechanism may have different effects on metabolism and gastric function compared to single-action medications. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating one pathway. Clinical trials show both support weight loss: the SURMOUNT-1 trial (Mounjaro) demonstrated average weight loss of 20.9% at the 15mg dose over 72 weeks, whilst the STEP-1 trial (Wegovy) showed approximately 14.9% average weight loss at 2.4mg over 68 weeks. Both used alongside reduced-calorie diet and physical activity. For people with hyperthyroidism, the dual mechanism of Mounjaro may theoretically affect metabolism differently, though individual response varies significantly. Your prescriber will assess which medication, if any, is appropriate based on your thyroid medication stability and overall health status.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Hyperthyroidism, Weight Loss Medication, And Clinical Assessment
Hyperthyroidism (overactive thyroid) accelerates metabolism, making weight management complex — some people with untreated hyperthyroidism experience weight loss despite increased appetite, whilst others gain weight. Before starting Mounjaro, Wegovy, or any GLP-1 medication, your prescriber must know: your current thyroid hormone levels, what antithyroid medication you're taking, whether your thyroid is stable, and any history of thyroid eye disease. Both medications require a UK prescription following clinical assessment — this assessment includes full medical history review. A prescriber will consider whether your thyroid is adequately controlled first, as uncontrolled hyperthyroidism may complicate weight loss medication use. Your GP can refer you to a specialist weight management service, or you can access private prescribing through a licensed UK prescriber after online consultation. Individual suitability depends on your specific thyroid status, not on the medication type alone.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Can I take Mounjaro if I have hyperthyroidism?
Mounjaro is prescription-only and requires individual clinical assessment. A prescriber must review your thyroid status, current medication, and whether your hyperthyroidism is stable before deciding. Some people with well-controlled hyperthyroidism may be eligible, but this is an individual decision. Always inform your prescriber of any thyroid condition.
Is Wegovy safer than Mounjaro for thyroid conditions?
Neither medication is inherently "safer" — both are prescription-only and require clinical assessment. Wegovy is a single GLP-1 agonist whilst Mounjaro is dual GIP+GLP-1, meaning different mechanisms. Your prescriber will decide which (if any) suits your thyroid status. Safety depends on individual health factors, not medication choice alone.
Will weight loss injections affect my thyroid medication dose?
Weight loss medications may affect how quickly your body processes other medications, including antithyroid drugs. If prescribed either Mounjaro or Wegovy, your GP may need to monitor your thyroid levels more frequently and potentially adjust doses. Always inform your prescriber of all current medications.
What's the difference between Mounjaro's tirzepatide and Wegovy's semaglutide?
Mounjaro (tirzepatide) activates both GIP and GLP-1 pathways, whilst Wegovy (semaglutide) activates GLP-1 only. Clinical trials show both support weight loss, though Mounjaro showed slightly higher average weight loss in SURMOUNT trials. Both require prescription and clinical assessment — your GP decides which is appropriate for you.
How do I start weight loss medication if I have hyperthyroidism?
Contact your GP first to discuss your thyroid status and weight management goals. You can be referred to an NHS tier 3 weight management service (though waiting lists vary), or access private prescribing through a UK-licensed prescriber following online consultation. Either route requires full medical assessment before prescription.
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