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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Thyroid Health and Mounjaro: What the Evidence Shows
If you have thyroid concerns and are considering Mounjaro for weight management, it's important to understand how this medication works and whether it's suitable for you. Mounjaro (tirzepatide) is a prescription-only GLP-1 and GIP receptor agonist licensed in the UK for weight management in adults. This guide explains the relationship between Mounjaro and thyroid health, compares it to Wegovy, and helps you understand what to discuss with your GP or prescriber before starting 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 Works and Thyroid Considerations
Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways to help regulate appetite and support weight loss. Unlike Wegovy (semaglutide), which is a GLP-1 agonist only, Mounjaro's dual mechanism may offer different effects — though clinical benefits are comparable in weight loss outcomes.
Regarding thyroid health: GLP-1 medications like Wegovy have well-established safety data in thyroid monitoring, and there is no evidence that GLP-1 receptor agonists cause thyroid disease or dysfunction. However, gradual weight management itself can temporarily affect thyroid hormone levels in some people. If you have an existing thyroid condition (such as hypothyroidism managed with levothyroxine), your GP may need to monitor your thyroid function more closely during weight loss, as your medication dose might need adjustment.
- Mounjaro is administered as a once-weekly injection using a KwikPen device
- Doses are titrated from 2.5 mg up to a maximum of 15 mg weekly
- Average weight loss in clinical trials: 15–21% of body weight over 72 weeks alongside diet and physical activity
- Both medications require a clinical assessment before prescription
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Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
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Mounjaro vs Wegovy: Mechanism, Efficacy, and Prescriber Considerations
Mounjaro and Wegovy are both licensed in the UK for weight management, but their mechanisms differ. Mounjaro's dual action may theoretically provide different metabolic effects compared to Wegovy's single GLP-1 pathway. In clinical trials — SURMOUNT-1 for Mounjaro and STEP-1 for Wegovy — both showed significant weight loss: Mounjaro averaging 20.9% at the maximum 15 mg dose, Wegovy averaging 14.9% at 2.4 mg.
For people with thyroid conditions, both are considered appropriate with GP oversight. Your prescriber will assess which is more suitable based on your individual health profile, existing medications (including thyroid replacement therapy), and any concerns about drug interactions. The choice between Mounjaro and Wegovy is not determined by thyroid status alone — both require prescription, clinical assessment, and baseline health checks including thyroid function if clinically indicated.
- 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 hypothyroidism?
Yes, most people with well-controlled hypothyroidism can take Mounjaro with GP oversight. Your GP should monitor your thyroid function (TSH levels) during weight loss, as your levothyroxine dose may need adjustment. Always inform your prescriber of all medications, including thyroid replacement therapy, before starting Mounjaro.
Does Mounjaro cause thyroid problems?
There is no evidence that GLP-1 receptor agonists or Mounjaro cause thyroid disease. However, gradual weight management can temporarily affect thyroid hormone metabolism. If you have an existing thyroid condition, your GP may adjust your monitoring frequency during treatment.
How does Mounjaro compare to Wegovy for thyroid safety?
Both Mounjaro and Wegovy have similar safety profiles regarding thyroid health. Neither causes thyroid dysfunction. Both are prescription-only and both require clinical assessment. The choice between them depends on your individual health profile, not thyroid status alone. Your prescriber will recommend the most suitable option.
What should I tell my GP about my thyroid before starting Mounjaro?
Inform your prescriber of any thyroid condition (hypothyroidism, hyperthyroidism, thyroid nodules), current thyroid medications (levothyroxine, carbimazole), and your most recent TSH levels if available. They may recommend baseline thyroid function tests and more frequent monitoring during weight loss treatment.
How can I track my weight loss safely alongside thyroid treatment?
Use a free progress tracker to log your weight loss and any symptoms (fatigue, mood changes, temperature sensitivity) that might indicate your thyroid medication needs adjustment. Share this with your GP during routine reviews. Sustainable loss of 0.5–1 kg per week is recommended and helps minimise metabolic disruption.
Ready to take control of your weight loss journey?
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