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
Free tracker included with every download this week
Mounjaro Side Effects & Thyroid — How It Compares to Wegovy
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections available in the UK, but they work differently and carry different side effect profiles. Thyroid monitoring concerns have emerged with GLP-1 medications, though they are not officially listed as common side effects. This guide explains the clinical differences between these two injections, what the evidence shows about thyroid effects, and how your prescriber will help you choose the right option for your individual health needs.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
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.
Clinical Differences and Side Effect Profiles
Mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 agonist only. This dual mechanism in Mounjaro may affect the body differently, potentially influencing metabolic processes and side effect frequency.
Common side effects both share: Nausea, diarrhoea, vomiting, constipation, and stomach pain occur in both medications. In clinical trials, these were often mild to moderate and improved over time as the body adjusted.
SURMOUNT vs STEP trial data: In SURMOUNT-1 (Mounjaro), gastrointestinal side effects were very common but declined with continued use. In STEP-1 (Wegovy), similar patterns emerged. Neither trial identified thyroid dysfunction as a major adverse event, though post-market vigilance continues.
Dose titration impact: Mounjaro titration (starting 2.5mg) may cause different side effect timing than Wegovy (starting 0.25mg), affecting individual tolerance profiles.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
How It Works
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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Thyroid Concerns and Individual Suitability
Thyroid monitoring with GLP-1 medications remains an area of clinical discussion. While the UK MHRA and NICE have not flagged thyroid dysfunction as a primary concern, some patients have reported thyroid-related symptoms. Your prescriber will assess your thyroid function before starting either medication, particularly if you have a history of thyroid disease or thyroid cancer.
The choice between Mounjaro and Wegovy depends on your personal health profile, not medication superiority. Both are prescription-only and require clinical assessment. Use our free GLP-1 weight loss calculator to understand potential outcomes with either medication, and discuss specific thyroid concerns with your GP or licensed prescriber before commencing treatment.
- 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.
Does Mounjaro affect thyroid function more than Wegovy?
Clinical trials have not identified significant thyroid dysfunction differences between the two. Mounjaro's dual mechanism is distinct, but thyroid effects are not officially recognised as a primary side effect for either. Your prescriber will monitor your baseline thyroid function before starting either medication and will advise on ongoing monitoring if you have thyroid concerns.
What side effects are most common with both Mounjaro and Wegovy?
Nausea, diarrhoea, vomiting, and constipation are the most commonly reported side effects with both medications. These typically occur early in treatment and often improve as your body adjusts. Gastrointestinal symptoms affect more than 1 in 10 people but are usually mild to moderate in severity.
Can I switch from Mounjaro to Wegovy if I experience thyroid issues?
Switching between medications should only be managed by your prescriber. There is no direct equivalent dose conversion between the two. If you have concerns about thyroid effects with either medication, discuss this with your GP or licensed prescriber, who can assess whether a switch is appropriate or if monitoring is sufficient.
How do I access Mounjaro or Wegovy in the UK?
Both medications are prescription-only and available through NHS specialist tier 3 weight management services or private online prescribing following clinical assessment. Your prescriber will assess your eligibility based on BMI, weight-related health conditions, and overall health profile, including thyroid function.
Which medication causes fewer side effects — Mounjaro or Wegovy?
Clinical trial data shows similar frequencies of common side effects for both medications. Individual responses vary significantly, and what suits one person may not suit another. Your prescriber will choose based on your personal health factors, not on side effect frequency alone. Our GLP-1 weight loss calculator can help you understand potential outcomes with either option.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.