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 Conditions and Mounjaro — What You Need to Know
If you have a thyroid condition and are considering Mounjaro for weight management, it's important to understand how tirzepatide interacts with thyroid health. Mounjaro is a prescription-only GLP-1 and GIP receptor agonist approved in the UK for weight management in eligible adults. However, thyroid disease requires careful consideration before starting any new medication. This guide covers NHS guidance, eligibility criteria, and what you should discuss with your GP before beginning tirzepatide treatment.
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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.
Mounjaro, Thyroid Disease, and NHS Eligibility
Mounjaro (tirzepatide) is licensed in the UK for weight management in adults with a BMI of 30 or above, or 27 or above with weight-related health conditions. Thyroid disease itself is not a direct contraindication to Mounjaro, but your individual thyroid status must be assessed by a prescriber before treatment begins.
- Hypothyroidism: If you're stable on levothyroxine (thyroid replacement), Mounjaro may be suitable, but your TSH and free T4 levels should be regularly monitored. Some people report thyroid hormone absorption changes with GLP-1 medications.
- Hyperthyroidism or Graves' disease: Requires specialist assessment. Weight loss medication may be considered, but thyroid function must be controlled first.
- Thyroid cancer history: Discuss with your endocrinologist or oncologist before starting Mounjaro.
Access through NHS specialist tier 3 weight management services includes medical assessment covering thyroid function. Your GP can refer you, though waiting lists vary significantly by region.
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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.
Monitoring and Safety Considerations
If you have thyroid disease and start Mounjaro, your prescriber should monitor thyroid function regularly, particularly in the first few months. GLP-1 medications can affect gut motility, which may impact how your levothyroxine is absorbed if you take it orally — this can slightly alter your thyroid hormone levels.
Common Mounjaro side effects like nausea and changes in appetite may also mask or mimic thyroid symptoms, so clear communication with your GP is essential. Always inform your prescriber about any thyroid condition, current medications, and thyroid blood test results before starting treatment. Use our free BMI calculator to assess your current status, and discuss all options — including NHS specialist services versus private prescription routes — with your healthcare team.
- 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, Mounjaro may be suitable if your thyroid condition is stable and well-controlled on levothyroxine. However, your prescriber must assess your individual case, review your recent TSH and thyroid function tests, and monitor your levels regularly during treatment. GLP-1 medications can occasionally affect thyroid hormone absorption, so ongoing monitoring is important.
Will Mounjaro affect my thyroid medication?
GLP-1 medications like Mounjaro slow stomach emptying, which can theoretically affect how quickly levothyroxine is absorbed. In practice, this is usually minor, but your GP may need to monitor your TSH levels more closely and adjust your levothyroxine dose if necessary. Take levothyroxine at least 30 minutes before food or other medications.
Is Mounjaro safe for thyroid cancer survivors?
This requires specialist assessment. If you have a history of thyroid cancer, discuss Mounjaro with your GP and endocrinologist or oncologist before starting treatment. They will consider your individual risk factors and whether the benefits of weight management outweigh potential concerns.
How often should my thyroid be checked on Mounjaro?
Your GP should check thyroid function (TSH and free T4) before starting Mounjaro, then monitor regularly — typically every 6–12 weeks during the first year, depending on your individual circumstances. More frequent monitoring may be needed if your thyroid levels change unexpectedly.
Can I get Mounjaro on the NHS if I have thyroid disease?
Yes, you may be eligible if you meet BMI criteria (≥30, or ≥27 with weight-related health conditions). Thyroid disease is not a barrier to NHS access, but it must be disclosed during assessment. Referral to your local tier 3 specialist weight management service is the NHS route. Access varies by region and waiting times can be lengthy.
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.