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 Underactive Thyroid — What You Need to Know
If you have an underactive thyroid (hypothyroidism) and are considering Mounjaro for weight management, it's important to understand how these conditions interact. Mounjaro (tirzepatide) is a prescription-only GLP-1 and GIP receptor agonist injection licensed in the UK for weight management in adults. This guide explains the relationship between Mounjaro and thyroid health, eligibility, and how to access it safely in the UK.
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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 Conditions, and Weight Management
Underactive thyroid (hypothyroidism) is a condition where your thyroid gland doesn't produce enough thyroid hormone, slowing your metabolism and often contributing to weight gain. Weight management becomes more challenging with hypothyroidism, but it remains possible with the right support.
Mounjaro works differently to thyroid medication — it's a dual GIP and GLP-1 receptor agonist that helps regulate appetite and support feeling fuller after meals, working alongside a reduced-calorie diet and increased physical activity. Tirzepatide does not treat or affect your underlying thyroid condition, so your thyroid medication will continue to work independently.
A key point: you can take Mounjaro while managing hypothyroidism, but your GP or prescriber must be aware of your thyroid condition before prescription. Some people with hypothyroidism have successfully lost weight with GLP-1 medications when their thyroid is well-controlled on levothyroxine or other thyroid replacement therapy.
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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.
UK Access, Eligibility, and What to Expect
To access Mounjaro in the UK, you'll need a prescription from a UK-licensed healthcare professional. NHS route: referral to a specialist tier 3 weight management service, though waiting lists vary by region and may be lengthy. Private route: online consultation with a UK-licensed prescriber, typically faster but involves costs ranging from approximately £130–£280 monthly depending on dose.
When you apply for Mounjaro, you must declare your hypothyroidism and your current thyroid medication. Your prescriber will assess whether Mounjaro is suitable for you. The starting dose is 2.5 mg once weekly, increasing gradually to a maximum of 15 mg. Track your progress using our free progress tracker to monitor weight changes alongside your thyroid management.
For eligibility, adults with a BMI of 30 kg/m² or above, or 27 kg/m² with weight-related health conditions (which may include poorly controlled hypothyroidism-related weight gain), may be considered. Use our free BMI calculator to check your current BMI. Combined with a reduced-calorie diet and increased physical activity, Mounjaro may help you achieve meaningful weight loss — though individual results vary. Always ensure your GP monitors both your thyroid levels and weight loss progress if you're on Mounjaro.
- 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 an underactive thyroid?
Yes, you can take Mounjaro alongside thyroid medication, provided your hypothyroidism is well-controlled and your prescriber is aware of your condition. Mounjaro does not treat or interfere with thyroid function — it works on appetite regulation. Your levothyroxine or other thyroid medication will continue to work independently. Always inform your prescriber of your thyroid condition and current medication.
Will Mounjaro affect my thyroid medication dosage?
Mounjaro itself does not directly affect thyroid hormone levels or your levothyroxine dose. However, as you lose weight with Mounjaro, your energy requirements may change slightly. Your GP may review your thyroid medication dose periodically as part of routine monitoring. This is normal practice — inform your GP that you're using Mounjaro so they can monitor both conditions together.
What's the cost of Mounjaro in the UK privately?
Private Mounjaro costs typically range from £130–£280 per month depending on your dose (starting at 2.5 mg, increasing to a maximum of 15 mg). Prices vary by prescribing service. NHS access is free but requires referral to a specialist weight management service — waiting times vary significantly by region. Compare timelines and costs when deciding between NHS and private routes.
How do I access Mounjaro on the NHS if I have hypothyroidism?
Ask your GP for a referral to your local tier 3 specialist weight management service. You'll need a BMI of 30 or above, or 27 or above with weight-related health conditions. Declare your hypothyroidism during assessment. NHS availability and waiting lists vary considerably by region (ICB). Private prescription is an alternative if NHS waiting times are long in your area.
Should I use MyWeightCheck's GLP-1 calculator if I have underactive thyroid?
Yes — our free GLP-1 weight loss calculator can help you estimate potential weight loss at different Mounjaro doses. Remember that hypothyroidism may affect your baseline metabolism, so actual results may vary from projections. Use the calculator alongside your GP's advice as part of your weight management planning. Track real progress with our progress tracker once you start treatment.
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.