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
Can I Take Mounjaro With Levothyroxine?
If you're taking levothyroxine for an underactive thyroid and considering Mounjaro for weight management, you'll need to discuss this combination with your GP. While there's no absolute contraindication between Mounjaro (tirzepatide) and levothyroxine, Mounjaro may affect how your body absorbs oral medications — including levothyroxine — which is why careful monitoring is essential. Your prescriber will need to know about all your current medications before assessing your eligibility for this prescription-only injection.
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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.
How Mounjaro Affects Medication Absorption
Mounjaro works partly by slowing gastric emptying — the rate at which food and medications move through your stomach into the small intestine. This means oral medications like levothyroxine may be absorbed more slowly when you start Mounjaro, particularly at higher doses during dose titration. Your GP or prescriber may recommend:
- Taking levothyroxine at a consistent time each day, ideally on an empty stomach
- Maintaining a fixed interval between your levothyroxine and other oral medicines
- Having your thyroid function monitored more frequently — TSH and free T4 blood tests
- Possibly adjusting your levothyroxine dose if absorption patterns change
Many UK patients successfully take both medications, but individual response varies. Your prescriber will consider your thyroid history and current levothyroxine dose before deciding if Mounjaro is suitable for you.
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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.
What to Tell Your NHS or Private Prescriber
When seeking Mounjaro through NHS specialist services or private online prescribing, always disclose that you're taking levothyroxine. Provide details including your levothyroxine dose (typically 50–200 mcg daily in the UK), how long you've been on it, and your most recent TSH result if available. Your prescriber will use this information to assess whether Mounjaro is appropriate for you and may request baseline thyroid function tests before you start. If you're eligible and begin treatment, regular thyroid monitoring becomes especially important to ensure your levothyroxine remains at the right level.
- 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.
Will Mounjaro stop my levothyroxine from working?
Mounjaro doesn't prevent levothyroxine from working, but it may slow its absorption because it reduces how quickly food and medications move through your stomach. This is why consistent dosing and timing are important. Your GP can monitor your thyroid function with blood tests to ensure your levothyroxine remains effective.
Should I take levothyroxine and Mounjaro at the same time?
No. Take levothyroxine on an empty stomach, typically first thing in the morning, at least 30–60 minutes before food or other medications. Space Mounjaro separately — it's a once-weekly injection, so timing is less critical, but discuss the exact schedule with your prescriber.
Can I get Mounjaro on the NHS if I take levothyroxine?
Yes, you can potentially access Mounjaro through NHS tier 3 specialist weight management services if you meet BMI and health criteria. However, your thyroid condition and levothyroxine use will be part of the clinical assessment. Some regional services assess thyroid stability before proceeding; others don't — it depends on your local NHS integrated care board.
Will I need more thyroid tests if I start Mounjaro?
Likely yes. Your prescriber may recommend TSH and free T4 blood tests at baseline, then again after 6–8 weeks and periodically thereafter. This ensures your levothyroxine dose remains optimal while you're adjusting to Mounjaro. More frequent monitoring is a reasonable precaution given the potential for altered medication absorption.
What if my thyroid symptoms worsen after starting Mounjaro?
Contact your GP immediately. Symptoms like fatigue, weight gain, or cold intolerance despite taking levothyroxine may indicate your dose needs adjusting. Your GP can arrange urgent thyroid function tests. Don't stop either medication without medical guidance — both levothyroxine and Mounjaro require proper prescriber oversight.
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.