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 Levothyroxine — What You Need To Know
If you take levothyroxine (thyroid medication) and are considering Mounjaro (tirzepatide) for weight management, it's important to understand how these two medications interact. Mounjaro is a prescription-only GLP-1 and GIP receptor agonist that may slow the absorption of other oral medications, including levothyroxine. This means your thyroid hormone levels could be affected. Always inform your prescriber if you take levothyroxine before starting Mounjaro, as dose timing and monitoring adjustments may be necessary. Your GP or prescriber will assess whether Mounjaro is suitable for you individually.
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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.
How Mounjaro Affects Levothyroxine Absorption
Mounjaro slows gastric emptying — the rate at which food and medications move from your stomach to your small intestine. This delay can reduce how much levothyroxine your body absorbs, potentially lowering your thyroid hormone levels and affecting the effectiveness of your thyroid medication.
Levothyroxine is taken orally and requires proper absorption in the gut to maintain stable thyroid hormone levels. If Mounjaro reduces this absorption, you may experience:
- Lower TSH and T4 levels than expected
- Return of hypothyroidism symptoms (fatigue, weight gain, cold sensitivity)
- Need for thyroid function monitoring and possible dose adjustments
This interaction is one reason prescribers take a detailed medication history before starting tirzepatide or when considering whether Mounjaro is appropriate for you. Individual suitability depends on your specific thyroid condition and medication stability.
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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.
Safe Use And Prescriber Guidance
If you're on levothyroxine and wish to start Mounjaro, your GP or specialist will likely recommend:
- Timing separation — taking levothyroxine and Mounjaro at different times of day
- TSH monitoring — regular blood tests to check thyroid hormone levels after starting Mounjaro
- Levothyroxine dose review — your prescriber may adjust your thyroid medication dose if needed
- Wegovy as an alternative — if absorption concerns are significant, your prescriber might suggest semaglutide instead, though individual assessment is always required
Never adjust your levothyroxine dose without guidance from your GP. Weight loss medications like Mounjaro should always be used alongside reduced-calorie diet and increased physical activity for best results.
- 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 may reduce levothyroxine absorption, but it won't completely stop it. Your prescriber will monitor your thyroid levels with blood tests and adjust doses if necessary. Timing your medications separately (e.g., levothyroxine in the morning, Mounjaro at a different time) can help. Never stop taking levothyroxine without medical guidance.
Should I take levothyroxine and Mounjaro at the same time?
No. Levothyroxine works best on an empty stomach, typically taken first thing in the morning. Mounjaro injections are once-weekly. Your GP will advise on optimal timing to minimise absorption interactions and maintain thyroid hormone stability.
Can I use Wegovy instead if I take levothyroxine?
Wegovy (semaglutide) is a GLP-1-only medication, whereas Mounjaro is a dual GIP+GLP-1 agonist. Both can affect oral medication absorption, though the mechanism differs slightly. Your prescriber will assess which is more suitable for you based on your thyroid condition and individual health profile. Both require clinical assessment.
Will I need more frequent thyroid blood tests on Mounjaro?
Yes, likely. If you start Mounjaro, your GP will typically monitor your TSH levels more frequently initially to ensure your levothyroxine dose remains appropriate. Once stable, monitoring intervals may return to normal. Always discuss monitoring plans with your healthcare provider.
What's the difference between Mounjaro and Wegovy for this situation?
Mounjaro is a dual GIP+GLP-1 agonist (tirzepatide), starting at 2.5mg and increasing to 15mg maximum. Wegovy is GLP-1-only (semaglutide), reaching 2.4mg maximum. Clinical trials show Mounjaro achieved approximately 15–21% average body weight loss versus Wegovy at around 15%. Both slow gastric emptying and may affect levothyroxine absorption, so both require prescriber assessment and thyroid monitoring.
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