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 Thyroid Medication — What You Need to Know
If you take thyroid medication and are considering Mounjaro (tirzepatide) for weight management, it's important to understand how these medications interact. Mounjaro may affect how your body absorbs oral medications, including thyroid treatments, which means your thyroid medication dose may need adjusting. Your GP or prescriber must know about all medications you're taking before starting tirzepatide. This guide covers NHS guidance, interaction risks, and what to expect when combining these treatments.
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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 Thyroid Medication Absorption
Mounjaro (tirzepatide) is a once-weekly injection that slows gastric emptying — the rate at which food and medication move through your stomach into the small intestine. This mechanism, while helping with weight management, can delay the absorption of oral medications, including thyroid replacements like levothyroxine.
Thyroid medication must be absorbed efficiently to maintain stable hormone levels. If Mounjaro slows this absorption, your body may receive lower effective doses of thyroid medication, potentially leading to:
- Underactive thyroid symptoms (fatigue, weight gain, mood changes)
- Changes in TSH levels requiring dose adjustment
- Reduced effectiveness of your thyroid treatment
This doesn't mean you cannot take both medications together — it means your GP or endocrinologist must monitor your thyroid function more carefully. Blood tests (TSH and free T4) will likely be ordered more frequently when starting or adjusting tirzepatide doses. Your UK prescriber should always be informed of your thyroid condition and current levothyroxine dose before tirzepatide is initiated.
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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.
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NHS Prescribing Guidance and Practical Advice
NHS guidance: Tirzepatide is a MHRA-approved prescription medicine for weight management in adults. If you access Mounjaro through NHS tier 3 specialist weight management services, your care team will coordinate with your GP and any endocrinologist managing your thyroid. Private prescribers should also ask detailed questions about your thyroid history.
Practical steps to take:
- Tell your GP or prescriber about your thyroid condition and current levothyroxine dose
- Request thyroid function tests (TSH) before starting Mounjaro
- Schedule follow-up TSH testing 6–8 weeks after starting or increasing Mounjaro doses
- Take levothyroxine as advised — ideally 30 minutes before food, on an empty stomach
- Report any symptoms (fatigue, temperature sensitivity, mood changes) to your GP
- Keep your GP and weight management team informed of all your medications
FAQ:
Using our free GLP-1 weight loss calculator can help you explore projected outcomes with Mounjaro while your GP manages your thyroid health in parallel. Calculate your healthy weight range with our BMI calculator to understand your individual targets.
- 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 affect my levothyroxine dose?
Mounjaro may reduce absorption of levothyroxine by slowing gastric emptying. Your levothyroxine dose may need adjusting, which is why TSH monitoring is important. Your GP or endocrinologist will decide if a dose change is needed based on blood test results after starting tirzepatide.
Do I need thyroid tests before starting Mounjaro?
Yes. Your prescriber should request baseline TSH and free T4 tests before you start tirzepatide, especially if your thyroid is underactive. This establishes your baseline and helps identify any changes caused by Mounjaro. Follow-up tests should be scheduled 6–8 weeks after starting or each dose increase.
Can I take levothyroxine and Mounjaro at the same time?
Not simultaneously. You should take levothyroxine 30 minutes before breakfast on an empty stomach. Since Mounjaro is injected once weekly (not oral), timing isn't a direct conflict — but the slowed gastric emptying from Mounjaro can reduce levothyroxine absorption, which is why monitoring is essential.
Should I tell my GP about Mounjaro if I'm on thyroid medication?
Absolutely. Your GP must know you're considering or taking Mounjaro alongside levothyroxine. If you access Mounjaro privately, inform your prescriber of your thyroid condition. Your GP should coordinate your thyroid monitoring during treatment.
What symptoms suggest my thyroid medication needs adjusting?
Contact your GP if you experience persistent fatigue, weight gain despite Mounjaro, cold intolerance, dry skin, constipation, or low mood. These can indicate underactive thyroid. Your GP will check TSH levels. Don't adjust levothyroxine yourself — always seek medical guidance.
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.