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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Can You Take Thyroxine With Mounjaro?
If you're taking thyroxine for an underactive thyroid and considering Mounjaro (tirzepatide) for weight management, it's important to understand how these medications interact. Thyroxine and Mounjaro can be taken together, but there are important UK prescribing considerations your GP or prescriber needs to know. This guide explains what you need to tell your healthcare team, why the interaction matters, and how NHS and private prescribers manage this combination 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.
How Thyroxine and Mounjaro Interact
Thyroxine (levothyroxine) is a thyroid hormone replacement used to treat hypothyroidism (underactive thyroid). Mounjaro is a GIP and GLP-1 receptor agonist — a dual-action weight management injection licensed in the UK.
The key interaction concern is that Mounjaro slows gastric emptying (how quickly food and medications leave the stomach). This can potentially affect how quickly thyroxine is absorbed. Additionally, Mounjaro may cause weight loss, which can affect thyroid hormone requirements — some patients need dose adjustments after significant weight loss.
However, this combination is not contraindicated. Many UK patients take both medications safely. What matters is:
- Informing your prescriber of your thyroxine dose before starting Mounjaro
- Taking thyroxine on an empty stomach (as always) — at least 30 minutes before food
- Monitoring thyroid function with blood tests after weight loss begins
- Your GP may need to adjust your thyroxine dose as your weight changes
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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.
NHS and Private Prescribing Considerations
When accessing Mounjaro through NHS specialist tier 3 weight management services, your medical history — including thyroxine use — will be reviewed. The NHS prescriber will assess whether Mounjaro is appropriate given your thyroid status.
If accessing Mounjaro through private online prescribing, you must declare your thyroxine dose during the online health assessment. A UK-licensed prescriber will then decide if the combination is suitable for you. Neither NHS nor private routes should proceed without full medication disclosure.
After starting Mounjaro: Ask your GP to recheck your thyroid function (TSH blood test) approximately 6–8 weeks after starting, and again after each significant weight loss milestone. Weight loss sometimes requires a lower thyroxine dose; conversely, some patients need no adjustment. Use our free BMI calculator to track weight changes, and discuss results with your GP at each check-up.
- 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 thyroxine from working?
No. Mounjaro may slow how quickly thyroxine is absorbed, but taking thyroxine on an empty stomach (30+ minutes before food) minimises this effect. Most UK patients on both medications absorb thyroxine adequately. However, your GP should recheck your thyroid function via blood tests after starting Mounjaro to ensure your dose remains correct.
Do I need to change my thyroxine dose when starting Mounjaro?
Not necessarily at the start. However, as Mounjaro causes weight loss, your thyroxine requirements may change — weight loss sometimes lowers the dose needed. Your GP will monitor this with TSH blood tests at 6–8 weeks and after major weight loss. Dose adjustments are made only on the basis of blood results, not weight loss alone.
Can I take thyroxine and Mounjaro at the same time of day?
No. Thyroxine must be taken on an empty stomach, ideally first thing in the morning. Mounjaro is a once-weekly injection (no time restrictions). Take thyroxine at least 30 minutes before breakfast, then you can eat and inject Mounjaro at any time that week — separation is not critical for the injection.
What should I tell my GP or prescriber before starting Mounjaro?
Disclose your thyroxine dose (and TSH level if you know it), when you were diagnosed with hypothyroidism, and any recent thyroid function test results. If accessing Mounjaro privately, the online health form will ask about all medications — list thyroxine with your exact dose. This information helps the prescriber assess your suitability.
Will my thyroxine dose need adjusting if I lose weight on Mounjaro?
Possibly. As body weight decreases, some patients need a lower thyroxine dose to maintain normal TSH. Your GP will identify this via blood tests, not by guessing. After significant weight loss (e.g., >10% body weight), request a TSH recheck. Use our free progress tracker to log weight changes — this helps you and your GP spot patterns and timing for dose reviews.
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.