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 Hyperthyroidism — What You Need to Know
If you have hyperthyroidism and are considering Mounjaro (tirzepatide) for weight management, it's important to understand how this medication may interact with your thyroid condition. Mounjaro is a prescription-only GIP and GLP-1 receptor agonist licensed in the UK for weight management in adults, but certain medical conditions require specialist assessment before starting treatment. This guide explains what you need to discuss with your GP or prescriber, and how hyperthyroidism may affect your eligibility for this medication.
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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, Hyperthyroidism, and Medical Safety
Mounjaro works by activating GIP and GLP-1 hormone pathways to reduce appetite and support weight loss. However, if you have hyperthyroidism or any thyroid condition, you must inform your prescriber before starting treatment. Hyperthyroidism increases your metabolic rate and heart rate, which may be affected by GLP-1 receptor agonists.
The clinical concern is not that Mounjaro directly causes thyroid problems, but that:
- GLP-1 medications may increase heart rate in some people — hyperthyroidism already increases this
- Weight loss itself can affect thyroid hormone levels and medication requirements
- Your GP or prescriber needs to assess whether the combined effects are safe for you individually
- Your thyroid function tests (TSH, Free T4) may need monitoring during treatment
A UK-licensed prescriber will review your thyroid history, current medications (including thyroid treatments), and TSH levels before deciding if Mounjaro is appropriate. This is not a blanket contraindication — it requires individual clinical assessment.
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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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What to Tell Your Prescriber About Your Thyroid
Before starting Mounjaro, always disclose to your GP or online prescriber:
- That you have hyperthyroidism or any thyroid condition
- Whether your thyroid is controlled with medication (levothyroxine, carbimazole, propranolol, etc.)
- Your most recent TSH and Free T4 results if available
- Any symptoms of thyroid problems (palpitations, tremor, heat intolerance, anxiety)
- All medications you take, including thyroid treatments
Your prescriber may request recent blood results or ask you to get thyroid function tests before proceeding. If you're taking a beta-blocker for hyperthyroidism symptoms, your prescriber will also assess whether Mounjaro is compatible with that. Honest communication about your medical history is essential for safe prescribing.
- 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 hyperthyroidism?
Possibly, but only with specialist assessment. Hyperthyroidism is not an absolute contraindication, but your GP or prescriber must review your thyroid control, current medications, and blood tests first. If your thyroid is well-controlled and you're stable, Mounjaro may be suitable — but this requires individual clinical judgment.
Will Mounjaro affect my thyroid medication dose?
Weight loss itself can affect thyroid hormone requirements, so your TSH levels may change during treatment. Your GP may need to adjust your levothyroxine or other thyroid medication. This is another reason thyroid monitoring is important during and after Mounjaro treatment.
What if my hyperthyroidism is not well-controlled?
If your thyroid is not well-controlled (TSH abnormal, symptoms present), most prescribers will ask you to stabilise your thyroid condition first before starting Mounjaro. Once your thyroid is stable on medication, reassessment for weight loss treatment can be considered.
Does Mounjaro dose titration affect thyroid function?
The dose titration schedule (starting at 2.5 mg and increasing weekly doses up to 15 mg) is designed to minimise side effects, but does not directly affect thyroid function. However, as your weight changes with each dose increase, your thyroid hormone requirements may gradually shift — another reason for monitoring.
Should I get thyroid tests before starting Mounjaro?
Yes, absolutely. A UK-licensed prescriber will either request recent thyroid function tests (TSH, Free T4) or ask you to get them done before starting. Current, accurate thyroid results are essential for safe prescribing and dose adjustment decisions.
Ready to take control of your weight loss journey?
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