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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Does Mounjaro Affect Your Thyroid?
Thyroid concerns are a common question when considering Mounjaro (tirzepatide) for weight management. While Mounjaro is not known to directly damage the thyroid gland, there are important thyroid-related considerations you should understand before starting this prescription-only medication. This guide explains what current evidence shows about Mounjaro, tirzepatide, and thyroid health in UK adults, and when to speak with your GP about personal thyroid risk.
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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.
What the Clinical Evidence Shows About Mounjaro and Thyroid
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist approved in the UK for weight management. In the SURMOUNT clinical trials, which evaluated tirzepatide at doses from 5 mg to 15 mg once weekly, thyroid tumours were observed in animal studies — specifically in rodents. However, these animal studies do not automatically translate to human risk.
- Mounjaro carries a Black Triangle (▼) warning from the MHRA, meaning it requires additional monitoring for safety
- The product information notes a theoretical risk based on animal data, not human clinical trial evidence
- No cases of thyroid cancer were reported in human SURMOUNT trials
- GLP-1 medications (like Wegovy/semaglutide) have been in clinical use for over a decade without established thyroid cancer links in humans
- Individual risk assessment by your prescriber is essential before starting
If you have a personal or family history of thyroid cancer or Multiple Endocrine Neoplasia (MEN) syndrome, inform your prescriber — Mounjaro may not be 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.
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Thyroid Monitoring and What You Should Know
If your prescriber deems Mounjaro appropriate for you, monitoring may be recommended. Inform your GP immediately if you experience: persistent neck lumps or swelling, difficulty swallowing, hoarseness, rapid heartbeat, or unexplained weight loss despite the medication. Your GP can arrange thyroid blood tests (TSH, free T4) if needed.
Weight loss itself can affect thyroid hormone levels — when you lose weight, your metabolism changes and thyroid function may shift slightly. This is separate from direct medication effects. Use our GLP-1 weight loss calculator to estimate potential weight changes, and discuss thyroid monitoring with your prescriber as part of your overall care plan.
- 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.
Does Mounjaro damage the thyroid gland directly?
No direct damage has been reported in human clinical trials. However, animal studies showed potential thyroid tumour risk, which is why Mounjaro carries a Black Triangle warning. Your prescriber will assess whether Mounjaro is appropriate based on your individual health history, especially if you have thyroid cancer history or MEN syndrome.
Can I take Mounjaro if I have thyroid disease?
This depends on your specific thyroid condition and history. If you have hypothyroidism (underactive thyroid) controlled with levothyroxine, Mounjaro may be suitable — but inform your prescriber. If you have had thyroid cancer, Mounjaro is generally not recommended. Always disclose full thyroid history before starting any weight loss medication.
What thyroid symptoms should I watch for on Mounjaro?
Monitor for neck lumps, swelling, difficulty swallowing, hoarseness, persistent cough, rapid heartbeat, or unexplained weight loss. These can indicate thyroid problems. Seek medical attention if you notice any of these. Routine symptoms like tiredness may relate to weight loss itself rather than thyroid issues, but discuss with your GP.
Will Mounjaro affect my thyroid medication (levothyroxine)?
Mounjaro may slow how quickly oral medications are absorbed in the stomach. If you take levothyroxine for hypothyroidism, your GP may recommend taking it on an empty stomach and spacing it several hours from other medicines. Inform your prescriber of all medications — thyroid function may need rechecking after starting Mounjaro.
How is Mounjaro different from Wegovy regarding thyroid risk?
Both are GLP-1-type medications, but Mounjaro (tirzepatide) is a dual GIP+GLP-1 agonist — a newer mechanism. Wegovy (semaglutide) is a single GLP-1 agonist with longer clinical history. Both carry theoretical thyroid risks based on animal data. Discuss specific thyroid concerns with your prescriber when choosing between tirzepatide and semaglutide options available on UK prescription.
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