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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Thyroid Cancer and Mounjaro — What You Need to Know
Mounjaro (tirzepatide) carries an important safety warning related to thyroid cancer risk. If you have a personal or family history of thyroid cancer, or medullary thyroid carcinoma (MTC), you should not take Mounjaro. This guide explains the thyroid cancer warning, what the evidence shows, and how to discuss this with your GP before starting any weight loss medication. Always inform your healthcare provider of your full medical history.
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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.
Thyroid Cancer Risk and Mounjaro — The Safety Warning
Mounjaro carries a black box warning (additional MHRA monitoring) regarding thyroid C-cell tumours, based on preclinical studies in animals. Here's what this means in UK practice:
- Animal studies: Tirzepatide (Mounjaro's active ingredient) caused thyroid C-cell tumours in rodent models at high doses. These findings are mandatory to report in the UK product information.
- Human evidence: No cases of thyroid cancer have been directly linked to tirzepatide use in human clinical trials. The SURMOUNT trials did not show increased thyroid cancer in participants.
- Precaution vs. proven risk: The warning exists because of animal findings, not because thyroid cancer has occurred in people taking the medication. This is a precautionary measure by regulators.
- Contraindication: Mounjaro is contraindicated (not safe to use) if you have personal or family history of medullary thyroid carcinoma (MTC) or multiple endocrine neoplasia (MEN) syndrome type 2.
Your prescriber will assess your thyroid history before offering Mounjaro.
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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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Who Should Not Take Mounjaro Due to Thyroid Risk
Mounjaro is not suitable if you have:
- Personal history of medullary thyroid carcinoma (MTC) — a rare form of thyroid cancer
- Personal history of multiple endocrine neoplasia (MEN) syndrome type 2
- Family history of MTC or MEN-2 in close relatives
You must disclose any thyroid cancer history, including family history, to your GP or prescriber before starting Mounjaro. If you're unsure about your family medical history, discuss this with relatives or ask your GP to review your records. Patients with a family history but no personal diagnosis may still be suitable for Mounjaro, depending on individual risk assessment — this is a decision for your prescriber.
- 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 cause thyroid cancer?
No cases of thyroid cancer have been directly attributed to Mounjaro in human clinical trials. The warning is based on animal studies where high-dose tirzepatide caused thyroid tumours in rodents. Regulatory agencies (MHRA in the UK) require this warning as a precaution, not because the risk has been proven in people.
What is medullary thyroid carcinoma (MTC)?
Medullary thyroid carcinoma is a rare type of thyroid cancer that develops from C-cells in the thyroid gland. It can occur sporadically or run in families (hereditary MTC). If you have a personal or family history of MTC, you cannot take Mounjaro because of the animal study findings showing C-cell tumour risk.
Can I take Mounjaro if my family member had thyroid cancer?
This depends on the type of cancer and your relationship. If a close relative had medullary thyroid carcinoma (MTC) or MEN syndrome type 2, you should inform your prescriber immediately. Your GP can assess your individual risk. Mounjaro is contraindicated only if you have personal MTC or MEN-2, or a strong family history of these conditions.
How should I disclose thyroid history to my prescriber?
Be honest and detailed. Tell your prescriber about: personal thyroid cancer diagnosis (especially MTC), family history of MTC or MEN-2, previous thyroid surgery or biopsy, and any thyroid nodules you've had. If you're unsure, ask your GP to check your medical records before any weight loss treatment consultation.
Are there alternative weight loss medications if I can't take Mounjaro?
Yes. Wegovy (semaglutide), another GLP-1 receptor agonist, also carries a thyroid warning but may be considered depending on your individual risk profile. Saxenda (liraglutide) is another option. Your GP can discuss alternatives based on your thyroid history. NHS specialist weight management services can also advise on non-medication approaches.
Ready to take control of your weight loss journey?
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