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, Wegovy, and Underactive Thyroid: What You Need to Know
If you have an underactive thyroid (hypothyroidism) and are considering prescription weight management medication, you may wonder whether Mounjaro (tirzepatide) or Wegovy (semaglutide) is more suitable for your situation. Both are prescription-only GLP-1 or dual-action weight loss injections licensed in the UK, but they work differently and may affect people with thyroid conditions in different ways. This guide compares the two medications and explains what prescribers consider when assessing thyroid patients for weight loss treatment.
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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 and Wegovy Work: Mechanism Differences
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways. This dual mechanism may help regulate appetite, reduce food cravings, and support feeling fuller after meals. It also slows gastric emptying (how quickly food leaves the stomach).
Wegovy (semaglutide) is a single GLP-1 receptor agonist — it activates only the GLP-1 pathway. Clinical trials showed Wegovy helped participants lose an average of approximately 15% of body weight over 68 weeks, whilst Mounjaro KwikPen users in the SURMOUNT trial achieved average weight loss of 15–21% depending on dose.
In terms of thyroid function specifically, neither medication directly affects thyroid hormone production. However, because both slow gastric emptying, they may affect how quickly your body absorbs oral thyroid replacement medication (levothyroxine). This is why prescribers need to assess your individual situation and may recommend spacing doses apart.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Prescriber Assessment: Clinical Considerations for Thyroid Patients
Both Mounjaro and Wegovy require a UK prescription following a clinical assessment. Your prescriber will review your medical history, current thyroid medication, and how well controlled your hypothyroidism is before recommending either medication.
Key considerations include: whether your TSH and free T4 levels are stable, any history of thyroid cancer or thyroid nodules (which may affect eligibility), and whether you're taking other medications that could interact. If you're eligible for either medication, your prescriber will advise which is more suitable based on your individual health profile. Both medications are available through NHS specialist weight management services or private online prescription services following assessment. Use our free BMI calculator to check your current BMI and understand your eligibility threshold for prescription weight loss treatment.
- 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 or Wegovy if I have an underactive thyroid?
Many people with hypothyroidism can take either medication, but you need clinical assessment first. Your prescriber will review your thyroid function (TSH levels), current levothyroxine dose, and overall health. Neither medication directly damages thyroid function, but both slow stomach emptying and may affect how your body absorbs oral thyroid medication.
Which is better for thyroid patients — Mounjaro or Wegovy?
Neither is inherently "better" — individual suitability depends on your personal health profile, thyroid stability, and other factors. Mounjaro uses a dual GIP+GLP-1 mechanism and showed higher average weight loss in SURMOUNT trials. Wegovy uses single GLP-1 mechanism with established long-term data. Your prescriber will recommend the more appropriate option for you.
Will these injections affect my thyroid medication absorption?
Both Mounjaro and Wegovy slow gastric emptying, which may affect how quickly your body absorbs levothyroxine. If prescribed either medication, your GP or prescriber may recommend taking your thyroid medication at a different time of day or monitoring your TSH levels more frequently. Always inform your prescriber about any thyroid medication you're taking.
How do I know if I'm eligible for Mounjaro or Wegovy with hypothyroidism?
UK eligibility requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions. Having a well-controlled underactive thyroid doesn't exclude you, but your TSH must be stable. Your GP can refer you to NHS tier 3 weight management services, or you can access private prescription services following online clinical assessment.
What's the difference between Mounjaro dose titration and Wegovy dose escalation?
Mounjaro KwikPen starts at 2.5 mg weekly, escalating to a maximum of 15 mg. Wegovy starts at 0.25 mg, escalating to 2.4 mg maximum. Both increase gradually — minimum 4 weeks on each dose. Prescribers may adjust escalation speed for thyroid patients to monitor absorption and TSH stability more carefully.
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