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
Free tracker included with every download this week
Can You Take Mounjaro If You Have An Underactive Thyroid?
If you have an underactive thyroid (hypothyroidism) and are considering Mounjaro for weight management, you're right to check compatibility first. Mounjaro (tirzepatide) is not contraindicated in hypothyroidism, but it requires careful assessment by your prescriber. Underactive thyroid can affect metabolism and weight, so your GP or prescriber needs to know about your thyroid condition before starting any weight loss medication. This guide explains what you need to know about taking Mounjaro with thyroid disease, and how it compares to alternatives like Wegovy.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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 vs Wegovy: Thyroid Considerations and Clinical Differences
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only GLP-1-based weight loss medications, but they work slightly differently. Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1-only agonist. Neither is specifically contraindicated in hypothyroidism, but both require individualised clinical assessment.
In clinical trials, Mounjaro showed average weight loss of approximately 15–21% of body weight, while Wegovy showed around 14.9% average loss. However, individual results vary significantly. Both medications slow gastric emptying, which can affect the absorption of thyroid replacement medication (levothyroxine) — this is why your prescriber must know you have hypothyroidism.
- Mounjaro: dual mechanism (GIP + GLP-1)
- Wegovy: single mechanism (GLP-1 only)
- Both require prescription and clinical assessment
- Both affect medication absorption timing
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Thyroid Health and Weight Loss Medication: What Your Prescriber Needs to Know
When discussing Mounjaro or Wegovy with your GP or prescriber, always mention your hypothyroidism and current levothyroxine dose. GLP-1 medications slow how quickly food (and medications) pass through your stomach, which can reduce levothyroxine absorption if taken at the same time.
Your prescriber may recommend taking levothyroxine at least 30 minutes before Mounjaro or Wegovy, or at a different time of day entirely. You may also need thyroid function tests (TSH and free T4) monitored more closely once you start weight loss medication, as weight loss itself can affect thyroid hormone requirements. Neither medication directly affects thyroid function, but weight loss can improve thyroid control — your prescriber will individualise this based on your health.
- 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.
Is Mounjaro safe to take if I have hypothyroidism?
Mounjaro is not contraindicated in hypothyroidism, but it requires clinical assessment. Your prescriber must know about your thyroid condition before prescription. Neither Mounjaro nor thyroid medication directly harm each other, but Mounjaro can affect levothyroxine absorption, so timing of doses is important. Your GP will assess suitability based on your specific situation.
Will Mounjaro affect my levothyroxine absorption?
Yes, Mounjaro slows gastric emptying, which can reduce how quickly levothyroxine is absorbed. To minimise this, take levothyroxine at least 30 minutes before your Mounjaro injection, or at a different time of day. Your prescriber may also recommend more frequent thyroid function tests to ensure your levothyroxine dose remains optimal during weight loss.
Should I take Wegovy instead of Mounjaro if I have an underactive thyroid?
Both medications affect medication absorption similarly, so neither is inherently "safer" than the other with hypothyroidism. The choice between Mounjaro and Wegovy depends on your individual health, prescriber recommendation, and clinical assessment. Both are prescription-only and require discussion with your GP or licensed prescriber about your thyroid condition.
Will weight loss from Mounjaro affect my thyroid levels?
Weight loss itself can improve thyroid control and may reduce your levothyroxine requirements. This is why thyroid function tests should be monitored if you start Mounjaro — your prescriber may need to adjust your levothyroxine dose as you lose weight. Regular TSH and free T4 tests help ensure your thyroid remains optimally managed.
How do I access Mounjaro in the UK if I have hypothyroidism?
Mounjaro is prescription-only and available through NHS specialist weight management services (tier 3) or private online prescribing following a clinical assessment. Your prescriber will assess your eligibility based on BMI and health conditions. Tell them about your hypothyroidism, current levothyroxine dose, and recent TSH levels — this information is essential for safe prescribing and dose management.
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.