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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Can I Take Mounjaro With Underactive Thyroid?
Mounjaro (tirzepatide) may be prescribed to UK adults with a BMI of 30 or above, or 27 or above with weight-related health conditions. If you have an underactive thyroid (hypothyroidism), you may wonder whether it's safe to use this prescription weight loss injection. The short answer is: it's possible, but requires careful assessment and monitoring by your GP or prescriber. This guide explains what you need to know about combining Mounjaro with thyroid conditions.
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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 and Thyroid Function — What You Need to Know
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist that works by helping regulate appetite and supporting feelings of fullness. It does not directly affect thyroid function in the way some weight loss medications might. However, there are important considerations if you have an underactive thyroid.
People with hypothyroidism often gain weight more easily, and weight loss medication may be an option. However, your prescriber needs to know:
- Whether your thyroid condition is well-controlled on current medication
- Your current levothyroxine (or equivalent) dose
- Your recent thyroid function test (TSH and free T4) results
- Any symptoms of thyroid imbalance you still experience
Mounjaro doesn't directly interfere with levothyroxine absorption in the way some medications do. However, significant weight loss can affect how your body uses thyroid hormone, potentially requiring dose adjustments. This is why ongoing monitoring is essential.
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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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Before Starting Mounjaro — What Your GP Needs to Know
Always tell your prescriber that you have an underactive thyroid before starting Mounjaro or any weight loss medication. They need a complete picture of your health. Your GP may:
- Review your thyroid medication dose and whether it's optimised
- Request recent thyroid function tests to establish a baseline
- Monitor your TSH and free T4 levels more frequently during treatment
- Adjust your levothyroxine dose if needed as you lose weight
- Screen for other weight-related conditions that may qualify you for Mounjaro (such as type 2 diabetes, high blood pressure, or cardiovascular disease)
Use our free BMI calculator above to check whether you may be eligible for prescription weight loss treatment, then discuss your results and thyroid condition with your GP.
Starting a Mounjaro KwikPen requires a valid UK prescription from a licensed prescriber following a clinical assessment. The assessment will include questions about your medical history, including thyroid conditions. If your thyroid is well-controlled and your prescriber agrees it's safe, Mounjaro may be an option. During treatment, you'll typically have thyroid function tests every 6–12 months, or more frequently if your dose needs adjustment. Weight loss itself may mean you need less levothyroxine over time, so regular monitoring protects your health. Never adjust your thyroid medication on your own — always involve your GP or 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 affect thyroid function directly?
No, Mounjaro does not directly interfere with thyroid hormone production or levothyroxine absorption. However, significant weight loss can affect how your body uses thyroid hormone, so ongoing monitoring is important if you have hypothyroidism.
Will Mounjaro interact with levothyroxine?
There is no direct pharmacological interaction between Mounjaro and levothyroxine. However, always inform your prescriber about all medications, including thyroid medication. Weight changes may require levothyroxine dose adjustments, which your GP will monitor.
Can I get Mounjaro on the NHS if I have underactive thyroid?
Possibly. NHS access to Mounjaro (tirzepatide) is through tier 3 specialist weight management services. You'll need referral from your GP and assessment of your eligibility. Hypothyroidism alone doesn't prevent access, but your thyroid must be stable and well-managed before starting.
How often will my thyroid be monitored on Mounjaro?
Your GP will likely recommend thyroid function tests (TSH and free T4) every 6–12 months during Mounjaro treatment, or more frequently if weight loss is significant or if your levothyroxine dose changes. This ensures your dose remains appropriate.
What if my thyroid medication needs adjusting while on Mounjaro?
This is a normal part of weight loss treatment. As you lose weight, your body may need less levothyroxine. Your GP will monitor your TSH levels and adjust your dose as needed. Never self-adjust thyroid medication — always involve your healthcare team.
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.