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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Hypothyroidism and Mounjaro — A Practical Guide
If you have hypothyroidism and are considering Mounjaro (tirzepatide) for weight management, it's important to understand how the medication may interact with your thyroid condition. Hypothyroidism itself can make weight loss more challenging, and combining it with weight loss treatment requires careful medical assessment. This guide explains the key considerations, how Mounjaro compares to Wegovy, and what you should discuss with your GP before starting any weight loss medication.
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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 vs Wegovy — Key Differences for Thyroid Health
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — meaning it activates two hormone pathways in the gut to help regulate appetite and blood sugar. Wegovy (semaglutide) activates only the GLP-1 pathway. In clinical trials, Mounjaro showed slightly higher average weight loss (approximately 15–21% of body weight) compared to Wegovy (approximately 15% on average), though individual results vary significantly.
For people with hypothyroidism specifically, the key difference is mechanism: Mounjaro's dual action may affect metabolic rate more noticeably than Wegovy's single mechanism. Both medications slow gastric emptying and reduce appetite, but Mounjaro's GIP component has additional metabolic effects that some prescribers monitor more closely in thyroid patients. Clinical trials (SURMOUNT for Mounjaro, STEP for Wegovy) did not primarily focus on thyroid disease, so individual suitability depends on your specific thyroid status and other health factors.
- Mounjaro starting dose: 2.5 mg weekly, escalating to maximum 15 mg
- Wegovy starting dose: 0.25 mg weekly, escalating to maximum 2.4 mg
- Injection frequency: Both are once-weekly injections via KwikPen or pre-filled pen
Neither medication is insulin. Both are prescription-only and require a clinical assessment before being prescribed in the UK.
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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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Hypothyroidism and Weight Loss Medication — What Your Prescriber Will Consider
If you have hypothyroidism, your GP or prescriber will assess whether your thyroid is well-controlled (TSH levels stable on your current levothyroxine dose) before approving Mounjaro or Wegovy. Uncontrolled hypothyroidism can actually cause weight gain and slow metabolism, so optimising thyroid management is typically the first step. Once your thyroid is stable, weight loss medication may be suitable, but your prescriber may recommend additional thyroid function monitoring while you're on GLP-1 or GIP/GLP-1 treatment.
Both medications can cause nausea and appetite suppression, which may mask or worsen hypothyroid symptoms like fatigue. Use our free BMI calculator to establish your current status, and our GLP-1 weight loss calculator to explore projected outcomes — then share these with your GP when discussing options. Individual suitability is determined through clinical assessment, not on medication choice alone.
- 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 if I have hypothyroidism?
Possibly, but it requires prescriber approval. Your thyroid function must be well-controlled on levothyroxine first. Mounjaro is not contraindicated in hypothyroidism, but additional thyroid monitoring may be recommended during treatment. Your GP or prescriber will assess your individual case and thyroid blood test results before deciding.
Will Mounjaro affect my thyroid medication dosage?
Mounjaro may indirectly affect thyroid medication absorption because it slows gastric emptying. Some patients require levothyroxine dose adjustments. It's essential to inform your prescriber of your hypothyroidism and current levothyroxine dose, and to have your TSH levels monitored regularly while starting Mounjaro.
Is Wegovy safer than Mounjaro for hypothyroidism?
Not necessarily. Both are prescription-only medications requiring clinical assessment. Wegovy's single GLP-1 mechanism may be considered less metabolically complex, but neither medication is contraindicated in hypothyroidism. Your prescriber will recommend whichever is more suitable based on your thyroid status, other health conditions, and personal factors.
Should I wait until my thyroid is perfectly controlled before starting Mounjaro?
Yes, in most cases. Hypothyroidism should be optimised (stable TSH) before starting weight loss medication. Uncontrolled thyroid disease can cause fatigue and metabolic dysfunction, making weight loss difficult. Stabilise your thyroid first, then discuss weight loss options with your GP.
Do Mounjaro and Wegovy interact with levothyroxine?
Both medications slow gastric emptying, which may affect oral medication absorption including levothyroxine. Take levothyroxine as directed by your prescriber (usually on an empty stomach), and inform your prescriber you're considering Mounjaro or Wegovy. Thyroid function should be monitored with regular blood tests.
Ready to take control of your weight loss journey?
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