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
Will Thyroxine Cause Weight Loss?
Thyroxine (levothyroxine) is a prescription thyroid hormone replacement used to treat hypothyroidism in the UK. While thyroxine itself is not a weight loss medication, correcting low thyroid function can sometimes support weight management. However, weight changes with thyroxine are highly individual and depend on how well your dose is balanced. If you're experiencing unexpected weight changes after starting thyroxine, always discuss this with your GP rather than adjusting doses yourself.
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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.
How Thyroxine Affects Your Weight
Thyroxine replaces thyroid hormone in people with hypothyroidism (underactive thyroid). Your thyroid controls your metabolic rate — how quickly your body burns calories at rest. When thyroid hormone is low, your metabolism slows, which can contribute to weight gain and difficulty losing weight.
When you take thyroxine and your thyroid function normalises, your metabolic rate should return to normal. This may make it easier to lose weight through diet and exercise — but thyroxine is not a weight loss medication itself. Some people do lose weight after starting thyroxine, but others see no change, and a small number gain weight if their dose becomes too high (overtreatment).
The amount of weight change depends on:
- How low your thyroid function was before treatment
- How well your dose is balanced
- Your age, genetics, and overall health
- Your diet and activity level
Your GP will adjust your thyroxine dose based on blood tests (TSH levels), aiming for the right balance — not too little and not too much.
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.
Weight Changes and Thyroxine — What's Normal?
Some weight loss or stabilisation in the first few months after starting thyroxine is normal as your metabolism settles. However, thyroxine is not prescribed for weight loss — it's prescribed to restore normal thyroid function. If you're looking for weight loss support beyond managing your thyroid health, your GP can discuss other options with you, including lifestyle changes, dietitian referral, or — for eligible adults with a BMI of 30 or above, or 27 or above with weight-related health conditions — prescription weight loss medication.
Use our free BMI calculator above to understand your current weight status. If you're looking to lose weight alongside thyroid management, our free progress tracker can help you log changes over time. Always discuss any weight concerns and thyroid medication adjustments with your GP — never alter your thyroxine dose without medical guidance.
- 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 thyroxine help me lose weight?
Thyroxine is not a weight loss medication. However, if you have hypothyroidism, correcting your thyroid function with thyroxine can normalise your metabolism, making it easier to lose weight through diet and exercise. Weight changes vary by individual.
Will I lose weight after starting thyroxine?
Some people experience modest weight loss or stabilisation after starting thyroxine as their metabolism settles. Others see no change. Weight loss is not assured. Your GP will monitor your thyroid levels and adjust your dose to ensure it's balanced — too much thyroxine can cause weight gain.
How long does it take to see weight changes with thyroxine?
Thyroid function typically stabilises within 6–8 weeks of starting or adjusting thyroxine. Some weight changes may happen during this time, but metabolism can take longer to fully adjust. Consistent results usually appear after 3–6 months on a stable dose.
Is thyroxine different from weight loss injections?
Yes. Thyroxine is a thyroid hormone replacement for hypothyroidism. Weight loss injections like Wegovy (semaglutide) are GLP-1 medications prescribed specifically for weight management in eligible adults. They work through different mechanisms and are prescribed for different reasons.
What should I do if I'm gaining weight on thyroxine?
If you're gaining weight despite taking thyroxine, contact your GP. You may need a blood test (TSH check) to see if your dose needs adjusting — either too little or too much thyroxine can affect weight. Never adjust your dose yourself.
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.