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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T3 and Weight Loss — What UK Patients Should Know
T3 (liothyronine) is a thyroid hormone replacement medication prescribed for hypothyroidism, but many people wonder whether it can help with weight loss. While T3 does affect metabolism, it is not licensed for weight loss in the UK and should only be used under medical supervision for thyroid conditions. If you're interested in evidence-based weight loss support, there are other prescription options available through the NHS or private prescribing. This guide explains how T3 works, what the evidence shows, and what alternatives may be suitable for you.
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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.
How T3 Works and Its Effect on Weight
T3 (liothyronine) is a synthetic form of the thyroid hormone triiodothyronine. It plays a crucial role in regulating your metabolism — essentially how quickly your body burns calories at rest. When thyroid hormone levels are low (hypothyroidism), metabolic rate slows, which can contribute to weight gain or difficulty losing weight.
Some people assume that taking T3 will boost metabolism and lead to weight loss. However, T3 is only prescribed to restore normal thyroid function in people with hypothyroidism, not to artificially speed up metabolism in people with normal thyroid function. Taking excess T3 can be dangerous, causing heart palpitations, anxiety, and other serious side effects.
The NHS recognises that thyroid conditions can affect weight, but T3 is a UK pharmacy as a replacement therapy — a way to restore normal function — not as a weight loss medication. If your weight has increased alongside thyroid symptoms, treating your thyroid condition appropriately may help normalise weight, but this is different from using T3 as a weight loss tool.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Evidence-Based Weight Loss Alternatives in the UK
If you're looking for weight management support in the UK, there are several evidence-based options. Prescription weight loss medications such as GLP-1 injections (Wegovy) and dual GIP/GLP-1 injections (Mounjaro) are licensed for weight management and available through both NHS specialist services and private prescribing. These medications work through different mechanisms than thyroid hormones and have clinical trial evidence supporting their use for weight loss.
You can estimate potential results using our free GLP-1 weight loss calculator. Additionally, the NHS offers tier 1–3 weight management programmes including dietary support and behaviour change interventions. Your GP can assess which option suits your individual circumstances and refer you to appropriate services. Check your personal BMI using our BMI calculator to understand your starting point.
- 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.
Will taking T3 help me lose weight if I don't have a thyroid problem?
No. T3 is a prescription medication licensed only for treating hypothyroidism (low thyroid function). Taking T3 without a thyroid condition can be dangerous and cause serious side effects including heart problems, anxiety, and tremors. It is not approved for weight loss in people with normal thyroid function.
I have hypothyroidism and have gained weight — will T3 treatment help me lose weight?
Treating hypothyroidism appropriately with thyroid hormone replacement may help normalise your weight, especially if weight gain was caused by low thyroid function. However, weight management still typically requires attention to diet and activity alongside medication. Your GP can assess whether your current thyroid treatment is optimised.
What are the proven weight loss medications available on the NHS in the UK?
Prescription weight loss medications licensed in the UK include Wegovy (semaglutide), Mounjaro (tirzepatide), and Saxenda (liraglutide). These are available through NHS tier 3 specialist weight management services or private prescribing. All require clinical assessment and are used alongside lifestyle changes.
Can I get T3 prescribed privately for weight loss?
Whilst T3 can be prescribed privately, it should only be used under medical supervision for genuine thyroid conditions. Prescribers have a duty to ensure medications are used safely and appropriately. Using T3 for non-medical weight loss is not recommended and carries health risks.
What should I do if I want help with weight loss in the UK?
Start by speaking to your GP, who can assess your health, measure your BMI, and discuss appropriate options. The NHS offers free weight management support through tier 1–2 programmes. If you're eligible, your GP can refer you to specialist services where prescription medications may be considered.
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.