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
T3 Weight Loss — What You Need to Know
T3 (liothyronine) is a synthetic thyroid hormone sometimes discussed in weight loss contexts, but it is not a licensed weight loss treatment in the UK. If you're exploring prescription weight management options, GLP-1 medications like Wegovy (semaglutide) and Mounjaro (tirzepatide) are the NHS-approved choices for eligible adults. This guide explains T3, compares it to licensed weight loss medications, and walks you through how to access prescription weight management in the UK.
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.
T3 Thyroid Hormone and Weight Loss — The Facts
T3 (liothyronine) is a synthetic form of the thyroid hormone triiodothyronine. It is licensed in the UK for treating thyroid deficiency and hypothyroidism, not for weight loss. Some people misunderstand T3 as a weight loss tool because thyroid hormones do affect metabolism, but using T3 specifically for weight loss is off-label and not recommended by the NHS.
Thyroid hormones increase metabolic rate, which can theoretically aid weight loss. However, misusing T3 carries significant risks: it can cause heart palpitations, anxiety, muscle loss, and dangerous electrolyte imbalances. The NHS does not recommend T3 for weight management in people with normal thyroid function.
If you have an underactive thyroid (hypothyroidism) and weight gain is a symptom, proper thyroid replacement therapy under GP supervision is the correct approach — not excessive T3 dosing.
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.
Licensed Weight Loss Options in the UK — Better Alternatives
The NHS and MHRA have approved specific prescription medications for weight management: Wegovy (semaglutide) and Mounjaro (tirzepatide). These are GLP-1 and GIP/GLP-1 receptor agonist injections, licensed for adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions.
Clinical evidence shows these medications help: Wegovy trials showed average weight loss of around 15% of body weight, whilst Mounjaro demonstrated 15–21% loss at different doses. Both are once-weekly injections requiring prescription and clinical assessment. Access is available through NHS specialist weight management services or private prescribing. Use our GLP-1 weight loss calculator to explore potential outcomes with licensed medications.
Weight loss is most effective alongside lifestyle changes — reduced-calorie diet and increased physical activity. The NHS recommends losing 0.5–1 kg per week for sustainable results. Your GP can assess your individual health, discuss whether prescription weight loss treatment is suitable, and refer you to specialist services if appropriate. If you're concerned about your weight or thyroid health, start by speaking with your GP rather than seeking unregulated treatments like off-label T3.
- 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 T3 approved for weight loss in the UK?
No. T3 (liothyronine) is licensed only for treating thyroid deficiency, not for weight loss. Using it off-label for weight management is not recommended by the NHS and carries health risks including heart problems and muscle loss. Licensed alternatives like Wegovy and Mounjaro are the evidence-based options.
What is the difference between T3 and Wegovy for weight loss?
T3 is an off-label thyroid hormone; Wegovy is a licensed GLP-1 injection approved for weight management. Wegovy has clinical trial evidence showing average clinically studied weight loss. T3 is not designed for weight loss and misuse can be dangerous. Wegovy requires a prescription and clinical assessment.
Can I get weight loss medication on the NHS in the UK?
Yes. If you meet eligibility criteria (BMI ≥30, or 27–29.9 with weight-related conditions), you can be referred to a specialist tier 3 weight management service. Mounjaro and Wegovy are now available through some NHS services, though availability varies by region and waiting times can be lengthy.
What are the safest prescription weight loss options in the UK?
Wegovy (semaglutide) and Mounjaro (tirzepatide) are licensed, clinically tested options. Both are once-weekly injections requiring prescription. They work by reducing appetite and supporting feeling fuller. Individual results vary, and they work best alongside diet and exercise changes.
How do I know if I'm eligible for prescription weight loss treatment?
Eligibility depends on BMI and health conditions. Use our free BMI calculator above to check your personal BMI. Adults with BMI ≥30, or 27–29.9 with conditions like diabetes or high blood pressure, may be eligible. Speak to your GP for a personalised assessment and potential NHS referral or private prescribing options.
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.