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
How Thyroid Problems Affect Weight Loss
Your thyroid plays a crucial role in how quickly your body burns calories, which is why thyroid problems can make weight loss feel much harder. An underactive thyroid (hypothyroidism) slows your metabolism and can lead to weight gain, even with reduced calorie intake. An overactive thyroid (hyperthyroidism) can cause gradual weight management. Understanding how your thyroid affects weight management is essential for setting realistic goals and finding the right support. If you suspect a thyroid issue, your GP can arrange a simple blood test to check your thyroid function.
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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 Your Thyroid Affects Weight Management
Your thyroid is a small gland in your neck that produces hormones controlling your metabolism — the rate at which your body burns energy. When thyroid hormones are low (hypothyroidism), your metabolic rate drops significantly, making weight loss more challenging even with a calorie deficit. You may experience fatigue, slower digestion, and increased hunger, which can undermine weight loss efforts. Conversely, an overactive thyroid (hyperthyroidism) speeds up metabolism but can cause nutritional deficiencies and other health problems. NHS weight loss guidelines remain the same regardless of thyroid status — aiming for 0.5–1 kg per week is still the target — but achieving this may require additional support if your thyroid is not functioning optimally. Once thyroid medication (such as levothyroxine) is properly dosed, many people find weight management becomes more achievable, though it may take several weeks or months for metabolism to stabilise.
- Hypothyroidism slows metabolism and increases hunger
- Hyperthyroidism speeds metabolism but can cause other health issues
- Thyroid medication helps stabilise metabolic rate
- Weight loss may be slower with untreated thyroid problems
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.
Getting Thyroid Support and Weight Loss Help from the NHS
If you think your weight gain or difficulty losing weight might be linked to your thyroid, start by discussing this with your GP. They can order thyroid function tests (TSH and free T4 levels) to establish whether your thyroid is the underlying issue. If hypothyroidism is diagnosed, treatment with levothyroxine replacement therapy usually improves metabolism and makes weight management easier. The NHS also offers free weight management support through tier 1 and tier 2 services, including digital tools, behaviour change programmes, and dietitian advice. For adults with a BMI over 30 or 27–29.9 with weight-related health conditions, specialist tier 3 weight management services may be available. Use our free BMI calculator to check where you stand, and our progress tracker to monitor changes as your thyroid treatment takes effect.
- 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 an underactive thyroid prevent weight loss?
Yes. An underactive thyroid (hypothyroidism) significantly slows your metabolism, making weight loss much harder even with a calorie deficit. However, once thyroid hormone levels are corrected with medication, weight loss becomes more achievable. It may take several weeks for thyroid medication to be properly balanced, so patience is important.
Will treating my thyroid help me lose weight?
Treating hypothyroidism with levothyroxine typically improves metabolism and energy levels, making weight loss easier. However, medication alone does not cause weight loss — you still need a calorie deficit and lifestyle changes. Once your thyroid is optimised, standard weight management approaches (diet and activity) become more effective.
What blood tests check thyroid function?
Your GP will typically order TSH (thyroid-stimulating hormone) and free T4 (thyroxine) blood tests. These measure how well your thyroid is working. TSH levels between 0.5 and 5 mIU/L are generally considered normal, though some people feel better with TSH towards the lower end of this range.
Should I use weight loss injections if I have a thyroid problem?
GLP-1 weight loss medications like Wegovy or Mounjaro can be used alongside thyroid treatment, but this requires careful assessment by your prescriber. Thyroid medication and weight loss medication may interact, so your GP or specialist must know about both conditions before prescribing.
How long does it take for thyroid treatment to improve weight loss?
Most people notice improved energy and metabolism within 4–6 weeks of starting levothyroxine, though full effects may take 8–12 weeks. Weight loss typically becomes noticeably easier once thyroid levels are stable, but this varies between individuals. Your GP may adjust your dose during this time to optimise results.
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.