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 Does Thyroid Function Affect Weight Loss?
Your thyroid plays a crucial role in regulating your metabolism and weight. When thyroid function is impaired—whether due to hypothyroidism, hyperthyroidism, or other conditions—weight loss becomes significantly more challenging. This guide explains how thyroid health affects your weight management journey, what NHS support is available, and practical strategies to help you reach a healthy weight whilst managing your thyroid condition. Understanding this connection is the first step towards sustainable weight loss in the UK.
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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.
Understanding Thyroid Function and Metabolism
Your thyroid gland produces hormones that control your basal metabolic rate — the number of calories your body burns at rest. When thyroid hormone levels are low (hypothyroidism), your metabolism slows significantly, making weight loss much harder even with calorie restriction and increased physical activity.
Hypothyroidism is the most common thyroid condition affecting weight. Symptoms include fatigue, slower metabolism, fluid retention, and difficulty losing weight. Hyperthyroidism (overactive thyroid) produces the opposite effect — gradual weight management, increased hunger, and difficulty maintaining weight.
- Low thyroid hormones = slower metabolism and easier weight gain
- Optimised thyroid levels = normal metabolic rate and more effective weight loss
- Thyroid medication (levothyroxine) restores hormone balance
Even with correct thyroid treatment, some people find weight loss slower than expected. This is because thyroid conditions often cause other metabolic changes including insulin resistance and increased appetite.
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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.
Weight Management Strategies With Thyroid Conditions
If you have a thyroid condition, effective weight management requires a multi-faceted approach. First, ensure your thyroid medication is optimised — your GP will monitor your TSH levels to confirm your dose is correct. Under-treatment is a common reason people struggle to lose weight despite dieting.
Beyond medication, weight loss medication used alongside NHS weight management services can be effective for people with thyroid conditions. Prescription weight loss treatments like GLP-1 injections can help regulate appetite and support sustainable weight loss through weight management UK programmes.
- Work with your GP to confirm thyroid levels are optimised
- Use our free BMI calculator to track your healthy weight range
- Ask your GP about referral to tier 3 NHS weight management services
- Consider our GLP-1 weight loss calculator if eligible for prescription support
- 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 treating my hypothyroidism help me lose weight?
Yes, optimising thyroid medication is essential for weight loss. When your thyroid hormone levels are correct, your metabolism functions normally, making calorie deficit more effective. Your GP will adjust your levothyroxine dose based on blood tests. Weight loss may still require dietary changes and increased activity, but it becomes achievable once thyroid function is normalised.
Can I use weight loss injections if I have a thyroid condition?
Yes, if you meet the criteria. GLP-1 medications like Wegovy are prescription-only and can be used by people with thyroid conditions, provided your thyroid is well-managed on medication. A prescriber will assess your suitability during a clinical assessment. Always inform them about your thyroid condition and current medications, as these may interact.
Why am I not losing weight despite treating my thyroid?
Several factors may be involved: your thyroid medication may not be fully optimised (TSH levels should be checked), you may have insulin resistance (common with thyroid disease), or calorie intake may need adjustment. Some people also retain fluid with thyroid conditions. Your GP can investigate further and may refer you to specialist weight management services through the NHS.
What's the difference between hypothyroidism and hyperthyroidism for weight?
Hypothyroidism (underactive) slows metabolism, causing weight gain and difficulty losing weight. Hyperthyroidism (overactive) speeds metabolism, causing gradual weight management and difficulty maintaining weight. Both conditions impair normal weight regulation. Treatment with medication restores balance and allows normal weight management.
Should I follow a specific diet with thyroid disease and weight loss?
No single diet is "best" for thyroid disease, but evidence supports a balanced approach: adequate protein, consistent meal timing, and sufficient iodine and selenium intake. Very low-calorie diets are not recommended without GP supervision. Focus on sustainable changes rather than restrictive dieting. Your GP or a registered dietitian can provide personalised guidance.
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.