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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Thyroid Diet For Weight Loss — What You Need To Know
If you've been struggling to lose weight despite diet and exercise efforts, thyroid function could be a factor. The thyroid gland regulates metabolism, and an underactive thyroid (hypothyroidism) can slow weight loss significantly. This guide explains how thyroid health influences weight management in the UK, what foods may help support thyroid function, and when to see your GP for assessment. Understanding your thyroid can be key to sustainable weight management.
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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 Thyroid Function Affects Weight Loss
Your thyroid produces hormones that control how quickly your body burns calories — your metabolic rate. When thyroid hormone levels are low (hypothyroidism), your metabolism slows, making weight loss more difficult even with a reduced-calorie diet. Conversely, an overactive thyroid (hyperthyroidism) can cause unintended weight loss.
Common thyroid conditions affecting weight:
- Hypothyroidism — most common cause of thyroid-related weight gain in the UK
- Hashimoto's thyroiditis — autoimmune condition causing progressive thyroid failure
- Thyroiditis — inflammation of the thyroid gland
- Post-thyroidectomy — following surgical removal of the thyroid
If you suspect your thyroid may be affecting your weight, your GP can arrange a blood test to measure TSH (thyroid-stimulating hormone) and thyroid hormone levels. This is the only way to definitively assess thyroid function. Many people find that once thyroid levels are optimised with appropriate treatment, weight management becomes significantly easier.
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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
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Foods That May Support Thyroid Health
While diet alone cannot treat thyroid disease, certain nutrients support optimal thyroid function and may help with weight management when combined with appropriate medical treatment.
- Selenium-rich foods: Brazil nuts, eggs, fish, chicken — support thyroid peroxidase enzyme function
- Iodine sources: Fish, seaweed, dairy, eggs — essential for thyroid hormone production
- Iron-rich foods: Red meat, lentils, spinach — required for thyroid hormone conversion
- Zinc sources: Oysters, beef, pumpkin seeds, chickpeas — supports thyroid hormone metabolism
- Protein: All meats, fish, legumes, dairy — helps maintain muscle during weight loss and supports hormone balance
Avoid excessive soy, cruciferous vegetables in very large quantities, and ultra-processed foods — these may interfere with thyroid function if consumed excessively. A balanced, whole-food approach supports both thyroid health and sustainable weight management.
- 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 I lose weight if I have an underactive thyroid?
Yes, but it may be slower and require more effort. Optimising your thyroid medication with your GP is the first step. Once hormone levels are balanced, combined with a reduced-calorie diet and physical activity, weight loss becomes more achievable. Our free BMI calculator can help you track progress over time.
What's the connection between thyroid and metabolism?
Your thyroid produces T3 and T4 hormones that directly control your basal metabolic rate — how many calories your body burns at rest. Low thyroid hormone levels reduce metabolic rate significantly, making weight loss harder. This is why thyroid testing is important if you're struggling with weight management.
Should I change my diet if I have thyroid problems?
If you have diagnosed thyroid disease, discuss dietary changes with your GP or a registered dietitian. A balanced diet rich in protein, selenium, iodine, iron, and zinc supports thyroid function. Avoid extreme calorie restriction without medical guidance — it can further stress thyroid hormone production.
How long does it take to lose weight after thyroid treatment starts?
This varies significantly. Some people notice improved metabolism within weeks of starting thyroid medication, others take several months. Once thyroid levels stabilise, sustainable weight loss typically follows the NHS guidance of 0.5–1 kg per week when combined with a calorie deficit and physical activity.
When should I see my GP about thyroid and weight?
See your GP if you've gained weight unexpectedly, feel unusually tired, feel cold, or if you're struggling to lose weight despite genuine dietary and exercise efforts. They can arrange a simple blood test to check your thyroid function and advise on next steps for weight management.
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.