What is hypothyroidism?
Hypothyroidism, or an underactive thyroid, is a condition where the thyroid gland doesn’t produce enough thyroid hormones. These hormones regulate metabolism, energy levels, and body temperature.
It’s one of the most common endocrine conditions in the UK, affecting an estimated 3.5% of the population, around 2.2 million people.2 Women are five to ten times more likely to be affected than men.
The most common cause is autoimmune thyroiditis (Hashimoto’s disease), where the immune system attacks the thyroid gland. It’s typically treated with daily levothyroxine, a synthetic form of the thyroid hormone T4.
Hypothyroidism and weight
Weight gain is one of the most well-known symptoms of hypothyroidism. However, the amount of weight directly caused by an underactive thyroid is often smaller than people expect.
Research suggests that hypothyroidism typically causes around 2.3 to 4.5 kg of weight gain, and most of this is fluid retention rather than fat.3
When thyroid levels are corrected with levothyroxine, most people lose less than 2 kg, which supports the idea that the gain was largely water and salt. Still, this doesn’t mean many people don’t struggle with their weight due to an underactive thyroid.
Hypothyroidism causes fatigue, low mood, and reduced motivation, all of which can make it harder to stay active and maintain healthy eating habits.
If you’re carrying significantly more weight than can be explained by thyroid function alone, Wegovy could be a useful addition to your treatment plan.
How Wegovy works
Wegovy contains the drug semaglutide, which mimics a hormone called GLP-1 that our body naturally produces in the gut after eating. It works by reducing appetite, increasing feelings of fullness, and slowing gastric emptying.
In clinical trials, people taking Wegovy lost an average of 14.9% of their body weight over 68 weeks, compared to 2.4% with placebo.4 Wegovy is injected once a week, and the dose is gradually increased over 16 weeks to reduce side effects.
A higher dose of Wegovy (7.2 mg) is now available and has been shown to support an average weight loss of around 20% after a year.
Important safety considerations
There’s a specific thyroid-related safety concern with Wegovy that’s important to understand, even though the actual risk to humans appears low.
In two-year rodent studies, semaglutide caused thyroid C-cell tumours, including medullary thyroid carcinoma (MTC).1
C-cells are specialised cells in the thyroid that produce calcitonin. In rodents, GLP-1 receptors on these cells are highly active, leading to overstimulation.
However, humans have very low GLP-1 receptor expression on thyroid C-cells.5 Studies in primates at over 60 times the human dose showed no C-cell changes.
Clinical data from large trials and systematic reviews haven’t found a consistent increase in thyroid cancer risk with GLP-1 receptor agonists.
Despite this reassuring evidence, Wegovy is contraindicated if you or a close family member has a personal or family history of MTC or MEN2.1 Your prescriber will ask about this before starting treatment.
Medication interactions
Levothyroxine
This is the most relevant interaction for people with hypothyroidism. Semaglutide slows gastric emptying, which can increase the absorption of levothyroxine.
One study found that oral semaglutide increased total T4 exposure by 33% when taken with levothyroxine.6 This means your body may absorb more of your thyroid medication than usual, potentially pushing thyroid levels higher than intended.
In practice, this doesn’t mean you can’t take both medications. It means your GP or endocrinologist should check your thyroid function (TSH and free T4) after starting Wegovy, typically at 4 to 8 weeks, and again once you reach the maintenance dose.
If your TSH drops below the normal range, your levothyroxine dose may need to be reduced. Without adjustment, over-replacement can lead to symptoms like palpitations, anxiety, and bone density loss over time.
Timing your medications
Levothyroxine should still be taken on an empty stomach, ideally 30 to 60 minutes before food.
Because Wegovy is a weekly injection rather than an oral tablet, there’s no direct timing conflict between the two. The interaction relates to Wegovy’s ongoing effect on gut motility, not a single-dose overlap.
Liothyronine (T3)
If you take liothyronine alongside levothyroxine, the same monitoring principles apply. Discuss your full thyroid medication regimen with your prescriber before starting Wegovy.
Symptom comparison: hypothyroidism and Wegovy side effects
Some symptoms of hypothyroidism overlap with common Wegovy side effects, which can make it harder to tell what’s causing what.
| Symptom |
Hypothyroidism |
Wegovy side effect |
What to do |
| Fatigue |
Very common |
Common, especially during dose escalation |
Check TSH levels if new or worsening |
| Constipation |
Common |
Very common |
Increase fibre and fluid intake; go for a walk or other physical activity |
| Nausea |
Uncommon |
Very common (affects up to 44%) |
Usually settles; eat smaller meals |
| Low mood |
Common |
Uncommon |
Check thyroid levels; speak to GP |
| Weight changes |
Modest gain (2-5 kg, mostly fluid) |
Significant loss (average 14.9%) |
Monitor thyroid levels as weight changes |
| Hair thinning |
Common |
Reported with rapid weight loss |
Ensure adequate energy, protein, and iron intake |
| Cold sensitivity |
Very common |
Not typical |
May indicate undertreated thyroid |
If you notice new or worsening fatigue, constipation, or low mood after starting Wegovy, it’s worth checking whether your thyroid levels have shifted rather than assuming it’s a Wegovy side effect.
Foods to focus on
Dietary guidance for managing hypothyroidism alongside Wegovy isn’t wholly different from our advice to eat a diet based on whole foods, minimising ultra-processed foods.
Still, there are specific nutrients worth paying closer attention to when you’re supporting thyroid function and managing your weight at the same time.
Iodine-rich foods
Iodine is essential for thyroid hormone production. The thyroid gland uses iodine to make T3 and T4, so consistent dietary intake is essential.7
Good UK sources include cow’s milk and dairy products, white fish such as cod and haddock, eggs, and shellfish. Seaweed contains iodine, but levels vary enormously, so it’s best consumed in small amounts rather than relied on as a primary source.
Selenium sources
Selenium supports the enzymes that convert T4 into the more active T3 hormone and helps protect the thyroid from oxidative damage.7
Brazil nuts are the richest dietary source; just two to three a day provide enough to support our daily needs. Other good sources include fish, eggs, chicken, and sunflower seeds.
Iron-rich foods
Iron deficiency is more common in people with hypothyroidism and can impair thyroid hormone synthesis.7
Include red meat, lentils, chickpeas, dark leafy greens, and fortified cereals. Pair plant-based iron sources with vitamin C (such as peppers or citrus) to improve absorption.
Fibre for digestive health
Constipation is common with both hypothyroidism and Wegovy.
We recommend increasing fibre gradually through vegetables, oats, beans, nuts, seeds, and ground flaxseed or chia seeds. Increase slowly to avoid worsening bloating, and drink plenty of water alongside.
Foods to be mindful of
High-fibre meals and calcium-rich foods taken at the same time as levothyroxine can reduce its absorption. The simplest approach is to take levothyroxine first thing in the morning and wait at least 30 minutes before eating.
There’s no need to avoid soy or cruciferous vegetables in normal dietary amounts. The evidence for these interfering with thyroid function relates to very high intakes, or to the context of iodine deficiency, not to typical portions within a balanced diet.
Managing reduced appetite on Wegovy
When your appetite is reduced, prioritise nutrient-dense foods at each meal. Start with protein (eggs, fish, Greek yoghurt, red meat, tofu, or chicken), add vegetables, and include healthy fats like extra-virgin olive oil, avocado, or nuts.
If you’re eating less overall, it’s especially important to ensure your meals are nutrient-dense, particularly for iodine, selenium, and iron.
Physical activity
Fatigue is one of the biggest barriers to exercise when you have hypothyroidism. We recommend starting at a manageable level and building gradually rather than waiting until you feel completely well.
Getting started
Any movement is better than none, and building small, consistent habits is a better approach than trying to go too hard, too soon and giving up entirely.
A good starting point is linking a new activity to something you already do, like a 10-minute walk after dinner. Research on habit formation suggests that anchoring a new behaviour to an existing routine helps it become automatic.11
As it starts to feel manageable, gradually do a little more. NHS guidelines suggest working towards 150 minutes of moderate activity per week, but there’s no rush to get there.
Aerobic exercise
A randomised controlled trial in women with hypothyroidism found that aerobic training improved exercise capacity and lipid profiles.8
Walking, swimming, and cycling are all suitable options. Moderate intensity, where you can talk but not sing, is a good target.
Resistance training
Resistance training is particularly relevant because it helps maintain muscle mass during weight loss, which supports metabolic rate (the amount of energy we burn each day).
The same trial, mentioned above, found that combined aerobic and resistance training produced the greatest improvements in TSH levels and quality of life.8
Bodyweight exercises like squats, lunges, and press-ups are a good place to start, or you could try resistance bands. Even one session a week is worthwhile, and you can add more as it becomes part of your routine.
Practical tips for exercising with fatigue
- Schedule exercise for when your energy is highest, often mid-morning for people with hypothyroidism
- Keep sessions short and consistent rather than long and infrequent
- Try ‘snacking’ on exercise during the day; doing calf raises while the kettle boils, or squats while you’re cooking
- Track your energy patterns over a few weeks to find your best times
- Don’t skip rest days; recovery is essential when your thyroid is underactive
- If fatigue worsens significantly, check your thyroid levels before pushing through
Mental health and hypothyroidism
Depression and anxiety are significantly more common in people with hypothyroidism. Studies estimate that 20 to 60% of people with an underactive thyroid experience some degree of depression, depending on how well the thyroid is controlled.9
How hypothyroidism affects mood
Thyroid hormones influence serotonin activity in the brain, which is one of the key neurotransmitters involved in mood regulation.
When thyroid levels are low, serotonin function can be disrupted, contributing to low mood, poor concentration, and irritability.
Fatigue also plays a major role. Persistent tiredness affects motivation, social engagement, and the ability to exercise, all of which are protective factors for mental health.
Thyroid treatment and mood
For many people, adequate thyroid treatment with levothyroxine improves mood symptoms. However, some people continue to experience depression even when their TSH is within the normal range.
If this applies to you, it’s worth discussing with your GP rather than assuming your thyroid is to blame.
When to seek additional support
Speak to your GP if you experience persistent low mood lasting more than two weeks, loss of interest in activities you normally enjoy, difficulty sleeping or sleeping excessively, or feelings of hopelessness. These may benefit from treatment beyond thyroid medication.
When to speak to your GP
- Before starting Wegovy, review your thyroid history and current levothyroxine dose
- 4 to 8 weeks after starting Wegovy, for a thyroid function test
- After reaching the maintenance dose (2.4 mg), for another thyroid check
- If you develop new symptoms such as palpitations, anxiety, tremor, or heat intolerance, which could suggest thyroid over-replacement
- If fatigue or low mood worsens rather than improves
- If you have any personal or family history of thyroid cancer
Frequently asked questions
Will Wegovy affect my thyroid levels?
It can. Semaglutide slows gastric emptying, which may increase levothyroxine absorption by up to 33%.6 This could lower your TSH, so your GP should monitor your thyroid function after starting Wegovy and adjust your levothyroxine dose if needed.
Can I take levothyroxine and Wegovy together?
Yes. There’s no need to change when you take levothyroxine. Continue taking it on an empty stomach in the morning as usual. Because Wegovy is a weekly injection, there’s no direct timing conflict. The interaction is due to Wegovy’s ongoing effect on gut motility.
Does hypothyroidism make it harder to lose weight on Wegovy?
If your hypothyroidism is well-treated and your TSH is within range, it shouldn’t significantly reduce Wegovy’s effectiveness.
Untreated or undertreated hypothyroidism can slow metabolism, so ensuring optimal thyroid levels gives you the best chance of a good response.
Should I worry about the thyroid cancer warning on Wegovy?
The thyroid C-cell tumour finding comes from rodent studies. Humans have very low GLP-1 receptor expression on thyroid C-cells, and clinical data haven’t shown a consistent increase in thyroid cancer risk.5
However, Wegovy isn’t suitable for people with a personal or family history of medullary thyroid carcinoma or MEN2.
How often should I have my thyroid checked while on Wegovy?
A thyroid function test at 4 to 8 weeks after starting, and again after reaching the 2.4 mg maintenance dose, is a reasonable approach. After that, continue with your usual annual thyroid monitoring unless symptoms change.
Will losing weight on Wegovy change my levothyroxine dose?
Possibly. Levothyroxine is dosed partly based on body weight, so significant weight loss can mean you need a lower dose.
Given the increased absorption, your GP may need to reduce your levothyroxine dose as you lose weight.
Can I eat normally with hypothyroidism and Wegovy?
Yes, though it helps to prioritise nutrient-dense foods, particularly those rich in iodine, selenium, and iron, which support thyroid function. When your appetite is reduced on Wegovy, making each meal count nutritionally becomes more important.
Is it safe to exercise with hypothyroidism while taking Wegovy?
Yes. Exercise is beneficial for both conditions. A combination of aerobic and resistance training has been shown to improve thyroid function markers and quality of life in people with hypothyroidism.8 Start gradually and build up, particularly if fatigue is a significant symptom.
What if my hypothyroidism is caused by Hashimoto’s disease?
Hashimoto’s disease is the most common cause of hypothyroidism in the UK. There’s no specific contraindication to using Wegovy with Hashimoto’s. The same monitoring advice applies regarding levothyroxine absorption and thyroid levels.
Take home message
Wegovy can be used safely alongside hypothyroidism treatment for most people.
The two main considerations are monitoring levothyroxine absorption, as semaglutide can increase T4 exposure by up to 33%, and the contraindication for anyone with a personal or family history of medullary thyroid carcinoma or MEN2.
With regular thyroid monitoring and good communication with your GP, most people with hypothyroidism can benefit from Wegovy in the same way as the general population.
Pairing medication with a balanced diet rich in iodine, selenium, and iron, along with regular physical activity, gives the best foundation for both thyroid health and weight management.
Second Nature’s programme combines medication support with personalised nutrition and lifestyle guidance from registered dietitians and nutritionists.
Meals are built around a balanced plate model: half vegetables, a quarter protein, a quarter complex carbohydrates, plus a serving of healthy fat.
A peer-reviewed study published in JMIR Formative Research found that active subscribers on Second Nature’s semaglutide-supported programme lost an average of 19.1% of their body weight at 12 months, with 77.7% achieving at least 10% weight loss.10
Second Nature's Mounjaro and Wegovy programmes
Second Nature provides Mounjaro or Wegovy as part of our Mounjaro and Wegovy weight-loss programmes.
Why choose Second Nature over other medication providers, assuming you're eligible?
Because peace of mind matters.
We've had the privilege of working with the NHS for over eight years, helping people across the UK take meaningful steps toward a healthier, happier life.
Our programmes are designed to meet people where they are, whether that means support with weight loss through compassionate one-to-one health coaching, or access to the latest weight-loss medications (like Mounjaro and Wegovy) delivered alongside expert care from a multidisciplinary team of doctors, psychologists, dietitians, and personal trainers.
At the heart of everything we do is a simple belief: real, lasting change comes from building better habits, not relying on quick fixes. We're here to support that change every step of the way.
With over a decade of experience, thousands of lives changed, and a long-standing record of delivering programmes used by the NHS, we believe we're the UK's most trusted weight-loss programme.
We hope to offer you something invaluable: peace of mind, and the support you need to take that first step.
References
- Electronic Medicines Compendium. (2026). Wegovy 0.25 mg solution for injection in pre-filled pen: Summary of Product Characteristics.
- Razvi, S. et al. (2019). Trends, determinants, and associations of treated hypothyroidism in the United Kingdom, 2005-2014. Thyroid, 29(1), 16-23.
- American Thyroid Association. (2024). Thyroid and weight.
- Wilding, J.P.H. et al. (2021). Once-weekly semaglutide in adults with overweight or obesity. New England Journal of Medicine, 384(11), 989-1002.
- Bjerre Knudsen, L. et al. (2012). GLP-1 receptor agonists and the thyroid: C-cell effects in mice are mediated via the GLP-1 receptor and not associated with RET activation. Endocrinology, 153(3), 1538-1547.
- Granhall, C. et al. (2021). Effect of oral semaglutide on the pharmacokinetics of thyroxine after dosing of levothyroxine. Clinical Pharmacokinetics, 60(10), 1333-1341.
- Ventura, M. et al. (2017). Selenium and thyroid disease: from pathophysiology to treatment. International Journal of Endocrinology, 2017, 1297658.
- Ahmad, A.M. et al. (2023). Effects of aerobic, resistance, and combined training on thyroid function and quality of life in hypothyroidism: a randomised controlled trial. Complementary Therapies in Clinical Practice, 53, 101795.
- Siegmann, E.M. et al. (2018). Association of depression and anxiety disorders with autoimmune thyroiditis: a systematic review and meta-analysis. JAMA Psychiatry, 75(6), 577-584.
- Richards, R. et al. (2025). A remotely delivered, semaglutide-supported weight management program: 12-month outcomes from a retrospective service evaluation. JMIR Formative Research, 9(1), e72577.
- Lally, P. et al. (2010). How are habits formed: modelling habit formation in the real world. European Journal of Social Psychology, 40(6), 998-1009.
