Weight Loss Injections and Your Thyroid: What You Need to Know Before Starting

• A direct answer on eligibility, taken from UK product information rather than from forums
• The levothyroxine detail that most articles get wrong, with the exact figure
• Explains the thyroid cancer question, including what the UK label does and does not say
• Online consultation with UK registered prescribers through a GPhC registered pharmacy

If you have a thyroid condition and are thinking about weight loss injections, it is understandable to wonder whether the two can be used together.

The answer is not simply "yes" or "no".

The UK product information for Wegovy (semaglutide) and Mounjaro (tirzepatide) does not list an underactive or overactive thyroid as a formal contraindication. However, your thyroid history is still important when a prescriber is deciding whether treatment is suitable for you. Both medicines have information relating to thyroid C cell tumours from animal studies, and both can affect the way the stomach empties.

There is also an important point if you take levothyroxine. The documented 33% increase in total thyroxine exposure relates to oral semaglutide, not Wegovy injection. The product information for injectable semaglutide does not list the same specific levothyroxine interaction. Tirzepatide can delay stomach emptying and may affect the absorption of some medicines taken by mouth.

So, if you have a thyroid condition, the important questions are usually about your current thyroid control, the medicines you take, your medical history and how your treatment will be monitored.

This guide explains what the UK product information says, what you should tell your prescriber, and what to keep an eye on if you are taking thyroid medication.

Key things to know

• Neither Wegovy nor Mounjaro lists thyroid disease itself as a formal contraindication in the current UK product information.
• The formal contraindication listed for both medicines is hypersensitivity, meaning an allergy to the active ingredient or one of the excipients.
• Both medicines have preclinical information about thyroid C cell tumours in rodents. The relevance of these findings to humans is not known, and the UK product information does not list thyroid cancer as a contraindication.
• Oral semaglutide has a documented interaction with levothyroxine. Total thyroxine exposure increased by 33% in a study, and monitoring of thyroid parameters should be considered. This finding relates to the oral formulation rather than the Wegovy injection.
• Tirzepatide delays stomach emptying and can affect the rate at which some oral medicines are absorbed. Monitoring is advised for certain medicines with a narrow therapeutic index.
• If you take levothyroxine, regular thyroid blood tests are important because your dose may need adjusting over time. NHS guidance recommends regular blood tests to make sure you are taking the right dose.
• Significant weight loss can sometimes affect the amount of levothyroxine a person needs, so any dose change should be based on thyroid blood tests and clinical advice rather than symptoms alone.
• Treatment suitability depends on an individual clinical assessment. Results vary between individuals.

Can you take weight loss injections with a thyroid condition?

Having a thyroid condition does not automatically mean that weight loss injections are unsuitable for you.

Wegovy contains semaglutide and Mounjaro contains tirzepatide. In their current UK product information, neither medicine lists thyroid disease itself as a formal contraindication.

That does not mean every person with a thyroid condition should start treatment.

Your prescriber will need to know what thyroid condition you have, what treatment you take, whether your condition is currently controlled and whether you have any other medical conditions or medicines that could affect your treatment.

For example, if you have recently been diagnosed with a thyroid problem and your treatment is still being adjusted, it may be sensible to get your thyroid management reviewed first. The same applies if you have symptoms that have not yet been investigated.

This is not because thyroid disease is automatically a reason to avoid weight loss injections. It is because your prescriber needs a clear picture of your health before deciding whether treatment is appropriate.

Weight loss injections and thyroid cancer risk

What does the UK product information actually say?

This is where a lot of online information can become confusing.

The current UK product information for Wegovy and Mounjaro does not list thyroid cancer as a formal contraindication. However, both medicines contain information about thyroid C cell tumours seen in animal studies.

For semaglutide, thyroid C cell tumours were seen in rats and mice in long term studies. The product information states that the relevance of these findings to humans is considered low, but cannot be completely excluded.

For tirzepatide, thyroid C cell tumours were also seen in rats. The product information states that the relevance of these findings to humans is unknown.

So it would be misleading to say either that these medicines are proven to cause thyroid cancer in people or that there is absolutely no thyroid cancer concern.

If you have a personal or family history of thyroid cancer, tell your prescriber. They can consider the type of cancer, your medical history and whether further specialist advice is appropriate.

Weight loss injections and levothyroxine

What interaction is actually documented?

This is an area where the type of medicine matters.

The current UK product information for oral semaglutide reports that total thyroxine exposure increased by 33% after a single dose of levothyroxine. The maximum concentration did not change, and the product information says that monitoring of thyroid parameters should be considered when semaglutide and levothyroxine are used together.

That finding is from oral semaglutide. It should not be presented as a documented levothyroxine interaction with Wegovy injection.

For Mounjaro, the product information explains that tirzepatide delays stomach emptying and can affect the rate at which medicines taken by mouth are absorbed. It recommends monitoring for some oral medicines where changes in absorption could be important.

If you take levothyroxine, tell your prescriber before starting treatment. Do not change your levothyroxine dose yourself.

If you are considering an oral GLP 1 treatment, you can read more about oral GLP 1 weight loss tablets and discuss whether this type of treatment is suitable for you.

What should I do if I take levothyroxine?

You do not need to panic about taking thyroid medication and a weight management medicine together. The important thing is to make sure both healthcare professionals know what you are taking.

1. Tell your weight management prescriber. Give them the name and dose of your levothyroxine and explain how you take it.
2. Tell the healthcare professional who manages your thyroid. Let them know if you are starting a weight management medicine so they can decide whether any additional monitoring is needed.
3. Keep taking levothyroxine as prescribed. Do not change the dose or stop it because you have started a weight loss injection.
4. Keep the timing consistent. Taking levothyroxine in the same way each day makes it easier for your healthcare professional to interpret changes in your thyroid blood tests.
5. Keep up with thyroid blood tests. NHS guidance recommends regular testing to make sure your levothyroxine dose remains appropriate.
6. Ask before changing other medicines or supplements. Some medicines and supplements can affect levothyroxine absorption, so mention everything you take.
7. Do not change your thyroid dose based on symptoms alone. Tiredness, changes in bowel habits, sleep problems and changes in weight can have several possible causes. Blood tests can help your healthcare professional work out what is happening.

Could my levothyroxine dose change as I lose weight?

It can.

The amount of levothyroxine a person needs can be affected by body size, and significant weight changes may mean the dose needs to be reviewed. NHS guidance recommends regular blood tests to check that the dose remains right.

This does not mean your dose will definitely need to change.

It also does not mean every new symptom is caused by your thyroid medication.

For example, diarrhoea, changes in appetite and sleep problems can occur for different reasons, including the side effects of weight management medicines or having too much thyroid hormone replacement.

That is why it is better to check your thyroid function rather than guessing what is causing the symptoms.

Underactive thyroid and Hashimoto's

Can you take Mounjaro or Wegovy with an underactive thyroid?

For many people, an underactive thyroid does not automatically prevent them from being considered for weight management treatment.

Hypothyroidism means your thyroid gland does not make enough thyroid hormone. It is commonly treated with levothyroxine, and regular blood tests are used to check that the dose is right.

If you have hypothyroidism, tell your prescriber about it and provide details of your treatment and recent thyroid blood tests if you have them.

If your thyroid condition is still being investigated or your medication is being adjusted, your prescriber may want to understand what is happening before starting another medicine.

Weight loss injections with Hashimoto's

Hashimoto's thyroiditis is an autoimmune condition that can cause an underactive thyroid.

Having Hashimoto's does not automatically mean you cannot use Wegovy or Mounjaro. The same practical questions apply: is your thyroid being treated, what medicines are you taking, are your thyroid levels being monitored, and are there any other health conditions that need to be considered?

Hashimoto's can also change over time, so do not assume that every change in your weight, energy or symptoms is caused by your weight management medicine.

If something feels different after starting treatment, tell your healthcare professional rather than trying to work out the cause yourself.

How to lose weight with an underactive thyroid

An underactive thyroid can make weight management more difficult, particularly if it is not adequately treated. Common symptoms include tiredness, weight gain and constipation. Once hypothyroidism is diagnosed and treated, regular blood tests help make sure the levothyroxine dose remains appropriate.

Weight management usually involves more than one change. NICE recommends using weight management medicines alongside a reduced calorie diet and increased physical activity where the medicine is appropriate.

You do not need to make extreme changes because you have hypothyroidism.

If tiredness makes exercise difficult, start with an amount of activity you can manage and build up gradually. If constipation is a problem, speak to your healthcare professional about suitable changes to your diet, fluids and activity.

Our guide to medical weight loss explains more about weight management treatment and how suitability is assessed.

Can you take weight loss injections with an overactive thyroid?

An overactive thyroid is not listed as a formal contraindication to Wegovy or Mounjaro, but it is important to tell your prescriber if you have hyperthyroidism or symptoms that could suggest it.

An overactive thyroid can cause symptoms such as a fast or irregular heartbeat, shaking, feeling unusually warm, sweating, anxiety, diarrhoea and unintentional weight loss.

Some of these symptoms can overlap with symptoms that may occur during weight management treatment.

If you are losing weight without trying or your thyroid treatment is not yet settled, it is important to have this assessed rather than assuming the weight change is simply a sign that your weight loss medicine is working.

Your prescriber can decide whether your thyroid needs further assessment or treatment before starting or continuing weight management treatment.

Do weight loss injections work if you have a thyroid condition?

Having a thyroid condition does not automatically mean that a weight management medicine will not work for you.

Wegovy and Mounjaro work through pathways involved in appetite, food intake and blood glucose regulation. They are not thyroid replacement medicines, and they should not be used as a way to treat an underactive thyroid.

Clinical trials have shown that both semaglutide and tirzepatide can lead to significant weight loss in adults with overweight or obesity when used alongside diet and physical activity. However, these trial results should not be treated as a prediction of what will happen to an individual person, particularly because the trials were not designed specifically to study people with thyroid conditions.

Your thyroid treatment still matters. If hypothyroidism is not adequately treated, symptoms such as tiredness and weight gain can make weight management more difficult. NHS guidance recommends regular blood tests to make sure the levothyroxine dose remains appropriate.

If you are struggling with your weight despite making consistent changes, medical weight loss may be something to discuss with an appropriately qualified healthcare professional.

Which weight loss injection suits people with thyroid conditions?

There is no single weight loss injection that is automatically the right choice for everyone with a thyroid condition.

Wegovy contains semaglutide, while Mounjaro contains tirzepatide. Both have different product information, dosing schedules and safety considerations.

These differences are useful to discuss with your prescriber, but they do not create a universal "best" option for people with thyroid conditions.

Your medical history, current medicines, thyroid control, BMI, other health conditions and treatment goals all need to be considered before a medicine is prescribed.

Who may not be suitable for weight loss injections?

It is better to think about this as who needs an individual assessment rather than a simple list of people who can or cannot take these medicines.

For Wegovy and Mounjaro, the formal contraindication listed in the UK product information is hypersensitivity to the active ingredient or an excipient.

There are also medical situations where extra care may be needed.

For example, Mounjaro should be used with caution in people with a history of pancreatitis and in people with severe gastrointestinal disease, including severe gastroparesis.

Pregnancy is another important consideration. Weight management medicines are not intended for use during pregnancy, and anyone who is pregnant, planning a pregnancy or breastfeeding should discuss this with their healthcare professional before treatment.

BMI requirements also depend on the medicine, the indication and the prescribing setting. For example, the licensed indication for both Wegovy and Mounjaro includes adults with a BMI of 30 kg/m² or above, or a BMI of 27 kg/m² to below 30 kg/m² with at least one weight related health condition. NICE recommendations for NHS use can have additional eligibility requirements.

Your prescriber should assess your individual circumstances rather than relying on BMI alone.

If you have a personal or family history of thyroid cancer, a thyroid lump, unexplained thyroid symptoms, an uncontrolled thyroid condition or another significant medical condition, make sure you mention it during your assessment.

Red flags: symptoms you should not ignore

Most symptoms that occur during weight management treatment are not emergencies, but some need medical attention.

Speak to a healthcare professional if you develop:

• A new lump or swelling in your neck, persistent hoarseness or difficulty swallowing
• A racing or irregular heartbeat, shaking, unusual sweating or heat intolerance
• Severe or persistent abdominal pain, particularly if it spreads through to your back
• Pain in the upper right side of your abdomen, fever or yellowing of the skin or eyes
• Weight loss that is happening without you trying to lose weight
• Significant changes in tiredness, feeling unusually cold, constipation or other symptoms that could suggest your thyroid treatment needs reviewing
• Persistent diarrhoea, difficulty sleeping, tremor or a racing pulse that could have several possible causes, including thyroid hormone levels that are too high

If you develop severe abdominal pain or another serious or rapidly worsening symptom, seek urgent medical advice.

Getting assessed properly

A good weight management assessment should be about more than your weight.

Tell your prescriber about your thyroid condition, any previous thyroid surgery or radioactive iodine treatment, your current thyroid medicines and doses, your other medicines and supplements, and any relevant family history.

If you take levothyroxine, it can also be useful to know when your thyroid function was last checked. Regular blood tests are used to make sure the dose remains appropriate.

If treatment is considered suitable, your prescriber should explain how to use the medicine, what side effects to expect and when you should have a follow up review.

You can read about the weight loss treatments available and the assessment process.

These medicines should only be used under the supervision of an appropriately qualified prescriber.

Medical Disclaimer

This article is for general information and does not replace personal medical advice or the advice of the healthcare professional managing your thyroid condition.

Weight management medicines are prescription medicines and should only be used under the supervision of an appropriately qualified prescriber. Treatment suitability depends on an individual clinical assessment, and results vary between individuals.

If you have a thyroid condition, tell your prescriber before starting treatment and keep attending any thyroid blood tests recommended for you.

Seek prompt medical advice if you develop a new neck lump, persistent difficulty swallowing or hoarseness, a racing or irregular heartbeat, severe abdominal pain, or unexplained weight loss.