If you take levothyroxine for an underactive thyroid and are considering Mounjaro, you may be wondering whether the two medicines can be taken together. Hypothyroidism and weight difficulties can occur at the same time, so this is a common question. However, the thyroid warning linked to Mounjaro is not the same as having an underactive thyroid.
Here is the short answer. Levothyroxine is not listed as a contraindication to tirzepatide, the active ingredient in Mounjaro. The two medicines may be prescribed together following an individual clinical assessment. What matters is making sure your prescriber knows about your thyroid condition, your current levothyroxine dose and any recent thyroid function results. Your prescriber can then decide whether any monitoring is needed. This page explains the main points to consider.
Key things to know
- Levothyroxine and Mounjaro are not listed as contraindicated together. Eli Lilly's own medical information confirms this.
- There is no documented direct pharmacological interaction between the two medicines. However, tirzepatide may affect the absorption of some oral medicines.
- The main consideration is absorption. Tirzepatide slows gastric emptying, and levothyroxine is a medicine where small changes in absorption can affect thyroid hormone levels.
- An underactive thyroid is not the same as the thyroid conditions listed as contraindications for Mounjaro. These include medullary thyroid carcinoma and Multiple Endocrine Neoplasia syndrome type 2 (MEN2).
- Weight loss can change how much levothyroxine a person needs. The direction and size of any change can vary.
- Your prescriber may recommend thyroid function testing after starting or increasing tirzepatide. Do not change your levothyroxine dose without medical advice.
Can You Take Mounjaro With Levothyroxine? The Short Answer
Yes, in many cases, subject to an individual clinical assessment. Eli Lilly's published medical information states that levothyroxine is not contraindicated with tirzepatide. Neither the UK Summary of Product Characteristics nor NICE guidance lists the combination as a contraindication. [Drugs.com]
The manufacturer does state that tirzepatide delays gastric emptying and may affect the absorption of oral medicines taken at the same time. This is particularly relevant to medicines with a narrow therapeutic index, such as levothyroxine. [Drugs.com]
The important question is therefore not simply whether you can take both medicines, but whether your thyroid function needs to be monitored. Your healthcare provider or prescriber can make that decision based on your medical history, current levothyroxine dose and recent thyroid function results.
An Underactive Thyroid Is Not the Thyroid Problem That Rules Mounjaro Out
This is a common source of confusion. Mounjaro has a thyroid warning, but that does not mean everyone with an underactive thyroid is unable to take it. It is important to tell your prescriber about your thyroid history so they can check whether the treatment is suitable for you.
Tirzepatide is contraindicated in people with a personal or family history of medullary thyroid carcinoma, and in people with Multiple Endocrine Neoplasia syndrome type 2. This warning comes from findings in rodent studies, where GLP 1 receptor agonists activated thyroid C cells. The relevance of these findings to humans has not been determined.
Medullary thyroid carcinoma is a rare cancer of the thyroid's C cells. Hypothyroidism, including Hashimoto's thyroiditis, is a different condition in which the thyroid gland does not produce enough thyroid hormone. Levothyroxine is a synthetic version of thyroxine (T4) used for thyroid hormone replacement.
Taking levothyroxine does not, by itself, indicate that you have a personal or family history of medullary thyroid carcinoma. If you have such a history, or a history of MEN2, tell your prescriber during your consultation.
Two different thyroid questions that get confused
| Underactive thyroid on levothyroxine | Medullary thyroid carcinoma or MEN2 | |
|---|---|---|
| What it is | Insufficient thyroid hormone production, treated with hormone replacement | A rare C cell cancer, or an inherited syndrome linked to its risk |
| Effect on Mounjaro eligibility | Not a listed contraindication | Contraindication where there is a personal or family history |
| What your prescriber needs | Current dose and recent thyroid function results | Full disclosure of personal and family history |
| What follows | Monitoring and possible dose review by your GP | Tirzepatide is not suitable |
Does Mounjaro Affect Levothyroxine Absorption?
Potentially, yes, although the concern is about absorption rather than a documented direct interaction. There is no documented direct pharmacokinetic interaction between tirzepatide and levothyroxine. They do not compete for the same metabolic pathways, and tirzepatide does not appear to change how levothyroxine is broken down. [Lilly]
The main consideration is gastric emptying. Levothyroxine is absorbed mainly in the small intestine, and its absorption can be affected by food, timing and other medicines. This is why it is usually taken on an empty stomach. Tirzepatide slows the movement of stomach contents into the small intestine, which may affect how consistently some oral medicines are absorbed. [Trimi Health]
The prescribing information also notes that the effect of tirzepatide on gastric emptying was greatest after a single 5mg dose and reduced after subsequent doses. This does not mean that everyone will experience a change in levothyroxine absorption. However, it is one reason your prescriber may consider thyroid function testing after you start treatment or increase your dose. [Drugs.com]
In simple terms, Mounjaro may affect how your body absorbs thyroid medication, even when there is no documented direct drug interaction.
Why Weight Loss Itself Can Change Your Levothyroxine Requirement
Weight loss can also affect how much levothyroxine a person needs. Levothyroxine dosing may take body weight into account, although the right dose depends on several factors. NICE guideline NG145 recommends considering a starting dosage of 1.6 micrograms per kilogram of body weight per day for adults under 65 with primary hypothyroidism and no history of cardiovascular disease, rounded to the nearest 25 micrograms. For adults aged 65 and over, or with a history of cardiovascular disease, NICE recommends starting at 25 to 50 micrograms per day with titration. [Specialist Pharmacy Service]
If your dose was originally calculated using a different body weight, your prescriber may want to review it as your weight changes. This does not mean that your dose is automatically wrong or that you need to change it.
Thyroid hormone requirements may also be influenced by lean body mass, rather than total body weight alone. Research has found that levothyroxine dose per kilogram of actual body weight can vary with BMI, while dose per kilogram of lean body mass may be more consistent. This means that a simple weight based calculation may not accurately predict how much levothyroxine someone needs after weight loss.
The direction of any change is not guaranteed. Some published reports involving GLP 1 receptor agonists describe reduced levothyroxine requirements as weight falls. However, a 2026 case report described a woman whose weight fell from 116kg to 100kg over three months on tirzepatide, while her TSH rose to 20.7 mIU/L despite confirmed adherence. The report suggested that impaired absorption may have contributed. [Boltpharmacy]
This does not mean the same thing will happen to everyone. Weight loss and changes in absorption may affect thyroid hormone levels in different ways. Your prescriber can use repeat TSH and thyroid hormone tests to decide whether your levothyroxine dose needs review. Do not change the dose based on weight loss alone.
Thyroid Monitoring When You Start Mounjaro on Levothyroxine
There is no single monitoring schedule specifically required for this combination in UK guidance. The approach below is based on general levothyroxine monitoring practice and published clinical commentary, rather than a formal protocol. Your GP or prescriber will decide what is appropriate for you.
What may be monitored and when
| Point in treatment | What is usually checked | Why it matters |
|---|---|---|
| Before starting tirzepatide | Recent TSH levels and free T4, current levothyroxine dose | Provides a baseline for comparing any later changes |
| Around 6 to 8 weeks after starting | Thyroid function tests, including thyroid stimulating hormone where appropriate | May help identify changes in thyroid hormone levels after starting treatment |
| After each increase in strength | Thyroid function, if your prescriber considers it necessary | A dose increase may affect gastric emptying and could change the absorption of some oral medicines |
| After substantial weight loss | Thyroid function | Weight changes may affect the amount of levothyroxine you need |
| Once stable | Routine intervals set by your GP | NICE advises maintaining TSH within the reference range and avoiding suppression |
General guidance only. Testing intervals and any dose changes are decisions for your GP or prescriber, not for the patient or an online pharmacy. Regular blood tests may help your prescriber assess whether your thyroid levels remain within the target range while you are taking Mounjaro.
One practical point worth raising with your GP is where your thyroid function tests will be arranged. These are usually managed through NHS primary care rather than an online weight management service. If you start tirzepatide privately, your GP may not know unless you tell them.
Practical Timing for Taking Levothyroxine Alongside Mounjaro
The usual advice for taking levothyroxine does not change simply because you are taking tirzepatide. However, keeping to a consistent routine may help your prescriber assess any changes in your thyroid levels.
- NHS advice is to take levothyroxine on an empty stomach, usually 30 to 60 minutes before breakfast, at the same time every day.
- Consistency matters. Taking your dose at a similar time each day can help keep your routine steady.
- Keep levothyroxine well separated from calcium, iron and antacids, which can reduce its absorption. Around four hours is commonly advised.
- Your Mounjaro injection day does not need to be coordinated with your levothyroxine timing. Tirzepatide has a half life of around five days and is given once weekly.
- If nausea in the early weeks makes your usual routine difficult, tell your prescriber rather than skipping or changing doses without advice. They can help you decide what to do.
Some people take levothyroxine at night instead. This can be an option, but it is worth discussing with your GP before changing your routine, particularly if your thyroid function is being monitored.
Symptoms That Get Confused Between the Two Treatments
When taking these medicines together, some symptoms may have more than one possible cause. Fatigue, constipation and feeling cold can be linked to an underactive thyroid, but they may also occur during weight loss or while taking tirzepatide. This is why it is useful to discuss persistent or unusual symptoms with your prescriber.
Overlapping symptoms and what they might mean
| What you notice | Could be thyroid related | Could be tirzepatide related | Sensible next step |
|---|---|---|---|
| Persistent fatigue | Under replacement, or changes linked to weight loss | Reduced calorie intake in early weeks | Ask your prescriber whether thyroid function testing is needed and review your food intake |
| Constipation | Under replacement | Slowed gastric emptying, a common side effect | Try fluids and fibre if suitable for you, and mention it at your review |
| Feeling cold or hair thinning | Under replacement | Rapid weight loss or low food intake | Discuss the symptoms with your prescriber and ask whether thyroid function testing is needed |
| Palpitations or anxiety | Over replacement | Not a typical tirzepatide side effect | Contact your GP promptly |
| Nausea | Not typical of an underactive thyroid | Common, especially during dose increases | Contact your prescriber if it is persistent or difficult to manage |
If you notice symptoms that may suggest your levothyroxine dose is too high, such as palpitations, tremor, unusual anxiety or heat intolerance, contact your GP rather than changing the dose yourself.
If You Have Type 2 Diabetes as Well as Hypothyroidism
If you have both conditions, there is one more thing to consider. Levothyroxine prescribing information warns that starting treatment in people with diabetes may affect blood glucose control and increase the need for diabetes medicines or insulin. If your levothyroxine dose changes while you are taking tirzepatide for type 2 diabetes, your prescriber may want to check your blood glucose levels alongside your thyroid function. [Drugs.com]
Metformin may also be relevant if you take it. It can be associated with lower vitamin B12 levels, which may cause fatigue. This can sometimes be confused with thyroid symptoms or side effects from Mounjaro. If you have ongoing fatigue, your GP can decide whether a blood test is appropriate.
What to Tell Your Prescriber Before You Start
A consultation for prescription weight loss treatment works best when your prescriber has a clear picture of your health and medicines. If you take levothyroxine, include:
- Your current levothyroxine dose and how long you have been taking it.
- Your most recent TSH and free T4 results, with dates if you have them.
- Your thyroid condition and whether your recent results have been stable or your dose has changed.
- Any personal or family history of medullary thyroid carcinoma, MEN2 or other thyroid cancer.
- Every other medicine and supplement you take, including calcium and iron, which can affect levothyroxine absorption.
- Whether you have diabetes and which medicines you take for it.
Sharing this information helps your prescriber assess whether Mounjaro is suitable for you and whether any monitoring may be needed. It also helps them avoid missing important details about your thyroid treatment.
Mounjaro Treatment, Prices and How Supply Works
Mounjaro is a prescription only medicine containing tirzepatide, a dual GIP (glucose dependent insulinotropic polypeptide) and GLP 1 receptor agonist made by Eli Lilly. In the UK, it is licensed as a weekly injection for weight management alongside a reduced calorie diet and increased physical activity, and for type 2 diabetes.
These gut hormones help regulate blood glucose and insulin release. Tirzepatide also affects appetite and food intake, which can contribute to weight loss. It is given by subcutaneous injection using the KwikPen device, which delivers four weekly doses per pen.
Mounjaro standard prices by strength
| Strength | Standard price per pen (4 weekly doses) | Approximate cost per weekly dose |
|---|---|---|
| 2.5mg | £174.99 | £43.75 |
| 5mg | £204.99 | £51.25 |
| 7.5mg | £254.99 | £63.75 |
| 10mg | £274.99 | £68.75 |
| 12.5mg | £304.99 | £76.25 |
| 15mg | £323.99 | £81.00 |
Standard prices shown for comparison. Prices update live and the price table on the product page always takes precedence. Treatment is subject to consultation and clinical approval.
EveryDayMeds is a UK registered pharmacy operated by OVIO LTD. Treatment is supplied only after clinical assessment by an appropriately qualified UK registered prescriber, who decides whether treatment is suitable and which strength is appropriate.
Clinical trials have found that tirzepatide can produce greater weight loss than some treatments that target only one of these pathways. However, results vary between individuals, and treatment does not guarantee a particular amount of weight loss.
Mounjaro is licensed through the UK healthcare products regulatory process. Genuine Eli Lilly medication is dispatched with temperature controlled delivery, since tirzepatide requires refrigerated storage before first use. Availability and dispatch times can change, so check the live status on the product page before ordering.
You can read more about Mounjaro injections, or compare the wider range of medically supervised weight loss treatments in the weight loss hub.
Medical Disclaimer
This article is for general information only and does not constitute medical advice, a diagnosis, or a recommendation to start, stop or change any medicine. Mounjaro is a prescription only medicine 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.
Levothyroxine doses, thyroid function testing intervals and any adjustment to thyroid treatment are decisions for your GP or specialist. Never alter your levothyroxine dose, timing or schedule on the basis of this page or because you have started weight loss treatment. Dosage figures quoted are published national guidance for prescribers, not personal recommendations.
If you experience palpitations, tremor, or severe or persistent symptoms, seek medical advice promptly.