What the Foundayo licence says about levothyroxine
Foundayo’s Summary of Product Characteristics, the official UK prescribing document, doesn’t mention levothyroxine or thyroid hormone.1
The licence does give specific instructions for medicines whose use may need changing alongside Foundayo:1
- Use non-oral contraception or add a barrier method for 30 days after starting Foundayo and for 30 days after each dose increase
- Halve the dose of simvastatin
- Use additional caution with rosuvastatin doses above 20 mg
There are no instructions to separate levothyroxine from Foundayo or change its dose when Foundayo is started.
Foundayo slows digestion, so some tablets may take longer to reach the bloodstream.
In one study, the highest concentration of paracetamol was 28% lower when it was taken with Foundayo, although modelling didn’t predict a clinically relevant change in how commonly used medicines are absorbed.1
Levothyroxine wasn’t included, so we don’t know whether Foundayo changes its absorption. We can only say that the UK licence doesn’t list an interaction.
How to take Foundayo and levothyroxine
The NHS advises taking levothyroxine with water on an empty stomach, 30 to 60 minutes before the first meal of the day, a caffeinated drink, or another medicine.2
A cautious morning routine if you start Foundayo is:
- Take levothyroxine with water when you wake up
- Wait 30 to 60 minutes before eating, drinking caffeine, or taking another medicine
- Take Foundayo with breakfast or at another consistent time later in the day (with lunch, for example)
You can take Foundayo at any time, with or without food or water.1 Not taking it with levothyroxine is a cautious practical choice because the combination hasn’t been studied, not a requirement in the Foundayo licence.
If your GP has already advised a different levothyroxine routine, don’t change it solely because you’re starting Foundayo. Ask your GP or pharmacist how to include Foundayo in your routine.
The NHS lists antacids, calcium and iron supplements, omeprazole, and several other medicines as treatments that may not mix well with levothyroxine.2 A pharmacist can tell you whether any of your regular medicines need separating.
Our compatibility checker for GLP-1 medications, which includes Foundayo, covers common combinations. It doesn’t replace advice from your GP or pharmacist.
Why the Wegovy pill is different
Advice to separate an oral GLP-1 medication from levothyroxine often refers to the Wegovy pill, which contains semaglutide. Foundayo contains a different drug and has different dosing instructions.
Semaglutide is a peptide drug that would usually be broken down in the stomach. The Wegovy pill must be taken on an empty stomach with up to 120 mL of water, followed by at least 30 minutes without food, drink, or another tablet.3
Foundayo contains a small-molecule drug rather than a peptide. It doesn’t need to be taken on an empty somtach, have a water limit, or wait before another meal.1
Researchers tested oral semaglutide with levothyroxine. The study involved 45 healthy adults who took oral semaglutide with a single 600 microgram dose of levothyroxine.
The total amount of thyroxine, the thyroid hormone in levothyroxine, reaching the blood increased by 33%, although the highest concentration was unchanged.4
The study used a high single dose of levothyroxine in people without thyroid disease. It supports monitoring when oral semaglutide and levothyroxine are combined, but it doesn’t show that Foundayo has the same effect.
| Medication |
How it’s taken |
Levothyroxine evidence |
| Foundayo |
Once a day, at any time, with or without food or water |
No listed interaction and no direct study; keeping levothyroxine in its own window is a cautious routine |
| Wegovy pill |
Once a day on an empty stomach, with up to 120 ml of water and a 30-minute wait |
A healthy-volunteer study found a 33% increase in total thyroxine exposure when taken together |
Our guide to taking the Wegovy pill with levothyroxine covers its morning routine. Our Wegovy pill and Foundayo comparison explains the wider differences between the tablets.
Second Nature has worked with the NHS since 2017. If you take levothyroxine and are considering Foundayo, take our 3-minute eligibility quiz, and a clinician will review your answers.
When thyroid tests may be needed
No guideline recommends needing more thyroid tests if you start Foundayo or other GLP-1 medications. However, substantial weight loss can change the levothyroxine dose you need.
So, we’d recommend telling your GP that you’re on Foundayo (ideally before you start) and asking them whether you need an additional test if your thyroid levels haven’t been checked recently, you develop symptoms of an incorrect levothyroxine dose, or your weight changes substantially.
The British Thyroid Foundation advises that people taking levothyroxine may need a thyroid blood test around two to three months after substantial weight loss, to check whether the dose needs changing.5
We can’t predict whether your levothyroxine dose will change based on how much weight you’ve lost. Your GP will use TSH, the main blood test used to check whether a levothyroxine dose is right, alongside your symptoms and other results.
With their permission, we contact our members’ GPs to let them know they’ve started Foundayo. Their GP remains responsible for thyroid testing and any change to their dose.
Symptoms that can occur when the levothyroxine dose is too high include:
- Feeling anxious or having difficulty sleeping
- Sweating or tremor
- A fast or irregular heartbeat
Symptoms of an underactive thyroid returning can include tiredness, feeling cold, constipation, and low mood. Contact your GP or pharmacist if these symptoms are new, persistent, or getting worse.
The NHS advises calling 999 or going to A&E if heart palpitations don’t go away, or occur with chest pain, shortness of breath, feeling faint, or fainting.
Will Foundayo work with an underactive thyroid?
There’s no evidence that stable, treated hypothyroidism, or an underactive thyroid, stops Foundayo (or other GLP-1 medications) from working. However, the main Foundayo trial hasn’t published results specifically for people taking levothyroxine.
ATTAIN-1 studied 3,127 adults living with obesity who didn’t have diabetes. At 72 weeks, Foundayo produced an average weight loss of 11.2% (or 12.4% for participants who stayed on the medication for the whole trial), compared with 2.1% for placebo.6
Foundayo mimics the GLP-1 hormone, which reduces hunger and food noise. It doesn’t treat an underactive thyroid or increase metabolic rate (how many calories we burn each day).
For people living with an underactive thyroid who find weight management difficult, reduced hunger and food noise may make weight loss particularly challenging. This could make Foundayo a helpful tool, but no research shows it works better for this group.
Foundayo won’t correct thyroid hormone levels if the levothyroxine dose is too low. If weight loss is slower than expected and symptoms such as tiredness, feeling cold, constipation, or low mood have returned, ask your GP whether your thyroid levels should be checked.
The thyroid cancer warning in the UK and U.S.
The U.S. Foundayo label says Foundayo shouldn’t be used by people with a personal or family history of medullary thyroid cancer, or multiple endocrine neoplasia type 2 (MEN 2).
The UK licence doesn’t list either history as a reason not to prescribe. Its only listed reason is an allergy to Foundayo or one of its ingredients.1
The concern comes from tumours in a specific type of thyroid cell, called a C cell, seen in rodents given GLP-1 medicines. The UK Foundayo licence says the relevance to humans is considered low, but it can’t be excluded completely.1
An underactive thyroid and Hashimoto’s disease aren’t medullary thyroid cancer, and neither is listed in the UK licence as a reason to avoid Foundayo.
If your thyroid was removed because of cancer, or you have any personal or family history of thyroid cancer, discuss it with your GP, thyroid specialist, or Foundayo prescriber before starting.
Frequently asked questions
Can you take Foundayo and levothyroxine at the same time?
You can take them on the same day, and there are no specific instructions to avoid taking them at the same time.
Taking levothyroxine alone on an empty stomach and Foundayo later in the day is a sensible approach to ensure that levothyroxine absorption.1
Does Foundayo affect levothyroxine absorption?
No direct study has tested this. Foundayo slows digestion, which can change how quickly tablets are absorbed, but the UK licence doesn’t list an interaction with levothyroxine.1
Do I need a thyroid blood test after starting Foundayo?
Not automatically. Ask your GP whether testing is needed based on your recent results, symptoms, and weight loss.
The British Thyroid Foundation suggests considering a test approximately two to three months after substantial weight loss.5
Will my levothyroxine dose change as I lose weight?
It may change, but a reduction isn’t automatic and shouldn’t be made in advance. Your GP will use blood-test results and symptoms to decide whether the dose remains suitable.
Can you take Foundayo with Hashimoto’s disease?
Hashimoto’s disease isn’t listed in the UK Foundayo licence as a reason not to prescribe it.1 Your thyroid levels should be adequately treated, and your GP can advise whether additional testing is needed.
Can you take Foundayo after having your thyroid removed?
Having your thyroid removed isn’t listed as a reason to avoid Foundayo. If it was removed because of cancer, discuss the type of cancer and your history with your thyroid specialist or prescriber before starting.
Why is the advice different for the Wegovy pill?
The Wegovy pill also needs to be taken on an empty stomach, like levothyroxine.
A healthy-volunteer study also found that taking oral semaglutide with levothyroxine increased total thyroxine exposure by 33%, while no equivalent study has tested Foundayo.4
Does an underactive thyroid make weight loss slower on Foundayo?
No Foundayo trial has published a direct comparison between people with and without hypothyroidism.
Stable, treated hypothyroidism isn’t known to prevent Foundayo from reducing hunger, but returning thyroid symptoms should be discussed with your GP.
Who manages my thyroid tests if I get Foundayo through Second Nature?
Your GP manages thyroid testing and changes to levothyroxine. With your permission, Second Nature can contact your GP to let them know that you’ve started Foundayo.
Take home message
You can usually take Foundayo with levothyroxine. No interaction is listed in the UK licence, but the combination hasn’t been studied directly.1
Still, we recommend taking levothyroxine on an empty stomach, then taking Foundayo later in the day. This is a cautious approach, but it will ensure levothyroxine absorption is unaffected.
While no guidelines state that you need a thyroid test after taking Foundayo, weight loss can change the levothyroxine dose you need, so we tell your GP if you start Foundayo and ask them whether testing is needed, especially if symptoms return or your weight changes substantially.
The British Thyroid Foundation suggests considering a test approximately two to three months after substantial weight loss.5
Stable, treated hypothyroidism isn’t known to stop Foundayo from reducing hunger and food noise.
However, no published evidence exists for people taking levothyroxine alongside Foundayo, and Foundayo doesn’t treat an underactive thyroid.6
U.S. thyroid-cancer warnings refer to medullary thyroid cancer and MEN 2, not an underactive thyroid or Hashimoto’s disease.
The UK licence is different, but anyone with a personal or family history of thyroid cancer should discuss it with their GP, specialist, or Foundayo prescriber.
At Second Nature, we combine GLP-1 medication with structured habit support from registered dietitians to help people maintain a whole-food eating pattern, stay active, and reduce the risk of weight regain if they stop medication in the future.
A peer-reviewed service evaluation in JMIR Formative Research found that active subscribers with 12-month data lost an average of 19.1% of their body weight. Across everyone with 12-month data, 77.7% lost at least 10%.7
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.
