How to time your medications
Levothyroxine is best absorbed on an empty stomach. The NHS and the British National Formulary advise taking it once a day in the morning, at least 30 to 60 minutes before breakfast or any caffeinated drink.1
Mounjaro is a subcutaneous injection given once a week, on the same day each week. You can inject at any time of day, with or without food, so the timing of your weekly injection doesn’t need to align with your daily tablet.
For most people, the simplest approach is to keep your levothyroxine routine and add Mounjaro on a chosen day.
Morning or evening dosing
Take your levothyroxine with water as soon as you wake up, and wait at least 30 minutes before breakfast, tea, coffee, or any other tablets. Some people prefer bedtime dosing, taken three to four hours after their last meal. Either approach works alongside Mounjaro.
What to keep separate from levothyroxine
Calcium, iron, and antacids reduce absorption, so leave at least four hours between these and your tablet. Soya and high-fibre meals can also interfere.1
Gastric emptying and levothyroxine absorption
Tirzepatide, the drug in Mounjaro, slows the rate at which food and tablets leave the stomach. The Mounjaro Summary of Product Characteristics notes this effect is most pronounced at initiation and dose increases, and attenuates over time.4
The same document advises monitoring patients on oral medications with a narrow therapeutic index. Levothyroxine fits: too little and thyroid symptoms return, too much and you can develop palpitations, anxiety, or sleep problems.
No published clinical trial shows tirzepatide meaningfully changes levothyroxine absorption. The sensible approach is to keep your usual timing, monitor your blood tests, and let your symptoms guide any conversation with your prescriber.
Symptoms worth watching for
If you’ve been stable on levothyroxine for years and start Mounjaro, these symptoms suggest your thyroid levels may have shifted:
- Returning fatigue, feeling colder, constipation, or low mood (levels may have dropped)
- A racing heart, anxiety, tremor, sweating, or trouble sleeping (levels may be too high, sometimes after weight loss has reduced your requirement)
None are emergencies on their own, but they’re worth flagging to your GP. A blood test usually clarifies what’s going on.
TSH monitoring on Mounjaro
NICE guidance on thyroid disease (NG145) recommends measuring TSH (the hormone that tells your thyroid how much to produce) every three months until stable, then once a year.5
Starting Mounjaro counts as a meaningful change. It makes sense to bring your TSH check forward to roughly six to eight weeks after starting, and again after each dose escalation. Discuss this with your GP rather than waiting for your annual review.
What you’re looking for is whether your TSH stays in the target range your prescriber has set for you. If it drifts up, your absorption may have changed. If it drifts down, you may have lost enough weight that your dose now needs reducing.
The standard test is TSH. If your symptoms are out of step with the result, your GP may also request free T4 (the main hormone your thyroid produces). NICE doesn’t recommend routine free T3 testing for primary hypothyroidism because the result rarely changes management.5
Weight loss and your levothyroxine dose
This is the bigger consideration for most people taking Mounjaro alongside levothyroxine. Lean body mass, not fat mass, drives how much thyroid hormone you need. As you lose weight, your dose requirement often falls.3
A 2005 study in the Journal of Clinical Endocrinology and Metabolism found that lean body mass was the strongest predictor of levothyroxine dosage.3 If you lose 10% to 15% of your body weight, your prescriber may reduce your levothyroxine to keep your TSH in range.
Mounjaro produces substantial weight loss in clinical trials. SURMOUNT-1 reported an average reduction of 20.9% on the 15 mg dose at 72 weeks.6 SURMOUNT-4 showed continued treatment maintained that loss.7
If your prescriber reduces your dose, they’ll usually retest TSH six to eight weeks later. Changes are typically small, often a 25 microgram step. Don’t adjust your own dose; thyroid hormone has a long half-life and it’s easy to overshoot.
If you have Hashimoto’s, weight loss won’t reverse the underlying autoimmune process. You’ll still need levothyroxine, the dose may simply be lower. Our guide on Mounjaro and hypothyroidism covers the broader picture.
Eating well on both medications
Mounjaro reduces appetite, but the aim isn’t to eat as little as possible. Eating three balanced meals a day helps preserve muscle, supports thyroid function, and keeps your energy levels even.
Second Nature’s balanced plate model is a useful starting point: half your plate as vegetables, a quarter as protein, a quarter as complex carbohydrates, plus a serving of fat.
Protein at every meal
Protein protects muscle mass during weight loss and keeps you fuller for longer. Include a portion at each meal: eggs, fish, chicken, turkey, lean red meat, Greek yoghurt, beans, lentils, or tofu.
Complex carbohydrates with fibre
Fibre-rich carbohydrates provide steady energy and support digestion, which helps because constipation is common in both hypothyroidism and the early weeks of Mounjaro.
Good options from Second Nature’s food lists include wholegrain or sourdough bread, rolled oats, brown rice, potato, sweet potato, quinoa, beans and lentils, wholemeal pasta or noodles, and wholemeal couscous.
Healthy fats from whole foods
Fat helps you absorb fat-soluble vitamins, supports hormone production, and adds satisfaction to meals. Choose olive oil, avocado, nuts, seeds, and oily fish such as salmon, mackerel, or sardines. Oily fish twice a week also provides iodine and selenium, two minerals your thyroid uses.
Vegetables and the balanced plate
Aim for half your plate as vegetables at lunch and dinner. Even when appetite drops, a smaller portion still counts. The myth that broccoli, kale, or cabbage harm thyroid function isn’t supported by the evidence at normal dietary intakes.
Eating 3 balanced meals when you’re not hungry
Mounjaro can leave you genuinely uninterested in food, especially in the first few weeks and after each dose increase. Skipping meals tends to backfire; you’re more likely to feel tired, weak, or nauseated later.
On low-appetite days, focus on small but nutrient-dense options:
- Greek yoghurt with berries and nut butter
- Scrambled eggs on sourdough
- A handful of nuts and a piece of fruit
- A smoothie with spinach, banana, Greek yoghurt, and protein powder
Batch-cooking on hungrier days means you’ll have meals ready when appetite drops.
Staying active
Movement supports weight loss, helps preserve muscle, and counteracts the fatigue that hypothyroidism can cause. Habits beat targets here.
Research on habit formation suggests it takes an average of 66 days for a new behaviour to feel automatic.8 The reliable way there is to start small and link the habit to something you already do, like a 10-minute walk after dinner.
Gradually do a little more as it feels manageable. The NHS recommends 150 minutes of moderate activity per week as a longer-term goal, but it isn’t where you need to start.
Aerobic activity
Walking, cycling, swimming, and gentle jogging all count. If hypothyroidism makes you tired, low-impact options like walking and swimming are easier to keep up with on a bad day.
Resistance training
Resistance training helps preserve muscle during weight loss on Mounjaro. You don’t need a gym; bodyweight exercises like squats, wall press-ups, and seated leg raises work well at home.
If juggling medications feels stressful
Managing two long-term medications, blood tests, and changes in appetite can feel like a lot. If you’re noticing low mood, anxiety, or persistent stress, mention it to your GP. They can check whether your thyroid levels need adjusting and help you access support.
Mounjaro versus other weight-loss medications
The table compares Mounjaro with the two other weight-loss medications most commonly prescribed in the UK.
| Feature |
Mounjaro (tirzepatide) |
Wegovy (semaglutide) |
Saxenda or generic liraglutide |
| Mechanism |
Dual GLP-1 and GIP receptor agonist |
GLP-1 receptor agonist |
GLP-1 receptor agonist |
| Dosing |
Once a week, subcutaneous injection |
Once a week, subcutaneous injection |
Once a day, subcutaneous injection |
| Average weight loss in trials |
Up to 20.9% at 72 weeks (SURMOUNT-1, 15 mg)6 |
Around 14.9% at 68 weeks (STEP-1, 2.4 mg) |
Around 5% to 8% at 56 weeks (SCALE) |
| Common side effects |
Nausea, diarrhoea, constipation, reduced appetite, most pronounced on initiation and dose escalation |
Nausea, diarrhoea, constipation, reduced appetite |
Nausea, diarrhoea, constipation, headache |
| Levothyroxine-relevant points |
Slows gastric emptying; effect strongest on initiation and at dose increases.4 Bring TSH check forward to 6 to 8 weeks after starting |
Also slows gastric emptying. Same monitoring approach applies |
Daily injection means more frequent gut effects, but each is shorter-lived. Same monitoring approach applies |
None of these medications has a direct interaction with levothyroxine. The shared consideration is delayed gastric emptying, which is why TSH monitoring after starting any of them makes sense.
Common questions
Can you take Mounjaro and levothyroxine on the same day?
Yes. Levothyroxine is a daily tablet and Mounjaro is a weekly injection, so they’ll often fall on the same day. Take your levothyroxine in the morning on an empty stomach, and inject your Mounjaro at any time that suits you. The timings don’t need to align.
Will Mounjaro stop my levothyroxine working?
It’s unlikely. There’s no listed drug interaction. Mounjaro slows gastric emptying, which could in theory affect absorption of any oral tablet, but the effect attenuates over time.4 A TSH check six to eight weeks after starting Mounjaro will pick up any meaningful change.
How often should I get my thyroid levels checked while on Mounjaro?
NICE recommends every three months until stable, then once a year.5 When starting Mounjaro or moving up a dose, ask your GP to bring forward a TSH check to roughly six to eight weeks later.
Will I need a lower dose of levothyroxine after losing weight?
Possibly. Lean body mass is the main driver of levothyroxine requirement, and significant weight loss often reduces the dose.3 Your GP will adjust your prescription based on your TSH, not your weight alone. Don’t change the dose yourself.
What if I forget my levothyroxine on the morning of my Mounjaro injection?
Take it as soon as you remember, ideally 30 minutes before your next meal. If it’s nearly time for the next day’s dose, skip the missed one. Levothyroxine has a long half-life, so a single missed dose isn’t usually a problem.
Can Mounjaro’s nausea affect my levothyroxine absorption?
If you vomit within 30 minutes of taking your tablet, the dose may not have been fully absorbed. Speak to your GP or pharmacist about whether to retake it. Persistent nausea is worth flagging to your prescriber.
Can I take Mounjaro if I have Hashimoto’s?
Yes. Hashimoto’s is the most common cause of hypothyroidism in the UK and isn’t a contraindication. The contraindication on Mounjaro’s label is specifically for medullary thyroid carcinoma (a rare cancer of the thyroid C-cells) or Multiple Endocrine Neoplasia type 2, which are different conditions.4
Should I switch to evening levothyroxine on Mounjaro?
Not unless your current routine isn’t working. Some prefer bedtime dosing, taken three to four hours after their last meal, but morning dosing remains the standard.
Are there other supplements to keep separate from levothyroxine?
Yes. Calcium, iron, multivitamins containing minerals, and antacids should be taken at least four hours apart from levothyroxine because they reduce absorption.1 Soya supplements and some heartburn medications also interfere.
Take home message
Mounjaro and levothyroxine can be taken together with no listed drug interaction. Keep your usual levothyroxine routine, taking the tablet on an empty stomach 30 to 60 minutes before breakfast, and inject Mounjaro at any time on its scheduled day.
The two practical considerations are gastric emptying, strongest at initiation and dose escalation, and weight loss reducing your levothyroxine requirement. Both are managed through TSH monitoring, ideally six to eight weeks after starting Mounjaro and after each dose increase.
Meaningful weight loss often leads to a small reduction in your levothyroxine dose. That’s an expected adjustment, not a sign of a problem.
How can Second Nature help?
Second Nature is a digital health programme that combines medication with structured habit-change support from registered dietitians and coaches. The approach is built around the balanced plate model: half vegetables, a quarter protein, a quarter complex carbohydrates, plus a serving of fat.
A 2025 study published in JMIR Formative Research found that active subscribers on Second Nature’s GLP-1-supported programme lost an average of 19.1% of their body weight at 12 months, with 77.7% achieving at least 10% weight loss.9
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
- NHS. (2024). Levothyroxine: a medicine used to treat an underactive thyroid (hypothyroidism).
- NHS. (2024). Underactive thyroid (hypothyroidism).
- Santini, F. et al. (2005). Lean body mass is a major determinant of levothyroxine dosage in the treatment of thyroid diseases. Journal of Clinical Endocrinology and Metabolism, 90(1), 124-127.
- Electronic Medicines Compendium. (2024). Mounjaro (tirzepatide) Summary of Product Characteristics.
- National Institute for Health and Care Excellence. (2019, updated 2023). Thyroid disease: assessment and management. NICE guideline NG145.
- Jastreboff, A.M. et al. (2022). Tirzepatide once weekly for the treatment of obesity. New England Journal of Medicine, 387(3), 205-216.
- Aronne, L.J. et al. (2024). Continued treatment with tirzepatide for maintenance of weight reduction in adults with obesity: the SURMOUNT-4 randomised clinical trial. JAMA, 331(1), 38-48.
- 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.
- Richards, R. et al. (2025). Improvements in cardiometabolic outcomes in a remotely delivered GLP-1RA-supported specialist weight management programme. JMIR Formative Research, 9(1), e72577.
