Losing Weight With Hashimoto's or an Underactive Thyroid

Yes, you can lose weight with Hashimoto's or an underactive thyroid. Once your thyroid levels are in range on treatment, the American Thyroid Association says the ability to gain or lose weight is the same as for people without thyroid problems. Treatment itself mostly takes off retained salt and water, often only a kilo or two, so if the scales haven't moved much since starting levothyroxine, that is normal. Levothyroxine isn't a weight-loss medicine. The steps that help are checking your dose, then food, movement and sleep you can keep up.

If you have been told "it's just your thyroid", or that weight loss is impossible with Hashimoto's, here is what the research actually shows, and what is worth checking.

How much weight is the thyroid responsible for?

Less than most people expect. The American Thyroid Association says about 2.3 to 4.5 kg of weight gain may be related to an underactive thyroid, depending on how severe it is, and that most of it is retained salt and water rather than fat. Treatment returns weight to roughly what it was before the thyroid became underactive.

In practice the change on treatment is often small. In a study of 101 people starting levothyroxine, the median weight change was about 0.1 kg. In a small study of people with a very underactive thyroid, about 4.3 kg came off over a year, almost all of it water. So if you have been on treatment for a while and are still carrying extra weight, it is unlikely the thyroid alone is the reason.

Is your metabolism slower on levothyroxine?

Only slightly, if at all. In a study that measured it directly, resting energy use in people on levothyroxine was about 4% lower than in healthy people, and there was no difference in food intake or body composition. A larger US survey found people on levothyroxine had a slightly higher body mass index on average, but they also reported being less active and were more likely to take other medicines that can affect weight, so it doesn't show the tablet is the cause.

Can you lose weight with Hashimoto's?

Yes. In a study of 100 people with Hashimoto's and obesity on levothyroxine, both groups lost weight and body fat on a reduced-energy diet over six months. In another study, a weight-loss medicine worked as well for people with a well-treated underactive thyroid as for people without one. There is no evidence that dieting is "pointless" with a thyroid condition.

What helps

1. Check your dose first. If your levels are off, your weight and energy will be harder to manage. Ask for a test, and remember that levels take about six weeks to settle after any change.
2. Eat in a way you can keep up. Enough protein, plenty of vegetables and legumes, wholegrains, and fewer drinks with added sugar. Higher-protein, reduced-energy eating can help with appetite and weight management in general. No special Hashimoto's diet has been shown to work better. Our guide to the Hashimoto's diet looks at the evidence.
3. Move regularly. The Australian guidelines suggest muscle-strengthening activity on two or more days a week alongside regular movement. In a trial of people on levothyroxine, 12 weeks of aerobic, strength or combined training improved quality of life.
4. Protect your sleep. Short sleep is linked with changes in hunger hormones that push appetite up.
5. Get support. A dietitian can tailor a plan to you.

Things worth checking with your GP

• Your thyroid dose, including after any brand change.
• Other medicines. Ask your doctor or pharmacist whether any of them can affect weight.
• Menopause. Around menopause, body fat tends to increase and muscle decreases, even when total weight changes little.
• Sleep and mood, which both affect appetite and energy.
• Iron levels, if tiredness is making activity hard. Your GP may check for other causes too.

Don't change your dose to lose weight

The Australian levothyroxine leaflet says it should not be used for weight loss, and that larger doses can cause serious side effects. The American Thyroid Association lists insomnia, palpitations, anxiety, muscle and bone loss and heart rhythm problems among the risks, and notes that any weight lost on extra hormone is usually regained. A six-month trial that changed people's doses found no improvement in weight or body composition.

Gluten-free and elimination diets

Going gluten-free hasn't been shown to make a meaningful difference to weight in Hashimoto's without coeliac disease, and the evidence is very uncertain. Be wary of elimination diets based on food intolerance tests. Our guide to gluten-free for Hashimoto's has more.

Weight-loss medicines and your thyroid tablet

If you start a weight-loss medicine, ask your doctor when to recheck your thyroid levels. Losing weight can change the levothyroxine dose you need, and some of these medicines may affect how tablets are absorbed. A large study found thyroid levels weren't rechecked any more often after people started these medicines, even though weight loss can change dose needs, so it is worth asking.

"Thyroid support" weight products

Be careful with supplements sold to "boost" the thyroid or metabolism. US testing found thyroid hormone in 9 of 10 "thyroid support" products in one study, which can push levels too high when you are already on levothyroxine. Our guide to supplements to avoid with an underactive thyroid explains more.

Where ThyroBase fits

ThyroBase is not a weight-loss product and makes no weight claim. It is a daily morning and evening nutrition powder, a food rather than a medicine, made with plant protein, greens and minerals, with no added iodine. It sits alongside your thyroid medication, never instead of it. Both serves contain minerals, so keep them several hours away from your tablet. If you are working on your weight, it is worth talking through how it fits your day with your dietitian. Here is who ThyroBase is, and is not, designed for.

Once your levels are right, your thyroid isn't standing in the way. Check the dose, then build habits you can keep.

Frequently asked questions

Can you lose weight with Hashimoto's?

Yes. Once thyroid levels are in range on treatment, the American Thyroid Association says the ability to gain or lose weight is the same as for people without thyroid problems. People with Hashimoto's on levothyroxine have lost weight on reduced-energy diets in studies.

Why can't I lose weight with an underactive thyroid?

Start by checking your levels are in range. After that, the thyroid is rarely the main barrier: treatment usually removes only a little retained water, and the rest depends on food, activity, sleep, other medicines and life stage. Your GP and a dietitian can help work out what is playing a part.

How much weight does an underactive thyroid cause?

The American Thyroid Association says about 2.3 to 4.5 kg may be thyroid-related, mostly salt and water rather than fat. In a study of people starting levothyroxine, the median weight change was only about 0.1 kg.

Will increasing my levothyroxine help me lose weight?

No. The Australian leaflet says levothyroxine should not be used for weight loss, and too high a dose can cause heart and bone problems. A trial that changed people's doses found no improvement in weight, and any weight lost on extra hormone is usually regained.

Do weight-loss medicines affect thyroid medication?

They can. Losing weight can change the levothyroxine dose you need, and some of these medicines may affect how tablets are absorbed. If you start one, ask your doctor when to recheck your thyroid levels.

The honest takeaway

With Hashimoto's or an underactive thyroid, your thyroid is usually responsible for only a few kilos, mostly water, and treatment takes care of that. After that, weight loss works the same way as for anyone else. Check your dose, steer clear of products that promise to boost your thyroid, and build eating, movement and sleep habits you can keep, with a dietitian's help if you can.

ThyroBase is a nutritional supplement and is not intended to diagnose, treat, cure, or prevent any disease. It is designed to sit alongside your thyroid medication, not replace it. This article is general information, not medical advice. Never start, stop or change a supplement or prescription medication without speaking to your doctor or pharmacist first.

References

Karmisholt J, Andersen S, Laurberg P. Weight loss after therapy of hypothyroidism is mainly caused by excretion of excess body water associated with myxoedema. Journal of Clinical Endocrinology and Metabolism. 2011;96(1):E99-103.
Lee SY, Braverman LE, Pearce EN. Changes in body weight after treatment of primary hypothyroidism with levothyroxine. Endocrine Practice. 2014;20(11):1122-8.
Samuels MH, Kolobova I, Niederhausen M, et al. Effects of altering levothyroxine dose on energy expenditure and body composition in subjects treated with LT4. Journal of Clinical Endocrinology and Metabolism. 2018;103(11):4163-75.
Samuels MH, Kolobova I, Smeraglio A, et al. Effects of levothyroxine replacement or suppressive therapy on energy expenditure and body composition. Thyroid. 2016;26(3):347-55.
Peterson SJ, McAninch EA, Bianco AC. Is a normal TSH synonymous with "euthyroidism" in levothyroxine monotherapy? Journal of Clinical Endocrinology and Metabolism. 2016;101(12):4964-73.
Chukir T, Yaghmour M, Almutairi T, et al. Weight and body composition outcomes with liraglutide in individuals with well-treated hypothyroidism: a retrospective case-control study. PLoS One. 2025;20(9):e0332091.
Osowiecka K, Myszkowska-Ryciak J. The influence of nutritional intervention in the treatment of Hashimoto's thyroiditis: a systematic review. Nutrients. 2023;15(4):1041.
Araújo EMQ, Coutinho-Lima CRO, Sousa AS, et al. Effects of gluten-free diet in non-celiac Hashimoto's thyroiditis: a systematic review and meta-analysis. Nutrients. 2025;17(21):3437.
Hauge C, Breitschaft A, Hartoft-Nielsen ML, et al. Effect of oral semaglutide on the pharmacokinetics of thyroxine after dosing of levothyroxine and the influence of co-administered tablets on the pharmacokinetics of oral semaglutide in healthy subjects. Expert Opinion on Drug Metabolism and Toxicology. 2021;17(9):1139-48.
Chen Y, Du F, Singh Ospina NM, et al. Patterns of TSH test after GLP-1 RAs initiation in patients on levothyroxine: a trial emulation study. Journal of Clinical Endocrinology and Metabolism. 2026;111(9):2536-47.
Kang GY, Parks JR, Fileta B, et al. Thyroxine and triiodothyronine content in commercially available thyroid health supplements. Thyroid. 2013;23(10):1233-7.
Leidy HJ, Clifton PM, Astrup A, et al. The role of protein in weight loss and maintenance. American Journal of Clinical Nutrition. 2015;101(6):1320S-9S.
Ahmad AM, Serry ZH, Abd Elghaffar HA, et al. Effects of aerobic, resistance, and combined training on thyroid function and quality of life in hypothyroidism: a randomized controlled trial. Complementary Therapies in Clinical Practice. 2023;53:101795.
Taheri S, Lin L, Austin D, et al. Short sleep duration is associated with reduced leptin, elevated ghrelin, and increased body mass index. PLoS Medicine. 2004;1(3):e62.
Greendale GA, Sternfeld B, Huang M, et al. Changes in body composition and weight during the menopause transition. JCI Insight. 2019;4(5):e124865.
Eutroxsig (levothyroxine sodium) Product Information and Consumer Medicine Information. Australian Commission on Safety and Quality in Health Care, Medicine Finder. 2025.
American Thyroid Association. Thyroid and weight. thyroid.org.
Australian Government Department of Health. Physical activity and exercise guidelines for adults. health.gov.au.