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Having started my career writing academic papers and studies for students in the US, Australia, and Canada, I naturally approach this subject with the thoroughness it deserves.
You’re eating less than before and staying active, yet the scale won't budge. Or worse, it’s slowly creeping up. If you have Hashimoto’s, you’ve likely heard, "It’s just your thyroid," followed by, "Just take your medication and you’ll be fine." Both statements are partially true, which is exactly why they’re so confusing.
The link between Hashimoto’s and weight gain is real, but it’s more nuanced than most people think. Here is what science says about how hypothyroidism slows down your metabolism, how much weight is actually "due to the thyroid," why the scale often stays stuck even when your medication is optimized, what actually helps, and my own personal experience with it.
Hashimoto’s thyroiditis is an autoimmune condition where the immune system attacks thyroid tissue. Over time, the gland produces fewer hormones, leading to hypothyroidism, or an underactive thyroid.
Hashimoto’s is the most common cause of hypothyroidism in both the US and the UK, and women are up to ten times more likely to be affected than men (NIDDK; NHS; Cleveland Clinic).
T4 and T3 hormones act as the body’s "thermostat." They determine your basal metabolic rate—the energy your body burns at rest—as well as heat production, fat breakdown, and how your muscles and liver utilize glucose.
A review published in the journal Physiological Reviews describes thyroid hormones as one of the primary regulators of energy expenditure in almost every tissue (Mullur, Liu, and Brent, 2014).
When there is a lack of hormones, the body burns fewer calories at rest. It is estimated that about 30% of total energy expenditure depends on thyroid hormones.
The authors of a review on the link between obesity and the thyroid point out that even a small change, of 5 to 10% of the energy you burn at rest, is enough to accumulate several kilograms over the years (Walczak and Sieminska, 2021). That is why the link between Hashimoto's and a slowed metabolism is real, it just acts slowly and silently.
This is the part that surprises many. According to the American Thyroid Association (ATA), in most patients with hypothyroidism, about 2 to 4.5 kilograms (5 to 10 pounds) can be attributed to the thyroid itself, depending on how pronounced the hypothyroidism is (American Thyroid Association). The Cleveland Clinic also lists "mild weight gain" among the symptoms of Hashimoto's, rather than a drastic jump in weight.
The same source explains that a large part of that excess is retained salt and water, not adipose tissue. That is good news: when hormone levels return to normal, that part of the weight is usually the first to go. If your weight gain with Hashimoto's is much greater than a few kilograms, other factors are almost certainly at play, which I write about below.
It is not just the thyroid that affects weight. Research shows that obesity itself can slightly raise TSH, partly through the hormone leptin secreted by adipose tissue, and through chronic low-grade inflammation (Walczak and Sieminska, 2021).
A review by Elizabeth Pearce concludes that weight gain in hypothyroidism is generally moderate and that thyroid hormones are not recommended as a weight-loss aid for people whose thyroid function is normal (Pearce, 2012).
The most common frustration I hear goes something like this: "My test results are good, but I still can't lose weight." Here is what the research says about that.
At the ATA congress, a study was presented that followed patients with newly diagnosed hypothyroidism after the introduction of levothyroxine. Just over half (52%) lost weight in the first two years, but the average loss was modest, about 3.8 kilograms (Lee, Braverman, and Pearce, ATA). Therefore, therapy removes the "brake" caused by hypothyroidism, but it does not melt fat tissue on its own.
Good news comes from a recent study published in the journal Scientific Reports. Researchers compared 69 obese women with hypothyroidism on levothyroxine therapy and 69 women of the same age and weight with a healthy thyroid gland. With diet, exercise, and medical supervision, both groups lost almost the same percentage of weight over about 19 months, around 5% (Gokkaya and Aydin, 2024). In other words, when hypothyroidism is well-regulated, your body can lose weight just as well as the body of a woman without Hashimoto's.
Many women hope that a higher dose will "speed up" their metabolism. A randomized, double-blind study of 138 people taking levothyroxine tested exactly that: doses were adjusted so that TSH was low-normal, high-normal, or slightly elevated. After six months, there was no significant difference in energy expenditure or body composition, so the authors do not recommend changing the dose for weight loss (Samuels et al., 2018). The dose is always determined by an endocrinologist, based on test results.
With Hashimoto's, pounds are often gained indirectly rather than directly. Chronic fatigue means less movement throughout the day, poor sleep increases hunger and cravings for sweets, and the feeling that "nothing is working" easily leads to cycles of strict dieting and overeating. None of this is your failure, but it is something you can influence.
Before you start any weight loss plan, it is worth making sure the foundation is in place.
If hypothyroidism is untreated or undertreated, the metabolism is indeed slower and weight loss is more difficult. Regular check-ups and dose adjustments are the first step, and that is the part handled by a doctor, not a diet (NHS).
Levothyroxine should be taken on an empty stomach, as certain foods and supplements can reduce its absorption (NIDDK). If you take your tablet with coffee, breakfast, or calcium and iron supplements, your lab results may fluctuate even if your dosage hasn't changed. Talk to your doctor about how and when to take your medication.
Foods and supplements high in iodine, such as seaweed, can trigger or worsen hypothyroidism in people with Hashimoto's (NIDDK). The same applies to products that promise to "wake up your thyroid," which often contain unknown amounts of hormones or iodine. Consult your doctor before taking any supplements.
I always blamed gluten for my rapid weight gain. It never occurred to me to check my thyroid hormones, not even when I was losing weight.
For two years, I tried everything. I was a strict vegan for seven months. I lost weight, but I also lost muscle, I was constantly bloated, I had the worst acne of my life, and the inflammation in my body remained.
The turning point came when I started eating with intention. Protein and healthy fats became my priority, along with whole foods and as little sugar and processed carbohydrates as possible. The more consistently I ate this way, the calmer I felt, the more energy I had, and the less body fat I carried.
With four workouts a week, I felt unstoppable. Then, at the beginning of 2025, everything collapsed at once. It was the most stressful period of my life, accompanied by exhaustion, a constant feeling of cold, digestive issues, and acne.
When I received my diagnosis, the doctor first asked me, "Have you gained weight?" That was the only symptom I didn't have, precisely because I had been watching my diet for a long time.
At 163 cm, I went from 75 to 57 kilograms, and today I weigh 60. It wasn't just my weight that changed. My body composition, skin, mood, and my relationship with food all transformed.
Hashimoto's didn't stop me from achieving a dream I'd had since high school. In May 2026, I won a silver medal in bikini fitness at the Balkan Cup in Kragujevac. The preparation was difficult, mostly due to the fatigue that comes with thyroid issues.
However, within six months, the inflammation in my body had completely disappeared. You don't need a stage. You need to wake up feeling rested and have energy for your day.
From my own experience, I also know this: chronic starvation doesn't pay off with Hashimoto's. What truly delivers results is consistency based on the 80/20 principle, not extremes. But for me, it was important to prove that even that was possible.
Once your medication is adjusted, the rules are the same as for everyone else, just with more patience and less room for extremes, but with some necessary sacrifices.
Drastic diets further slow down your metabolism, increase fatigue, and almost always end in regaining the weight. A moderate deficit, combined with meals that keep you full, yields less spectacular but lasting results. If you need a practical framework for your meals, check out my guide Hashimoto's diet: what to eat and what to avoid.
Hashimoto's is an autoimmune disease, and excess body fat itself maintains chronic low-grade inflammation (Walczak and Sieminska, 2021). That is why it makes sense to reduce the foods that trigger this inflammation the most: ultra-processed foods, added sugar and sweetened beverages, white flour products, processed meats, trans fats, and alcohol.
A review article published in the journal Nutrients recommends an anti-inflammatory diet for people with Hashimoto's, primarily based on plant-based foods and rich in omega-3 fatty acids, polyphenols, and antioxidants (Szczuko et al., 2022). The upside is that these are usually the most calorie-dense foods, so when you cut them out, a deficit comes much more easily and without starvation.
Gluten is a specific issue. A 2023 meta-analysis showed that a gluten-free diet lasting about six months slightly reduced antibodies and improved TSH, especially in people who also have a gluten-related disorder.
However, the authors emphasize that the evidence is not sufficient to recommend this diet to everyone who does not have celiac disease (Piticchio et al., 2023). That is why the order is important: first get tested for celiac disease, then make a decision with your doctor, and if you start an elimination diet, have a plan for reintroducing foods.
I wrote more about how to structure your meals in my guide on the Hashimoto's diet.
No supplement will cause weight loss on its own. However, deficiencies in certain nutrients can increase fatigue and make daily movement more difficult, so it is worth checking them and addressing them in a targeted manner rather than randomly.
Selenium is the most well-researched. A meta-analysis of randomized studies published in the journal Thyroid showed that selenium supplementation is associated with slightly lower TSH and lower TPO antibodies, mainly in individuals who had a selenium deficiency (Huwiler et al., 2024). However, a review of 75 randomized studies rates the certainty of this evidence as low and does not recommend routine use, but rather supplementation only in cases of proven deficiency (Li et al., 2023).
In addition to selenium, it is worth checking your vitamin D, ferritin (iron stores), and vitamin B12 levels with your doctor, as deficiencies are common and directly impact energy levels. Two things are important: take calcium and iron supplements several hours after your levothyroxine, as they reduce its absorption, and do not take iodine on your own (NIDDK).
A more detailed overview of forms and dosages can be found in the article Thyroid supplements: where to start.
Muscles are metabolically active tissue, and the more you preserve while losing weight, the higher your resting energy expenditure will be. Protein in every meal and two to three strength training sessions per week are the simplest ways to combat a slowed metabolism, regardless of whether you have hypothyroidism.
Walking, taking the stairs, and moving throughout the day often burn more energy than a workout itself. Additionally, regular sleep and reduced stress help keep hunger and energy levels more stable, which is especially important with Hashimoto's, as fatigue is a common symptom anyway.
Since hypothyroidism affects water retention, your weight may fluctuate from day to day. Weigh yourself at the same time, track your weekly average, and monitor your waist circumference rather than focusing on a single number.
If you have symptoms of hypothyroidism and haven't had your thyroid checked yet, or if your weight is changing rapidly despite treatment, consult your doctor.
This text is for informational purposes only and does not replace professional medical advice from an endocrinologist.
Sources
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