Losing weight with an underactive thyroid: what to realistically expect

Losing weight with an underactive thyroid goes slower, but it does go. A slow thyroid lowers your resting metabolic rate, so you gain more easily on the same eating pattern. The weight you put on is usually limited, often a few kilos, and a good part of it is fluid rather than fat.

Tip articles rarely say that last part out loud. They promise the weight falls off the moment your values are right. It does not, and that promise only makes the disappointment afterwards worse.

I would rather be straight about it.

Why is it harder to lose weight with an underactive thyroid?

Thyroid hormone helps set how fast your cells use energy. With an underactive thyroid your resting metabolic rate falls, so you burn fewer calories doing nothing. Add fluid retention, less inclination to move and sometimes loss of muscle mass. Those four reinforce each other.

The effect is real, but smaller than people assume. The drop in resting metabolic rate is a portion of your daily burn, not half of it.

Something else is at play too. If you are exhausted for months, you move less without noticing. Not a skipped gym session, but thousands of steps a day fewer. That adds up.

How much weight do you gain from an underactive thyroid?

Weight gain with an underactive thyroid stays within a few kilos for most women. It is rarely tens of kilos, and a substantial part is fluid held in tissue rather than stored fat. That explains why some women drop a little quickly after correction and then stall.

So that first quick fall is not fat loss. It is your body letting go of fluid.

I think that is the most important sentence in this article. Read those first kilos as fat loss and you assume the rest will follow, then conclude something is wrong with your body.

Will the weight come off by itself once your values are right?

Partly. With corrected values your resting metabolic rate moves back towards normal and you usually lose the fluid you were holding. The fat you built up in the meantime does not disappear with it. For that, the same rules apply as for anyone.

That last row is the important one. Thyroid hormone is not a weight-loss drug, and a dose above what you need brings palpitations, poor sleep and, over time, risks for heart and bone. What is known about that is in levothyroxine side effects.

Why can I not lose weight when my TSH is normal?

This is the most asked question on the subject, and the answer is usually that your thyroid is not the brake. One study in women looked at whether TSH variation within the normal range tracked with resting metabolic rate and with symptoms. The association turned out to be limited (Harsini et al., 2024).

Put differently: inside the normal range, your TSH usually does not explain your weight.

So the question becomes what does. In women, these four come up most:

• Low ferritin, which drags down your energy and your movement
• Perimenopause, which shifts fat distribution towards the abdomen
• Insulin resistance, which often runs unnoticed for years
• Sleep loss, which disturbs appetite regulation

For the menopause part we wrote weight loss in menopause and hormones and weight loss. For the metabolic part, insulin resistance in women is the starting point.

Make it concrete: your TSH is 1.9 mIU/L, your free T4 sits mid-range, and you have been at it for eight months with nothing to show. That conversation is probably not about your thyroid.

How can you lose weight with an underactive thyroid?

What works, works the same as for everyone: enough protein, enough fibre, strength training to keep muscle mass, and patience with the pace. The difference sits mainly in expectation, and in avoiding very strict diets, because those cost the muscle you want to keep.

Strength training is the most underrated part here. Muscle is the tissue that keeps your resting metabolic rate up, and it is exactly what you lose with a slow thyroid and with crash diets.

The Dutch Voedingscentrum works with a pace of roughly half a kilo a week for healthy weight loss. With an underactive thyroid you can sit comfortably below that without anything being wrong.

What I would say to women here: do not only measure your weight. If you strength train for eight weeks and the scale holds while your clothes get looser, something has changed.

Does Hashimoto itself affect your weight?

Hashimoto affects your weight mainly through thyroid function, not separately from it. As long as your TSH and free T4 are normal, there is no sign that the antibodies themselves steer your weight. The course of Hashimoto can produce a shifting picture though, with a short fast phase at the start.

In that phase women may actually lose weight, then gain it back as the thyroid slows. That swing is confusing, and it often gets blamed on the diet rather than the condition.

The background is in our article on Hashimoto. On food we wrote separately in the Hashimoto diet, where we grade each claim against what sits underneath it.

What to discuss with your GP

Take your weight history to that conversation, not just your current weight. When did it start, how fast did it move, and did it coincide with your symptoms or with a change in your cycle. Bring your last TSH and free T4 if you have them.

Ask as well whether ferritin, vitamin B12, vitamin D and your fasting glucose are known. In women those are the values that most often explain something that gets blamed on the thyroid.

If you want a starting point of your own, our Thyroid Complete looks at TSH, free T4, free T3 and anti-TPO, and the Tiredness panel at several causes of low energy at once. On hair loss in this same picture we wrote thyroid and hair loss.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.

References

• Harsini AR, et al. Are resting metabolic rate and clinical symptoms affected by variation of serum thyroid stimulating hormone levels within the normal range in healthy and women with hypothyroidism? A case-control study. Clinical Nutrition ESPEN, 2024. PMID 38777475
• Zijlstra LE, et al. Levothyroxine Treatment and Cardiovascular Outcomes in Older People With Subclinical Hypothyroidism: Pooled Individual Results of Two Randomised Controlled Trials. Frontiers in Endocrinology, 2021. PMID 34093444
• Ralli M, et al. Hashimoto's thyroiditis: An update on pathogenic mechanisms, diagnostic protocols, therapeutic strategies, and potential malignant transformation. Autoimmunity Reviews, 2020. PMID 32805423
• Voedingscentrum, information on healthy weight loss
• NHG guidance on thyroid conditions, Dutch College of General Practitioners