How Methimazole May Cause Weight Gain & What to do About it

Methimazole, the antithyroid blocking medication used to treat hyperthyroidism, can lead to weight gain in certain individuals.

But how does this work?

How can you gain weight if you have hyperthyroidism?

The answer has to do with how Methimazole works in the body and how it blocks thyroid function.

Learn more about Methimazole, how it can cause weight gain, and how to lose weight if you have hyperthyroidism here:

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What is Methimazole & How does it Work?

In order to understand how Methimazole leads to weight gain (and yes, it can cause weight gain) we need to understand how it works in the body.

Methimazole, as you probably already know if you are taking it, is a medication that is designed to treat hyperthyroidism.

Hyperthyroidism is a medical condition which results in excessive thyroid hormone production in the body.

This thyroid hormone, released from the thyroid gland, causes activation of thyroid hormone receptors in the body.

This activation ramps up the metabolism (1) (usually making patients lose weight), increases heart rate (causing palpitations), increased adrenaline release (making you feel jittery and anxious) and leads to diarrhea (through activation of the GI tract).

If you have hyperthyroidism then you probably understand all of these symptoms and have experienced them.

So where does Methimazole fit in?

Methimazole acts to block the effects of hyperthyroidism by inhibiting the PRODUCTION and ACTIVATION of thyroid hormone in your body (2).

It does this by blocking an enzyme known as thyroid peroxidase (3) which is responsible for producing thyroid hormone.

It also blocks the peripheral activation of T4 to T3 in your tissues reducing the amount of circulating and active T3 thyroid hormone.

The net result?

Decreased thyroid hormone production and activation in your body by blocking thyroid function.

This is all well and good but you have to realize a few very important points:

#1. Your thyroid, when working properly, helps manage your weight.

#2. Methimazole is dose-dependent and acts differently in each person.

#3. Over-blocking thyroid production may lead to a state of HYPOthyroidism

These three factors are important in understanding how Methimazole can lead to weight gain in certain people.

It has also been shown, in aminal studies, that long term use of Methimazole may cause weight gain and other harmful side effects to the body (4).

Animal studies do not always cross over perfectly to humans, but they can still act as a useful tool that we can learn from. 

These negative side effects, seen in animal studies, may be mediated through the anti-thyroid action of Methimazole.  

Like anything in life, we need to try and achieve a balance, and if the balance is tipped in favor of hypothyroidism over hyperthyroidism, then there may be negative consequences. 

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Does Methimazole Cause Weight Gain?

So what does this mean for you?

It means that it is entirely possible that taking Methimazole may block the normal thyroid function in the body and lead to weight gain.

And this should actually make perfect sense once you understand the physiology.

The treatment of hyperthyroidism is to BLOCK thyroid hormone.

But blocking thyroid production, in a perfect way, is easier said than done.

We know from studies such as this, that it is NOT uncommon for patients to gain around 5kg or 12 pounds when they start Methimazole (5).

This study showed that among 42 patients with Graves' disease that the average weight gain was around 12 pounds after starting Methimazole. 

Doctors know that this often occurs, but the question is why?

Your body, naturally, has many different feedback loops internal and external factors which regulate thyroid hormone activation and production very closely.

This is happening ALL the time in your body whether you realize it or not.

So does it make sense that you can put someone on a thyroid blocker like Methimazole and expect to perfectly balance thyroid hormone in the body?

Not really, at least not for every person.

There's also another very important point to consider:

Doctors would prefer patients to be HYPOthyroid over being HYPERthyroid due to the side effects of hyperthyroidism.

It's also widely believed that treating and managing hypothyroidism is easier when compared to hyperthyroidism.

For these reasons, it's not surprising that some patients may experience weight gain after taking Methimazole because their dose is either too high or they are sensitive to the medication.

So does Methimazole lead to weight gain?

It absolutely can, and it does this by slowing down your metabolism and reducing the amount of energy that the body produces at baseline.

It's well known that weight gain is a negative side effect of hypothyroidism and it is something that I've discussed in detail here.

It's also well known that having low T3 (a side effect of taking Methimazole) may lead to weight gain as well.

Now, some of these side effects, while unintended, may actually be necessary.

For instance:

We don't want to have excessive thyroid hormone production in the body causing long-term issues like heart problems (6) (atrial fibrillation or cardiac enlargement) or excessive bone loss leading to osteoporosis (7).

But, on the other hand, we also want to balance thyroid hormone to prevent weight gain, depression and cholesterol issues (all side effects and symptoms of hypothyroidism or an indication that your body is swinging from hyperthyroidism to hypothyroidism).

This balance may be difficult to achieve with Methimazole, but it is always worth attempting.

Sometimes it may be possible to simply reduce your dosage which will allow some thyroid hormone activation and production in your body.

A small change in your dose may be sufficient to restore some thyroid function (without causing the symptoms of hyperthyroidism) which can help manage your weight.

As a reminder:

Never alter your dose without consulting with your physician first! You may cause more harm than good if you adjust your medication in this way. 

How Hyperthyroidism Can Paradoxically Lead to Weight Gain

Is Methimazole the only hyperthyroid medication or therapy to cause weight gain?

Not at all, in fact, most hyperthyroid therapies and medications will eventually lead to weight gain.

It has been shown in studies that all treatments for hyperthyroidism result in "marked" weight gain (8).

This weight gain is worse if you go into hyperthyroid treatment already being overweight or if you have Graves' disease. 

The average weight gain in these patients ranges from around 10-15 pounds or an increase of 8.49 in BMI (a huge increase!). 

Other studies have shown that after hyperthyroidism treatment patients have a 32% increased risk of obesity and most of this weight gain occurs in the first 2 years after therapy (9).

So we know that this is not an isolated phenomenon, but how can it be? 

How can you actually gain weight if you have hyperthyroidism, isn't hyperthyroidism supposed to cause weight loss?

While the condition of hyperthyroidism does lead to weight loss, this is only true when it is not being treated.

Once you start treatment (whatever that may be) the goal of that therapy or medication is to block thyroid function.

Effectively this makes you HYPOthyroid (at least in most cases).

This can obviously be confusing for patients, but it doesn't have to be.

Having an understanding of how it works in your body will help you determine how to treat it.

Are there different degrees of thyroid suppression?

The answer is yes.

For instance:

If you have had your thyroid removed or if you underwent radioactive iodine ablation then you will have a more difficult time losing weight when compared to someone who is taking Methimazole or PTU.

The reason for this is that Methimazole only blocks a portion of thyroid function in the body.

T3 conversion booster results

While having your thyroid removed completely eliminates all thyroid function in the body and makes the person without a thyroid completely reliant upon thyroid medication for life.

Because of the way that thyroid function is naturally regulated by the body, it is much more difficult to try and "normalize" thyroid function with thyroid hormone replacement medication. 

Remember that your thyroid produces thyroid hormone constantly throughout the day. 

And the amount of hormone that each tissue needs is delivered based on that need. 

It's impossible to completely replicate this innate system in the body by taking thyroid hormone medication (though that shouldn't stop us from trying). 

But just because these therapies and medications may make weight loss difficult, it doesn't mean that losing weight is impossible.

How to Lose Weight with Hyperthyroidism

Is there a way to lose weight if you are taking Methimazole or other thyroid blocking medications?

The answer is yes, but it may be more difficult.

You have to consider that you will be at a disadvantage trying to lose weight if your thyroid is being blocked.

Blocking your thyroid will result in some suppression of normal metabolic function.

What do I mean?

Basically, if your thyroid is blocked sufficiently then you may be burning fewer calories at rest when compared to normal healthy adults.

This means that general exercise and dieting is usually not sufficient to effect significant weight loss in the person.

So what are you supposed to do?

It depends on each situation, but the goal should be to focus on your options.

If you are post-thyroidectomy (meaning you don't have a thyroid) then it's best to look at your current dose of thyroid medication and to optimize that dose to get your free T3 levels in a high enough range (you can read more here).

The same is true if you are status post radioactive iodine ablation which is another method of destroying the thyroid gland to treat hyperthyroidism.

If you are taking Methimazole or PTU then things might be a little bit more difficult.

These medications act to block the thyroid but they are highly dose-dependent.

So if you are on these medications you will need to take a close look at your current dose to determine if you are taking too much or to see if you can reduce your dose.

Reducing your dose will allow more thyroid hormone to function which may increase your metabolism.

But it will be a balance of trying not to block too much thyroid and not blocking enough so it can be difficult.

It may be tempting to opt for a therapy such as having your thyroid removed or getting radioactive iodine ablation to treat your hyperthyroidism but these options are not necessarily better.

Instead, you may want to focus on therapies to try and improve what little thyroid function is left in your body with techniques such as these.

You may also find benefit in trying to reduce inflammation if your hyperthyroidism is caused by Graves' disease (which is an autoimmune disease) with some strategies such as these

Just realize that attempting to lose weight with hyperthyroidism can be very difficult.

Wrapping it up

Methimazole can absolutely cause weight gain and it does this by blocking thyroid function and activation in the tissues in your body.

Over-blocking thyroid hormone may actually take you from hyperthyroidism to hypothyroidism which may then trigger symptoms such as hair loss, weight gain, fatigue, constipation and so on.

It may be possible to manage these symptoms by taking a close look at your dose and altering your dose if that makes sense.

You should look at your dose in conjunction with your current physician and your lab tests.

When optimizing thyroid function make sure you shoot for the optimal lab tests which you can find here.

Now I want to hear from you:

Are you suffering from weight gain while taking Methimazole?

Have you been able to lose weight despite it being difficult?

What strategies worked for you?

Share your thoughts and comments below!

References (Click to Expand)

Dr. Westin Childs

Dr. Westin Childs is a Doctor of Osteopathic Medicine. He provides well-researched actionable information about hormone-related disorders and formulates supplements to treat these disorders.He is trained in Internal Medicine, Functional Medicine, and Integrative Medicine. His focus is on managing thyroid disorders, weight loss resistance, and other sex hormone imbalances.You can read more about his own personal journey here.

38 thoughts on “How Methimazole May Cause Weight Gain & What to do About it”

  1. I was diagnosis-ed with both Graves disease and Hashimoto’s. I am currently on 10mgs of methimazole my dr keeps switching every month from 10mg – 20mg. This has been pretty miserable I no longer have the symptoms of heart palpitations and got rid a small amount of muscle weakness, as well as my eyes, quit swelling. But I gained 50 pounds in six months, massive hair loss, and terrible fatigue. My dr still either says my labs are normal or my labs or bad meaning that I went hyper again. This is an extremely frustrating process but I do not want to do RAI which seems to be my only other option. I would love there to be a third option my quality of life sucks. I’d love to find some strategies to feel better on methimazole and lose weight but I have yet to find any!

    • Hi Brandi,

      The third option would be to try alternative therapies such as diet, supplements, etc. to try and see if your hyperthyroidism is responsive. If so, then you may be able to avoid RAI or thyroidectomy.

  2. I have been on methimazole for 14 months and have gained 21 pounds since. I eat plenty of fruit, restrict sugars, don’t drink soda, etc and the weight just keeps piling on. I was diagnosed with Graves disease, and don’t know how to keep the weight off, short of starving myself and exercising to exhaustion.

    • Hi Courtney,

      The next step may be to evaluate your current dose of methimazole to see if it is suppressing your thyroid function more than necessary.

  3. I went on Methimazole in January of 2017. Diagnosed with Graves at 39 years old. Fifteen months later and a total weight gain of 50 pounds and I’m completely miserable. My labs have been getting significantly better since November and a number of negative side effects have come and gone. Except for this devastating weight gain. I have constant fatigue and am very depressed. I’ve done everything my Endocrinologist suggests. It’s just all so hopeless.

    • Hi Beth,

      It can definitely be difficult. Your thyroid controls up to 60% of your metabolism so even slight changes to it can alter your weight very quickly.

  4. I gained 30# after diagnosis of Graves and subsequent Methimazole therapy. I believe the weight gain was the result of inactivity because of feeling tired and achy, overconsumption related to the Hyper appetite and quitting smoking which was simultaneous. I insisted on only 5 mg low dose which may have delayed my initial progress and my numbers did improve some but I still had symptoms, after an increase to 10 mg I am symptom-free for the most part. For four weeks I applied my new energy to daily walks and moderate weight lifting, I began a ketogenic diet. I have lost ten pounds and my energy is improved. I believe the keto diet and exercise has helped my symptoms and that a calorie-reduced low carb diet will be effective even with thyroid problems. One aspect I should mention is that I consume my calories in a six or eight-hour window. Good luck.

    • Hi Susann,

      I would be careful with calorie restriction, especially with thyroid disease. Calorie restriction always works at first but it’s only temporary. The weight will always come back within 12 months or so and then the calorie restriction won’t work at that point.

  5. I have been on methimazole since 2012 after being diagnosed with Graves Disease. I gained 12 pounds instantly in 1 month. I have stayed the same weight since then despite exercising and diet. Nothing will change the weight. Is there anything you can recommend to assist in weight loss? I would only like to lose 15 pounds or so.

    • Hi Natalie,

      Your best bet is to try alternative therapies such as supplements, dietary changes, stress management and so on. Alternatively, you can also try altering your dose of methimazole under physician supervision.

  6. I was diagnosed with Grave’s almost 3 years ago when I was 18, and have been on and off of methimazole since then. The past 5 months, I have gained more than 20 pounds, although I am young, healthy, and very active. I have been really frustrated and upset, unable to find any possible explanation- except for maybe this. I am on higher doses of methimazole than ever after the lower doses failed to work, and I am also considering radioactive treatment though I am concerned for the effect this will have on my weight as well. I know getting my thyroid in check is more important than my weight, but as a young woman the huge weight gain has been incredibly upsetting and I am glad to see others have experienced this as well, as sometimes my friends and family think I am making it up.

    • Hi Trina,

      You are certainly not alone as almost everyone who undergoes thyroid treatment for hyperthyroidism gains weight. Thanks for sharing your story and sorry to hear about your struggles!

  7. I was diagnosed as hyperthyroid in October of 2017. I have maintained weight within 5 pounds my entire almost 60 years of life, other than my 3 pregnancies. I began Methimazole in November 2017. By February 2018, I was up 12 pounds. I completely altered my diet to clean eating and began working out 3-5 days each week. The pounds kept multiplying on.

    At my last blood draw, come to find out I am now hypothyroid. My Dr. told me to stop the medication.

    It’s been two weeks since quitting Methimazole. I have further restricted my diet and added weights to my exercise regimen. I am still gaining weight, even without the medication.

    I might add that my doctor stated there is zero correlation between Methimazole and weight gain. This is physiologically incorrect!

    • Hi Georgene,

      As you suggest, it would be a silly to say that something which blocks thyroid hormone function doesn’t lead to weight gain, considering your thyroid helps control the majority of your metabolism.

  8. Good morning,

    I stumbled across your page while looking for answers. I was diagnosed with hyperthyroidism in September of 2017 and was immediately placed on methimazole 5 mg (2 in morning and 1 in evening). By Dec, the doc was very impressed with my numbers – said that if it continued to be this way in 6 months, we would talk about reducing dosage. Fast forward to March, I started working out and watching what I eat (Paleo/Ketogenic). I started JuicePlus supplements in November 2017 (also D3 at the time of diagnoses per orders); and last week, I started taking calcium, magnesium and a turmeric supplement (for my bad knees). The working out is mostly bodyweight training, but I also added walking 2-4 miles daily. After 6 weeks of this, I still have not lost weight BUT I do feel exceptional (actually since I refuse to step on the scale I don’t know if I lost any weight but I thought clothes would be looser than they are now)! Also, I neglected to add that doc also prescribed propanol (10 mg) for as needed for the palpitations caused by hyperthyroidism. I stopped taking this when I ran out the end of April. Now I am seeing my heart rate rise again. Mostly in the 80s but not near as bad as when I found out (it was in the 100-110s). So – could the keto diet be aggravating my now underactive thyroid (due to meds)? Or is it because I quit the Propanolol? I am so confused. My next appt is June 5th. Sorry so long. 🙂

  9. I was diagnosed with Graves almost six years ago. My weight dropped from 220 to 140. I was happy to lose the weight as several chronic pain illnesses keep me from being able to exercise. Well, I decided that the benefit of the weight loss was greater than the benefit of the medication, so I’ve been at ridiculous levels since diagnosis. Because of my choice, I now have thyrotoxic myopathy. I don’t know yet how advanced it is, but after six years, my guess would be advanced enough. At this point, all I can do is hope and pray that the methimazole will regulate my thyroxine enough to halt the myopathy. I have young children, and because I was worried more about my weight than my actual health, I may not survive my choice. Please, please take your medication. It’s not worth the consequences if you don’t.

  10. I was diagnosed with hyperthyroidism with nodules on the right side of my thyroid in 2014. At that time, the doctor did not put me on any meds. I had some tests to see how my iodine levels were and they were normal. Fast forward to January of 2018. After a routine exam and blood work, which showed that I was still hyperthyroid, my doctor ordered an ultrasound and a follow-up with Endocrinologist. Ultrasound showed nodules on right and left with small nodules throughout. He put me on 10mgs of Methimazole. I do not want to take drugs if I don’t have to, but the heart palps and anxiety caused me to try. I decided not to take the full dose, because the doctor warned I could end up with hypothyroidism. Instead I cut the pill in half. I’ve been taking this dose since June 5, 2018. I felt so much better, but I am fatigued, especially in the middle of the day. I have gained some weight, but nothing drastic yet. I have been trying to do it naturally with supplements since 2014. I would love to find an alternative to taking Methimazole.

  11. Seems there is no way out of some weight gain on methimazole. I was diagnosed with hyper throid over a month ago. I was placed on 10mg Methimazole. The symptoms were overbearing with hair loss, palpitations and arm joint pains making it unbearable to sleep. I just starting to feel better and my long hair started showing some life back, I hope that I won’t experience any weight gain as I started reducing my doses on my own after my last blood test indicate T4 free returning to normal after 3 weeks but my T-SH is still low

    Do you think this was a wise decision? My next appointment is a month away.

    • Hi Donnette,

      I wouldn’t recommend making any changes to your methimazole dose without supervision or approval from your Doctor. You don’t want to risk putting yourself back into thyrotoxicosis.

  12. Hi! I was diagnosed with Grave’s disease in April this year and I went into thyroid storm short time after that in May. Since then I had been put on Methimazole daily. It feels terrible. I am constantly gaining weight, I started with 60 mg per day, now I am taking 5 mg, but doctors insist on increasing the dose again as my thyroid is not stable. But I cannot lose weight! I cut my daily food intake almost in half and still no luck! Do I need to starve myself and reject food at all to lose weight and become myself again?:(

  13. Looking for some answers-
    Graves disease and put on Methimazole 10mg a day.

    I have gained 12 lbs and CANNOT loose it. I workout 5 days a week and have changed my diet to be very clean 90 percent of the time.

    Any advice? Im only 33 and working out without seeing the scale change is very frustrating.
    Thanks 🙂

  14. I’ve just recently begun taking 5mg of Methimazole for Hyperthriodidms. After basically suffering for a long time, waking from my sleep in a complete state of Afib, joint pains, anxiousness. You name it. My concern is that I’m also in a medically supervised weight loss program designed to help me lose weight for an upcoming surgery. My diet and exercise are strict, Very, STRICT. So I must know, is there anything that I can do to ensure that I continue to lose weight while being treated with this medication?

  15. I was diagnosed with Graves in November 2017 and I started taking anti thyroid medication Thyrozol. I started to gain weight after 6 months on the medication. Luckily, as of end of December 2018 my hormone levels were normal with minimal dosage and my doctor took me off the medication. So far my hormone levels have stayed in the normal range. But it has been very difficult to shed off the extra pounds. I have been patient but sometimes feeling hopeless. Is it normal for it to take this long ? I have seen a bit improvement but would like to see more being 6 months off the meds. I understand that with antithyroid medication it slows down the metabolism, but what effect does coming off the medication have?

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