Methimazole Side Effects You Should Know

Methimazole Side Effects You Should Be Aware Of (What to Expect)

Methimazole: What You Need To Know

Are you taking methimazole for hyperthyroidism?

If so, there are some things you should know about this medication. 

For starters, how about the fact that it can have some serious consequences on your liver?

Or how about the fact that it can directly lead to weight gain?

These are just some of the potential side effects of methimazole. 

In this article, you will learn…

  • How methimazole works to blunt thyroid function
  • How this negatively impacts thyroid function and may lead to low thyroid symptoms
  • The side effects of lowering your thyroid and what that means for you
  • The mild short-term side effects associated with methimazole
  • The long-term more serious side effects associated with methimazole
  • And what other therapies you should do while taking methimazole

Let’s jump in: 

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Methimazole Helps Slow Down Thyroid Function

As someone with hyperthyroidism, you probably already have some idea of how methimazole works. 

Methimazole is what is known as an anti-thyroid medication. 

Its entire goal is to SLOW down thyroid function and to take you from the hyperthyroid state to the “normal” thyroid state. 

We call the normal thyroid state euthyroidism but you don’t necessarily have to know that for the purposes of this article. 

But what you should know is that it’s incredibly difficult for your doctor to take you from the hyperthyroid state to the normal thyroid state with methimazole. 

Instead, most hyperthyroid patients who take methimazole end up in a LOW thyroid state (known as hypothyroidism). 

The reason is simple:

It’s actually quite difficult to balance your hormones, especially your thyroid, with the use of a medication that you take by mouth once per day. 

Methimazole is like a blunt object when you really need a sharp scalpel. 

Despite this, doctors still use and recommend methimazole because it’s often thought that the low thyroid state (hypothyroidism) is safer than the high thyroid state (hyperthyroidism). 

Because of this, methimazole is often the drug of choice to treat pretty much any hyperthyroid condition (1). 

And there are a lot of people taking it. 

If you are one of them, you need to be aware of the potential side effects and consequences of this medication…

Side Effects + What To Look Out For

The side effects associated with methimazole are not as clear-cut as you might think. 

Because methimazole BLOCKS thyroid function you can expect it to have consequences that impact your thyroid. 

These consequences are in addition to any symptoms you may experience directly from the medication itself. 

Below I’ve split up the types of symptoms you may experience from methimazole into 3 different groups. 

The first group is symptoms you may experience from slowing down your thyroid. 

The second group is mild symptoms you may experience secondary to the medication.

And the third group is more serious symptoms you may experience if you use methimazole long-term. 

#1. Methimazole May cause Low Thyroid Symptoms

Remember:

The whole purpose of using methimazole is to slow down your thyroid to take you out of the hyperthyroid state. 

The problem here is that slowing down thyroid function isn’t a perfect science so it’s easy to slow down thyroid function too much. 

If you slow down thyroid function too much you may swing from a hyperthyroid state to a hypothyroid state and experience any of these symptoms:

These are all symptoms of having a LOW thyroid. 

It may seem weird that someone with hyperthyroidism can experience hypothyroid symptoms but believe me when I tell you that it happens all of the time. 

Do not confuse THESE side effects/symptoms with the side effects of the medication itself. 

They come from two very different reasons. 

#2. Mild Side Effects Directly from Methimazole

Like any prescription medication, methimazole can also directly cause side effects. 

These side effects are separate from the symptoms you may feel as you slow down your thyroid. 

Side effects of taking Methimazole may include any or all of the following:

  • Upset stomach
  • Nausea/vomiting
  • Rashes or itching of the skin
  • Headaches
  • Drowsiness
  • Dizziness
  • Muscle or joint pain
  • Swelling
  • Hair loss

These side effects tend to be on the more mild side and are often related to your dose. 

If you are experiencing any of these side effects then you can talk to your doctor about making changes to your dose. 

#3. More Serious Side Effects from Methimazole

What you may not realize is that methimazole is NOT intended to be used long-term. 

In fact, there are some serious side effects that may come about if you try to use methimazole long-term. 

These side effects come from organ damage. 

More serious side effects may include:

  • Liver damage
  • Yellow eyes or skin
  • Severe abdominal pain/stomach pain
  • Very dark-colored urine
  • Nausea/vomiting that doesn’t stop
  • Reduced urinary output (not urinating as much as you used to)

If you experience ANY of the side effects then you need to talk to your doctor immediately. 

Methimazole & Weight Gain Explained

Another side effect that patients often experience when starting methimazole is weight gain. 

How can you gain weight if you are hyperthyroid?

Shouldn’t hyperthyroid patients LOSE weight?

Well, yes, but also no. 

Your thyroid is one of the main things in charge of maintaining your weight. 

If your thyroid is in overdrive then your metabolism will be increased and you will lose weight (a side effect of hyperthyroidism). 

Conversely, if your thyroid is slowed down then your metabolism will be decreased and you will GAIN weight (a side effect of hypothyroidism). 

You can probably already see where I am going with this. 

What does methimazole do to your thyroid?

It SLOWS it down. 

And if you slow down your thyroid then you will necessarily slow down your metabolism in the process. 

So it is VERY possible that you can have hyperthyroidism but be treated with methimazole AND gain weight. 

In fact, it’s actually quite common. 

The mechanism isn’t that complicated either, it has everything to do with your thyroid. 

Unfortunately, it can be very difficult to try and lose weight while you are taking methimazole. 

Why?

Because you are fighting against a medication that is limiting your ability to succeed. 

It’s like trying to run a race with a crutch. 

It’s not impossible to lose weight while taking methimazole but it is very difficult. 

If you are trying to lose weight then your best bet is to try and get off of methimazole. 

Methimazole Shouldn’t Be Used Long-Term

If you are taking methimazole then you should be aware that this medication is NOT intended to be used long-term.

The usual recommended duration of therapy is somewhere around 18 to 24 months. 

And the reason is simple:

As much as one-half of patients with hyperthyroidism will experience a relapse in their condition and no longer need it. 

In this way, methimazole is really just used as a way to blunt the serious side effects of hyperthyroidism while giving your body time to fix whatever caused the hyperthyroid state to begin with. 

Recently, some doctors have been recommending that hyperthyroid patients take methimazole for a longer duration of time (2). 

This strategy requires a much smaller dose of methimazole but patients may have to take it for 60 to 120 months (3) (5 to 10 years). 

I’m not a fan of this approach and I will provide some reasons why it may not be the best option below. 

While methimazole is definitely effective at stopping the hyperthyroid state and reducing the symptoms associated with hyperthyroidism, long-term methimazole use may cause some serious consequences. 

It turns out that your body needs thyroid hormone (surprise, surprise) and that reducing it below normal levels can have consequences on many organ systems in your body. 

Long-term methimazole use may result in potential liver damage (4), changes to your bone structure (5), and more. 

It’s not a good idea to use methimazole unless absolutely necessary which brings me to my next point. 

What Else Can you Do?

If you aren’t a fan of the potential side effects associated with methimazole use then what are you supposed to do? 

Fortunately, there are a number of lifestyle changes that you can make which may have a positive impact on your thyroid. 

But let’s go back to the goal of methimazole for a minute to help you understand. 

To do this we need to talk about why doctors recommend methimazole. 

The entire goal when using methimazole is to slow down your thyroid function and allow enough time to pass for your body to heal. 

Using this strategy anywhere from 15 to 50% of patients may experience a relapse in their hyperthyroid symptoms and no longer need any therapy to treat their hyperthyroidism. 

Sounds pretty good, right?

It does. 

common side effects of methimazole

But what if there was a way to increase the chance that your body goes into remission so that you do not need to take methimazole for anywhere from 2 to 10 years (depending on which approach you use)?

That’s where natural therapies come in. 

Making changes to your diet, taking dietary supplements, managing your stress, ensuring that you get enough sleep, and so on, may increase your odds of going into remission. 

They also may allow you to get off of methimazole sooner rather than later (which would be ideal for most people). 

And these natural therapies are incredibly safe and they are things that you are able to do on your own. 

You don’t need permission from your doctor to eat more fruits and vegetables, avoid grains such as gluten, and make sure you sleep 8 hours per day. 

These types of therapies are often overlooked by conventional doctors even though they are highly effective. 

There are many therapies you can use depending on the type of hyperthyroidism that you have (Graves’ disease versus other conditions). 

Final Thoughts

Methimazole is an anti-thyroid drug that is designed specifically to slow down thyroid function. 

While it is great at managing the symptoms of hyperthyroidism (and consequences of this disease) it is not without side effects itself. 

Side effects from methimazole tend to come from its effects on thyroid function or directly from the medication itself. 

While methimazole can be effective it is not intended to be used long-term. 

Even while taking methimazole you can take advantage of natural therapies such as improving your diet, avoiding gluten, reducing your stress, and taking supplements to help put your hyperthyroidism into remission. 

These therapies are very safe and can be very effective. 

Now I want to hear from you:

Are you currently taking methimazole?

If so, what type of side effects are you experiencing? 

Do you think your side effects are coming from reducing thyroid function or directly from methimazole?

Have you tried using any natural therapies with methimazole? If so, which ones?

Leave your questions or comments below! 

#1. https://pubmed.ncbi.nlm.nih.gov/16584027/

#2. https://www.mdedge.com/clinicianreviews/article/150048/pituitary-thyroid-adrenal-disorders/long-term-methimazole-therapy

#3. https://pubmed.ncbi.nlm.nih.gov/31310160/

#4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4197171/

#5. https://pubmed.ncbi.nlm.nih.gov/28759749/

methimazole side effects you should know

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About Dr. Westin Childs

Hey! I'm Westin Childs D.O. (former Osteopathic Physician). I don't practice medicine anymore and instead specialize in helping people like YOU who have thyroid problems, hormone imbalances, and weight loss resistance. I love to write and share what I've learned over the years. I also happen to formulate the best supplements on the market (well, at least in my opinion!) and I'm proud to say that over 80,000+ people have used them over the last 6 years. You can read more about my own personal health journey and why I am so passionate about what I do.

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32 thoughts on “Methimazole Side Effects You Should Be Aware Of (What to Expect)”

  1. What supplements out there can help place Hyperthyroidism in remission? I was taking 10mg Methimazole daily for a year before being weaned off gradually. I am now on 5mg 5mg 3X a week. My symptoms have minimized and I have not had any of the serious side effects. Any information about supplements and any foods would be appreciated

    Thanks.

    Reply
    • Hello. I have recently moved to FL from Denver and was seeing a nurse practitioner that specialized in thyroid treatment. She practiced natural medicine. Since my move, I am going to a PA who while isn’t clinical, she isn’t natural either. She has me on 5 mg of methimazole daily. Anyway, I hate it. I have been fasting for healthy reasons and can’t lose a pound. I don’t sleep well and she also put me on estradiol and progesterone that made my stomach bloat.

      AST went from 18 to 41
      ALT went from 18 to 58
      And absolute eosinophil went from 2472 to 0
      I also have glumerular nephritis and my creatinine went from .90
      up to 1.36
      TSH-.01 to .03
      T3free 5.7 to 2.2
      T3 reverse 35 to 16

      The liver numbers are the most concerning. I have a multi modular goiter that has been biopsied and is benign. Do I hate the goiter in the middle of neck? Yes, it is quarter sized but am willing to put up with it instead of getting my thyroid out.

      Thoughts?

      Thank you,
      Suzanne Ankenbauer

      Reply
    • i have been on methimazole for two years. i have gained weight in spite or going to the gym and watching what i eat. i never had a weight problem. for the last six months, i have been experiencing joining pains. mainly when i stand up. walking and other forms of exercise do not bother me.. the pain is mainly in my lower back hips and knees, i am also on simvastatin 20 mg which also can cause joint pain.

      Reply
      • Hi Louise,

        Symptoms related to methimazole are almost always related to their impact on thyroid function. Methimazole reduces thyroid function and low thyroid CAUSES both joint pain as well as weight gain. So you are in a hypothyroid state right now from your methimazole.

        Reply
  2. I was diagnosed with Graves when I was 20. I have been on methimazole for the better part of 18 years with the exception of a few months before and during pregnancy.. My mother, her eldest sister and youngest brother all have graves, others in the family suffer from other autoimmune diseases. I have had several endocrinologists that tell me I’m in the normal range and that is the extent of their therapy. I still have a laundry list of symptoms/side effects. I have over the years changed my diet for the better (plant based for the most part) and supplement with vitD with K2, and probiotics. I’m sick and tired of being sick and tired.. They do not do not deal with it surgically in Canada unless it is cancerous and I’m concerned about radiation and being on the other end of the spectrum. I would love suggestions, articles, anything! There isn’t as much about hyperthyroid as there is about hypo…

    Reply
    • Hi Angelique,

      Many of the same treatments for Hashimoto’s apply to Graves’, so that would be the first place to start. The root causes of Graves’ are almost identical as outlined here: https://www.restartmed.com/hashimotos-vs-graves/

      Most practitioners are aware of this, so you just won’t find a ton of information on Graves’ over Hashimoto’s due to this fact and also that Hashimoto’s is more common. In addition, most cases of Graves’ end up with hypothyroidism due to thyroidectomy and/or RAI.

      Reply
  3. I was diagnosed with hyper thyroid in August of 2020 and have been taking methimazole 30mg per day up until January of 2021.
    I started thinking, how is my body going to adjust and balance out if I keep taking this medicine? I started having some of the side effects you mentioned above and wasn’t sure if these symptoms were from the drug or from the ailment itself. (very confusing.) My Dr. was useless. Said he never heard of such a thing as having symptoms of the ailment itself while on the medication! So I skipped 2-3 days of medication a few times and noticed I felt better not taking it than when I took it. Lowered dose to 20 mg. now taking 10mg and feel better. I plan to still have blood test done to check where the T3 and T4 levels are at.
    It makes me really question the competence these doctors. Are they working for the people, or selling as many drugs as possible, while getting paid to do so by the pharmaceutical companies! I am also taking supplements and herbal medicine to help with this condition.
    Thank you so much for this helpful information.

    Reply
    • Hi Deborah,

      Glad you found it helpful! Most doctors just prescribe the way they were taught to prescribe in residency/medical school and don’t think twice. Much of this I had to learn on my own after I got out.

      Reply
  4. Good morning Dr. Westin,

    I was diagnosed with Graves’ disease about 6 years ago and I went into remission a year after treatment with Methimazole and Atenelol. Last summer I had a mild case of Bells Palsy and it “reignited” the Graves Disease. I was immediately put back on 5mg of Methimazole and Atenelol. Here it is 8 months later and I have gained over 15 lbs, I constantly feel fatigued and swollen, and my eyes are greatly impacted by Graves. I’m being weaned off of Atenelol and my levels have normalized, but the doctor said I have at least another 6 months of Methimazole. I want to stop taking it and get my body and energy back now. I ordered your Thyroid daily essentials and I am starting to cut out gluten and dairy and increase my exercise. I am 47 years old and willing to do anything on the natural side to restore my Thyroid health. Am I on the right track? Should I do (take) anything else and/or eliminate things in my diet? Should I take other supplements?

    Thank you!

    Reply
  5. Hi,I am a 79 year old female, have been taking Methimazole for approximately 3 years, the dr started me off with 10mg, dropped to 5mg after a year, was feeling Great, last month went for my check up and blood work, he upped it back to 10mg 7 days a week, before it was 5mg for 5 days and skip two days, was feeling great, I have been having Constipation didn’t realize the meds is the culprit. Don’t sleep well at night.
    Went last week for another visit and lab work, after taking it for 1 month I started having acid indigestion, had never had this before, I am so afraid this will ruin my health! I don’t know what to do, very upset about it all’

    Reply
  6. Dr. Childs,
    I was diagnosed with Graves disease a year ago, my thyroid levels came up fairly qucikly and my endocriniologist weaned me down to 5mg of methimazole every other day. I feel lighheaded/dizzy and am very senstive to light. I started having ocular migraines about a month ago. Opthomologist says I am fine. Do you think my symptoms are from the meds?

    Reply
    • Hi Lois,

      It’s hard to say for sure. If they appeared around the time you changed your medication then they most likely are related.

      Reply
  7. Dr Childs,
    I was diagnosed with Graves’ disease in 2015. After a year of treatment with high dose of methimazole and levothyroxine (but my thyroid levels were never stable) I had to stop taking the meds and withing two months Graves came back. since 2016 I am on methimazole, started with 10 mg and am now on 3×2,5 mg and 4x 1,25 mg a week. I had a negative count of antibodies last July and I stayed on the lowest dose possible (from July to November on 1,25 daily, splitted) But in November I had a relaps of Graves again, but I could handle it with a low dose of methimazole. (My TSH is about 1 and FT4 is in range). I was reading about L-carnitine and I was wondering if I could replace my low dose of methimazole with L-carnitine and go on a diet without gluten or dairy. Just wondering what your thoughts were on this. I don’t want to have rai or a thryoidectomy, because my thyroid function is just fine.
    Another issue: I would love to translate some of your advice into Dutch, as in the Netherlands it is difficult to get good advice about Graves’ disease.

    Reply
  8. I am 77 years old. In 2019, I was diagnosed with Rocky Mt. Spotted Fever, Epstein Barr and pneumonia. During this period they discovered tumors on my thyroid. They were biopsied; not cancerous. (I also have AFIB, kidney failure). I was put on 2.5 milligrams of methimazole. In October 2020, I began having unusual symptoms…swelling and rash on my lower legs and constant chronic diarrhea. I believed the symptoms came from the methimazole. The diarrhea has been diagnosed as lymphatic colitis, a condition I had previously many years ago associated with another medication. Generally, my thyroid numbers are “within the range.” I stopped the methimazole on my own after a thyroid surgeon in a well known national clinic explained the side effects. I am changing thyroid doctors but can not see the new doctor until November. The previous doctor is pretty much unresponsive. Looking for help and suggestions. The Primary Doctor said to remain off the medication until I can see the specialist in November. I am coming out of my skin. I am looking for suggestions.

    Reply
  9. I have hyperthyroidism due to the large toxic nodule ,
    I have also two smaller non toxic nodules on the other side of the thyroid. I was also diagnosed with Hashimoto.
    Im on methimazole.
    What other options I have to get off methimazole??

    Reply
  10. newly dx with Graves. taking this medication for a couple of months, dosage had to change, Noticed swelling to face and joints are swollen to hands very painful, waking up in early morning with hands on fire and needle like sensation ( like carpel tunnel) . Pain to heels of feet. Looked for side effects on PDR found a diet I will defiantly try

    Reply
  11. I just started Methimazole last week after being diagnosed with Graves and having severe symptoms of weight loss, racing heart and muscle weakness. I’m really hoping to be able to watch my levels and know when I can start weaning off Methimazole before I swing hypo. I am also working with a naturopath and herbal tinctures and trying to watch my diet and eat the things that are recommended! Thank you for your helpful articles!

    Reply
  12. Before I started taking Methimazole, I had beautiful healthy hair. I told my endocrinologist if it would have any effect on my hair (my favorite feature) then I didn’t want it. I thought he prescribed a medicine for my hyperthyroid that would not affect my hair.
    After taking this drug for a few months, my hair started falling out when I shampooed. All I could do was cry. It this reversable? Will I gain my hair back if I stop using this drug?
    I explained this to my doctor but, he wanted to prescribe it for regrowing my hair back! That is insane. What can I do?

    Reply
    • Hi Baldy,

      Unfortunately, you will probably need to see a new doctor. Methimazole is given to SLOW down thyroid function. Both hyper and hypothyroid states can result in hair loss. It sounds like your doctor believes your hair loss is from hyperthyroidism even though it sounds like it’s more likely related to being overdosed on the methimazole (thus causing hypothyroidism).

      Reply
  13. My numbers started off so low, they hardly registered–mostly my TSH. Dr. said wanted to say I had Graves, but not all the numbers indicated that. Was put on 20mg methimazole and am now on 5mg/day. Have been on it for a total of 1 1/2 years. At last check my T3 & T4 were in normal range, TSH just registering. However, my hair is half of what it was when I started. A year ago, I was able to lose weight. Now, no matter what I do, it is just coming back on. Is this more likely to be from the methimazole vs thyroid? Not supposed to go back to Dr. until June. Thank you.

    Reply
  14. In 1998 I took a med called PTU. Levels returned to normal and stayed that way for 20 years. Current MD would not prescribe PTU and put me on Methimazol. 2 years later I’m still on methimazole. What are your thoughts about PTU. Should I try it again?

    Reply
  15. I am a 77 year old female, diagnosed in 1990 with hyperthyroidism, and in 2021 with Grave’s disease. Sometimes I am hyper and sometimes I am hypothroid. I have been on various dosages of Methimazole from 1990 to 2016 and from 2021 to present. I now take 2.5 mg (half of the 5 tablets) daily. From 2016-21 I was in remission. I have two doctors, a GP, General Practitioner and an Endocrinologist, and I usually have three blood tests a year. My doctors have never suggested any diet or supplements so for years I ate KELP and iodized salt because I heard that iodine was an important factor in thyroid problems. More recently I gave up the iodine and now take Selenium 200 mg and Acetyl L-Carnitine. Unfortunately I don’t have all of my test results. In Feb. 2022 my T4Free was 8.4 and in May 2022 my T4Free was 15.1 My symptoms are that I am extremely cold and tired and take several naps a day. I’ve had several sleep studies and was on CPAP machine for 4 months and it was a HORRIBLE EXPERIENCE that I wouldn’t want to wish on anyone. The endocrinologist says that my tests prove that my fatigue is NOT related to my thyroid problem, and I need to go to the hospital for an overnight sleep study. I am also very cold and can’t lose the weight I’ve gained with my thyroid problem. I’m due for another blood test in 6 weeks and then a telephone appointment with the endocrinologist when She just asks me a list of standard questions.
    Thank you.

    Reply
  16. Hi Dr. Childs,

    I was put on Neo-Mercazole in 2020 and part of 2021 (I can provide dosage and timelines if interested) to treat my Graves’ Disease. I was also put on Panafcourt (steroid) for 4 weeks to kill a virus which was likely causing my Graves’ Disease. I lost about 5kg at the peak of my Graves’ but once my thyroid blood tests results started to show signs of improvement and my Neo-Mercazole dose was reduced I started to gain weight (which was expected). I thought I’d initially only gain the 5kg I lost but I ended up gaining another 5kg on top of that and no matter how much diet or exercise I do, I cannot shift even 0.5kg. I have not taken any Neo-Mercazole since about Jun 2021 when I decided to take myself off it because I kept increasing weight. I do about 1 hour of cardio a day (intense cycling) and have really cleaned up my diet.

    What’s clearly happened here is that the medication I was on has done something to my thyroid and metabolism and I am struggling to get back to my pre-thyroid issue weight. My last blood test in Sep 2022 had my TSH at 1.05mIU/L, Free T4 at 12.0 pmol/L, and my Free T3 at 4.6pmo/L which are all in normal range. In fact all my thyroid blood tests have been normal since about mid-2021 but I seem to have struggles dropping the 5kg to get back to my normal.

    Has the medication damaged my thyroid function and this damage doesn’t show up in any blood tests? Any suggestions on how I can re-fire my metabolic rate so that I can go back to my normal weight? I’m actually on Cytomel at the moment (started at 10mcg for 4 weeks, then 20mcg for 1 week, and now 40mcg for 2 weeks so far) but it’s not made any difference yet. Is this change in my metabolism a direct result of the Neo-Mercazole and if so, will it ever fix itself over time or is the damage done for good?

    Thank you.

    Reply
    • Hi Dimitri,

      There is a small risk that some people who use this medication end up with permanent hypothyroidism. I’ve never seen it before but, apparently, it may occur.

      Reply

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