How Methimazole Leads to 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:

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 that results in excessive thyroid hormone production in the body.

This thyroid hormone, released from the thyroid gland, causes the 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 with humans, but they can still act as useful tools 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. 

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.

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!