Does Levothyroxine Cause Cancer? New Study Explained

Does Levothyroxine Cause Cancer? New Study Explained

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Is it time to start freaking out because you are taking levothyroxine?

Not quite!

As you probably know, levothyroxine is the most commonly prescribed medication used to treat hypothyroidism, and millions of thyroid patients use it every single month. 

Even though it’s probably not the best thyroid medication (and may not even be one that you are taking), the fact that new information suggests it may be harmful should matter to you. 

New research from a retrospective study has shown a link between cancer risk and the use of levothyroxine in thyroid patients. 

In this study, patients taking levothyroxine were found to have about a 50% increased risk of cancer compared to healthy controls. 

research study suggests that levothyroxine use imparts a 50% risk of cancer at any site

Even though this may sound terrible, there’s more to the story. 

Today we are going to dive into the details of this study and discuss what it means for you as a thyroid patient. 

Today, you will learn:

  • More about the details of this study and what types of cancers were seen in patients using levothyroxine. 
  • Why this type of study is not ideal for assessing true cancer risk among thyroid patients. 
  • What type of variables were not tested for and why we need to have this information in order to make definitive statements. 
  • Other possible drivers of cancer risk in thyroid patients. 
  • How this information applies to other thyroid medications. 
  • More about what the results of this study mean for thyroid patients going forward. 

Let’s dive in…


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Levothyroxine May Increase Cancer Risk by 50%, Study Shows

So, does levothyroxine cause cancer?

The answer to this question is more complicated than you might think which is why we need to spend some time discussing the results of this study and their impacts on thyroid patients. 

But before we discuss their impact, let’s first discuss what the study showed. 

You can find the full link to the study here (1), by the way, if you want to read it yourself. 

original article outlining cancer risk on levothyroxine users

Researchers from Taiwan gathered data from over 600,000 thyroid patients (and 2,400,000 million controls) to assess whether levothyroxine use was associated with an increased risk of cancer. 

the control group had 2.4 million patients and the case group had 600,000 thyroid patients

These researchers used a database called Taiwan’s Health and Welfare Data Science Center (2) database which is a massive database of health information among the residents of Taiwan. 

This database holds information about medical diagnoses, history of medication use, laboratory and diagnostic tests, dates of visits, lengths of hospitalizations, and much more. 

This database is constantly being updated so what the researchers did is they looked backward in time by selecting certain criteria. 

This allowed them to specifically look at thyroid patients who were taking levothyroxine for a defined period of time and compare their risk of cancer to healthy cancer-free controls. 

This is what is called a retrospective study which means that it uses existing data and compares one group of people to another. 

This isn’t the best form of study but it is a good way to put your foot in the door and to show possible connections. 

Once you show these connections you can then use further studies to confirm or deny your suspicions. 

So even though this study is certainly concerning, it doesn’t necessarily prove anything. 

When comparing healthy controls to thyroid patients using levothyroxine they found that levothyroxine users had an increased risk of the following cancers:

levothyroxine use and risk of various types of cancers
  • Brain cancer
  • Skin cancer
  • Pancreatic cancer
  • Female breast cancers

The data also showed that levothyroxine users had a reduced incidence of the following cancers:

  • Esophageal cancer
  • Cervical cancer

The total data showed that levothyroxine users had about a 50% increased higher risk of cancer at any site. 

This is certainly concerning information but it’s not damning information. 

So if you are a thyroid patient taking levothyroxine (or any other thyroid medication) sit tight so we can break down why this isn’t necessarily a problem and some lessons to learn from this study. 

Some Reasons Why You Don’t Necessarily Need to Worry

It may be tempting to brush this data aside, put your head down and keep taking your thyroid medication, but given the fact that it included hundreds of thousands of patients, it’s a good idea to address it head-on. 

But as you will soon find out, there is some nuance to this information that probably renders it less impactful than you might think. 

#1. Retrospective Studies Aren’t Perfect

The first has to do with the study itself. 

This study is what is referred to as a retrospective study (3) which means it’s not the ideal way to truly assess cancer risk in a patient population. 

disadvantages and flaws of restrospective studies

Retrospective studies take existing data and try to draw conclusions based on what happened in the past. 

As a result, this study has a few issues that should be addressed in future studies: 

  • The issue of data keeping and laboratory testing – The results are reliant upon data-keeping from those who took the data originally. This can be a problem because the information that you have to work with may not be all of the information you need. If you knew, for instance, that you were specifically assessing for cancer among patients taking levothyroxine you would be very concerned not only with the fact that patients were taking levothyroxine but how that levothyroxine dose was impacting other factors such as TSH, free T4, free T3 levels, pituitary hormones, cortisol levels, and so on. It may be the case, for instance, that those included in this study were severely underdosed compared to populations in the United States. The researchers note that the lack of information on levothyroxine dose influence is a big problem. 
  • The issue of confounding variables – Cancer is a very complex disease process (4) and one that we are still trying to understand. Because the data these researchers are looking at already existed, it’s difficult to control for factors that can greatly increase the risk of cancer such as family history (5), history of smoking/drinking, etc. The researchers had a lot of information to work with but they didn’t have all of the information that they would have liked. For instance, a known family history of cancer may have helped exclude certain people taking levothyroxine because of that confounding variable. Aside from the family history of cancer, the researchers weren’t able to take into account BMI (6), smoking status, or alcohol consumption (7) all of which certainly have some impact on cancer risk. It may be the case that thyroid patients are more likely to be overweight which accounts for their increased risk of cancer independent of their thyroid medication. 
  • The issue of the data source – Next and probably one of the more obvious issues is that of the data source. All of the information came from residents of Taiwan which means that it’s difficult to take that information and apply it to broad or diverse populations such as what is seen in the United States. It is well known, for instance, that certain ethnicities are more prone to developing certain cancers (8) while other ethnicities seem to have protection against certain types of cancers. In addition, location certainly can drive cancer risk given environmental factors (9) that may be present in some areas of the world and not others. It could very well be the case that levothyroxine use increases cancer in thyroid patients in Taiwan but not in the United States, for instance. 
  • The issue of limiting information on levothyroxine dosing – Lastly, the researchers were not able to assess whether thyroid patients were undertreated or overtreated with levothyroxine. All they know is that certain patients met the criteria for taking levothyroxine over a 3-year period and that they were included in the study. What they don’t know is how that dose of levothyroxine impacted their thyroid function. Were they taking too much levothyroxineToo little levothyroxine? We don’t know. It could certainly be the case that the actual risk of cancer thyroid medication has little to do with the medication itself and everything to do with the dose. 
additional studies outlining the connection between levothyroxine use and cancer risk

#2. More Research is Still Needed

While it’s good to have this sort of information, it really just opens the door to further research. 

Research on any given topic is complex and something that evolves over time. 

The type of information we are seeing here is really just saying “hey, there’s something that we need to look at”. 

What it’s not saying is “hey, levothyroxine causes cancer and all thyroid patients should stop taking it”. 

And this distinction is important. 

It’s sometimes tempting to look at information and try to draw definitive conclusions but this isn’t a study where we can do that. 

We can say that the information here is concerning but it needs to be taken in context with what is happening with thyroid patients around the globe. 

Right now we know that thyroid patients are dealing with a lot of issues, especially surrounding the issue of thyroid medication management. 

If you take one quick look at the research on the topic you will see that thyroid patients continue to suffer from a variety of medical conditions and thyroid symptoms despite taking their thyroid medication faithfully. 

join 80000 thyroid patients

If you were to take an objective look at that information by itself you would clearly be able to see there is a problem with thyroid medication management. 

In my mind, this study really just confirms that reality. 

We will talk more about it below but there’s a good chance that the results we are seeing here have more to do with how thyroid patients are being dosed than the impact that thyroid medication has on cancer risk. 

For instance, we know that thyroid function is necessary for regulating important cellular processes such as enzyme production (10), mitochondrial energy production, and immune function. 

If you are not taking enough levothyroxine then these important processes may not be functioning the way they should. 

We also know that a significant number of thyroid patients are unnecessarily taking thyroid medication when they don’t need it

If you are one of these patients then you might be causing more harm than good as your body tries to grapple with extra hormones that it doesn’t need. 

It seems more plausible that being underdosed or overdosed has more of an influence on cancer risk than the type of thyroid medication being used. 

But we simply don’t know for sure without more targeted research to help answer these important questions. 

More research drives more clarity and will also open the door to questions we didn’t think to ask originally. 

In other words, the topic is more complex than it appears and we don’t even know how complex it is without these future studies. 

#3. We Don’t Have Information on Other Thyroid Medications

It makes sense that researchers would use levothyroxine as their criteria for inclusion in this study but you should be aware that there are many other medications aside from levothyroxine! 

And each of these medications differs in the type and dose of thyroid hormone that they include. 

If you’ve spent any time on my blog (and probably many other thyroid blogs out there) then you’ll know that many thyroid patients prefer thyroid medications that contain a combination of T4 and T3. 

There are really only a handful of thyroid medications that contain T4 and T3 and these medications are prescribed in only about 10-15% of patients (estimated). 

The rest of the low thyroid patients out there are taking levothyroxine.

This is important because what type of thyroid medication you are taking does have some impact on how you view the results of this study. 

For instance, if you are someone taking a form of Natural Desiccated Thyroid like NP thyroid or Armour thyroid, does the information still apply to you?

What if you are taking a combination of T4 and T3 in the form of Tirosint and Cytomel

We just don’t have the answers. 

It makes sense that thyroid patients using these types of thyroid medications shouldn’t be lumped into the same group as those taking levothyroxine but we don’t have the data to say for sure. 

If you are someone using NDT formulations, T4 + T3 combos, or some other medication aside from levothyroxine, stay tuned for some information that may help drive your decision-making going forward. 

#4. Levothyroxine is Bio-Identical

Finally, it’s worth pointing out that levothyroxine is bio-identical to the thyroid hormone that your body produces naturally. 

The active ingredient found in levothyroxine is called thyroxine which is the same compound your thyroid gland produces when it is healthy and functioning normally. 

Yes, levothyroxine is considered synthetic in the sense that it is manufactured in a lab but the end result is still a bio-identical hormone. 

This is worth pointing out because it’s not like levothyroxine is a Frankenstein-like hormone like the hormones found in birth control pills. 

When researchers in the early 2000s found a link between estrogen and breast cancer (11), the medication in question was not bio-identical. 

premarin is not a bioidentical estrogen

And it’s been long suggested that bio-identical hormones are much safer compared to the altered but similar hormones produced by some pharmaceutical companies. 

After all, your body knows exactly what to do with thyroxine because you’ve been producing it since before you were born!

And even before birth, your body was able to utilize it because your mother supplied the thyroid hormone you needed before you could supply your own (12). 

does levothyroxine cause cancer? not excactly

Just because something is bio-identical doesn’t mean that it is automatically safe, though, and this comes back to the issue of dosing. 

When we look at something like insulin (which is also bio-identical), we see that excessively high doses of insulin may certainly increase cancer risk in those who take more than they need. 

The same goes for estrogen as estradiol. 

Excessively high doses of estradiol may increase your risk of uterine cancer even though it is bio-identical. 

This is all worth discussing because it brings us back to the idea that cancer risk with levothyroxine use may be more associated with dose than the medication itself. 

Lessons & Takeaways From This Study

Even though this study does have some flaws and shouldn’t be taken as gospel truth, it’s almost certainly going to have some impact on thyroid patients and thyroid doctors. 

Some will be good and some will be bad. 

Let’s talk about some takeaways, some lessons, and what you might expect going forward. 

#1. Don’t Take Thyroid Medication Unless You Need it

One major takeaway is the idea that it’s probably not a good idea to take levothyroxine (or any other thyroid medication) unless you absolutely need to be on it. 

Thyroid patients are often surprised when they hear that a pretty significant percentage of thyroid patients are taking thyroid medication when they don’t need to. 

Some studies estimate this number to be as high as 30%. 

Even if we assume that this information does not apply to you 100%, it’s still a good idea to spend some time figuring out if you fit into that 30% that doesn’t need to be on thyroid medication. 

The best-case scenario is that you realize you don’t need to be on it and the worst-case scenario is that you continue taking it while learning some valuable information along the way. 

As some of you may know, my wife has used T3 thyroid medication (without T4) to treat depression and metabolic issues and she was recently able to completely get off of her medication after being on it for years. 

I was convinced that she would need to take it for life but here we are. 

Even if you are someone that believes you need to be on thyroid medication for life it’s worth a discussion with your physician to see if a trial of tapering off makes sense. 

This doesn’t apply to those without a thyroid or those who are post-RAI but many other thyroid patients may be able to try. 

You can learn more about how to do that here

Caution: do not try to wean yourself off of your thyroid medication without physician supervision!

#2. It May Be More Difficult to Get On Thyroid Medication/Levothyroxine

This could be a good or a bad thing depending on how you look at it. 

The trend among doctors has definitely been to restrict thyroid medication to a select population of patients. 

In some sense, this is okay because you really don’t want someone to take thyroid medication unless they need it. 

And there have been several studies that suggest that taking thyroid medication in the subclinical state is probably not ideal and may result in more harm than good (13). 

thyroid treatment recommendations based on TSH levels

On the opposite side, there are some patients with conditions like early-stage Hashimoto’s who see significant benefits to early treatment from thyroid medication

I suspect that as this information becomes more widespread that it may push some doctors into being more selective in who they prescribe levothyroxine to. 

This will probably mean that it will be more difficult for patients with early-stage Hashimoto’s and early-stage hypothyroidism to get started on thyroid medication. 

It probably won’t have a huge impact on those already taking thyroid medication except for in select cases such as those with low thyroid symptoms who were started on low doses of thyroid medication as a trial.  

#3. Optimizing Your Dose is Probably More Important Than What Type of Medication You Take

There will be plenty of people who read this study who may feel trapped because they know they need to take thyroid medication but at the same time don’t want to deal with an increased risk of cancer. 

People in this situation may find more success in focusing less on what type of thyroid medication they are taking and more on optimizing their thyroid medication dose. 

What does this mean? 

It means focusing on taking thyroid medications that help you do a couple of things: 

Thyroid medications like levothyroxine contain only T4 which leaves many thyroid patients with adequate free T4 but less than optimal free T3. 

And when it comes to symptomatic management, T3 levels reign supreme. 

If you are someone taking levothyroxine but still experiencing low thyroid symptoms then make sure to test your free t3 and free t4 levels. 

These free thyroid hormone levels can give you insight into how your body is processing your thyroid medication. 

If you have adequate free T4 but low free T3 or total T3 then you can make the assumption that your body is not converting T4 into the active T3 thyroid hormone. 

Many factors influence your T3 levels and one of those is your thyroid medication. 

You can easily increase T3 levels by taking T3 thyroid medication in the form of Cytomel, liothyronineSR T3, or NDT. 

It’s my current belief that the actual risk of cancer from levothyroxine use stems from being underdosed or overdosed with the wrong type of thyroid medication. 

Using combination thyroid medications like T4 and T3 together more closely mimics what the healthy thyroid gland does in those without thyroid disease. 

Combining this strategy with the ones listed below should give you some peace of mind as it relates to your personal cancer risk. 

#4. There are Still Things You Can do to Decrease Your Personal Cancer Risk

Sometimes it’s difficult to look at information like this and take it all in. 

While we’ve focused specifically on levothyroxine and its potential impact on your personal cancer risk, it’s definitely not the only factor that influences that risk. 

There are still plenty of other factors that influence your personal cancer risk and many of those are within your control. 

You can’t control what thyroid medication your doctor gives you and you may not able to control your medical diagnoses or family history, but you can control things like how often you exercise and what foods you eat. 

And when it comes to cancer risk, these things matter. 

Weight gain, for instance, is associated with an increased risk of breast cancer (14), colorectal cancer (15), endometrial cancer, gallbladder cancer, kidney cancer, liver cancer, lung cancer, and the list goes on. 

obesity increases the risk of developing estrogen receptor positive breast cancer

Because the foods that you eat and how much you exercise are linked to your BMI and weight, these are things that you can control. 

Instead of allowing this information to stress you out, use it to help you make changes in your life that can help you become healthier and (hopefully) more resistant to cancer going forward. 

Two things that you can do right away include exercising regularly and eating a healthy whole-food diet. 

And as it turns out, I have free resources which can help you do both!

You can learn more about healthy diets to eat if you have hypothyroidism here and how exercise can naturally help improve thyroid function here

#5. You May be Able to Use This Study to Change Your Thyroid Medication

It’s not all bad news because the results of this study may actually make it easier to switch from levothyroxine to other forms of thyroid medications. 

Here’s how:

There are many thyroid patients taking levothyroxine still experiencing low thyroid symptoms who have been actively trying to get their doctor to change their thyroid medication for months or even years. 

They are often met with resistance by their doctor for a variety of reasons. 

If you were to bring this study up to your doctor with legitimate concerns over your own personal risk of cancer then that might be enough to persuade them to switch you to another thyroid medication. 

This option might work best for thyroid patients such as those who are post thyroidectomy and/or post RAI as they don’t have the same option as other thyroid patients to try and wean themselves off of thyroid medication. 

If that’s the case then the next best option may be to try and get on a combination of T4 and T3 thyroid medication to ensure optimization of thyroid function. 

Wrapping it Up

There definitely seems to be some connection between thyroid patients taking levothyroxine and their risk of developing certain cancers. 

That risk is out fully fleshed out, though, and it will require more research to completely figure out what is going on. 

I’m not entirely convinced the cancer risk seen in some patients is 100% associated with the use of levothyroxine but rather the impact that levothyroxine has on the body (for better or worse). 

If you are concerned about your own personal cancer risk then there are things that you can do right away to potentially help reduce that risk. 

Now I want to hear from you:

Are you currently taking levothyroxine?

Were you aware of the connection between levothyroxine and cancer?

Are you thinking about switching thyroid medications?

Are you planning on making any changes to your lifestyle? 

Leave your questions or comments below! 

Scientific References
















the link between levothyroxine use and cancer risk

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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 7 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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