Is there a link between thyroid medication and thyroid cancer risk? I got a question the other day that inspired me to put this article together. In it, I want to talk a little bit about thyroid hormone treatment and cancer, and thyroid health and cancer. I hope it gives you some clarity.
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Table of Contents
A Question From A Reader
Could Thyroid Medication Cause Cancer?
Are the Results of This Study Real?
How Bad is the Risk Between Thyroid Medication and Cancer?
Understanding Degrees of Hypothyroidism
Who Really Needs Thyroid Medication?
Types of Thyroid Medication and Cancer Risk
What Can You Do About It?
Breaking Down the Studies
Thyroid Medication and Cancer Risk: Next Steps
A Question From A Reader
This is a question I received from Jaci that I want to share with you in full:
“Hi Dr. C. I received an email on research that says levothyroxine causes cancer. Have you addressed this at all? Would love to hear more.”
This is a great question, and a big thank you to Jaci for sending it in. Yes, the study you forwarded did suggest a roughly 50% increase in cancer risk from thyroid medication.
But, your question deserves a better, more in-depth answer than that. I want to spend some time today unpacking the topic and how to think about it.
This is a big deal. Nobody wants to be put on thyroid medication in the first place, but what if it was potentially harmful and yet you have no choice but to take it?
You might find yourself asking yourself some questions including:
- Could thyroid medication cause cancer?
- Are the results of this study real and verifiable?
And, if the results are real:
- How bad is the risk?
- To whom does it apply?
- What can you do about it?
I’ll be sure to cover all of these points. On top of that, I’ll give you the details and the related studies, and close with some action steps you can take today.
If you’re in a rush, you can jump right to the summary! You can find it in the section titled “What Can You Do About It?” near the end of this article.
Based on my own experience, I have spent the past 20 years managing thyroid medications. I’ve started medications for many, and also stopped it for many. I’m neither pro-medication or anti-medication, but I am in favor of the truth. So, let’s get to the bottom of things.
Could Thyroid Medication Cause Cancer?
There have been several researchers and bloggers arguing that there is no way that thyroid hormones could cause cancer (and dismissed the previously mentioned study outright).
The logic is that since thyroid medications contain the same hormones that the body makes, they could not cause cancer. But, let’s first take a quick step back.
Hormones and Cancer
In the 1960s, the same argument was made for oral contraceptives and estrogen compounds. But, these days, we know a bit better.
Nowadays, we know that other hormones like estrogen and progesterone insulin are associated with cancer. And, we know that thyroid hormones can be, too.
In general, we know that hormones regulate cell growth. While many factors can give rise to cancer, abnormal cell growth signals are a part of the picture for many types.
Thyroid Hormones and Cancer
Let’s drill down and talk about thyroid hormones. Thyroid hormones regulate a lot, including:
- Metabolic rate
- Nerve condition
- Cell growth
Every part of the body is made up of distinct cells. Even though we live for somewhere between 70 to 100 years, our individual cells only live from several days to several months. Old cells are always dying and new cells are always taking their place.
Key Insight: Thyroid hormones are among many factors that regulate this process. This is why people have hair loss, dry skin, brittle nails, or ligament problems from hypothyroidism. The cells don’t regrow as quickly as they should.
And, we now know that thyroid hormones can play a role in cancer growth. Exact amounts of thyroid hormone are needed to prevent cancer cells from growing. Which means that too much (or too little) can be a big problem.
When taking medications especially, the amount in the tissues may not be the amount the body intends to have. An elevated TSH can also stimulate thyroid cancer. But, at the moment, there is still more to it than that.
Below are the connections between thyroid hormone and cancer…
Recent data has shown that:
- A high T3 equals a higher risk of breast cancer onset and its aggressiveness1
- A high T4 equals higher risk for any solid cancer, especially lung and breast cancer2
- Thyroid hormone works differently in cancer cells than in healthy cells3
- The amount of thyroid hormones in circulation (blood tests) does not always reflect the amounts in tissues4
- Brain cancer and its metastasis can be driven by too much or too little thyroid hormone5
- Hypothyroidism is associated with an lower risks of cancer including breast and lung cancer6
- Hypothyroidism seems to correlate with an increased risk of colorectal cancer and hepatocellular carcinoma7
Key Insight: Please keep in mind that thyroid hormones are not bad. Your body needs them. But, like so much in life, it’s a question of balance. Too much thyroid hormone raises the risk for many types of cancer. Too little raises the risk for colon and liver cancer, specifically.
Is Only ‘Too High’ or ‘Too Low’ a Problem?
One might think that as long as blood levels are normal, there should be no risk from thyroid medication. The problem is that many people who don’t need to take thyroid hormones are taking them.
Taking something you don’t need can be a problem. You can have too much. This is true even if your blood levels are completely normal.
Are the Results of This Study Real?
The question Jaci asked specifically referenced one study in particular. While we never want to put too much weight into study, it is important to take the results seriously.
And, I do take these results seriously for the following four reasons:
- It is not just a single study. Many others show the same thing.
- It is not a localized result. The risk shows up in different populations around the globe.
- The studies are consistent. Each shows the same level of risk and increases in cancers.
- These same risks show up time and again. Even when controlling for other factors.
Additionally, I truly feel that the risk is real. This is based on other studies that show:
- Thyroid hormones can cause cancer cells to appear and to be more aggressive.
- Those with even mild hyperthyroidism have risks for the same cancer types to similar degrees.
- Thyroid medication use correlates with an increased risk of death from all causes (not only cancer).
The Risk of ‘Total Mortality’
This leads us to ask another question: What do thyroid medications do to the risk for death causes from things other than cancer?
A recent Korean study (2021) compared death risk among those on thyroid medication against those who were not8.
Over two million people were monitored in this massive study. As an important note: the researchers excluded those who have had a cancer diagnosis.
Overall, the results showed a 14% increase in mortality even after factoring in cardiovascular disease and other risk factors. That’s super critical to keep in mind.
How Did Researchers Come to This Conclusion?
To figure this out, researchers compared a large number of people on thyroid medications to a similar group not on medication.
The tricky part became controlling for all of the other things that may cause cancer. While it is impossible to make the groups identical, researchers instead identify the risk differences and factor those into the results (we call this approach ‘odds adjusted’).
Key Insight: Not everyone on thyroid medication gets cancer, it’s not some sort of guarantee. But, more so than would otherwise be expected. The risks are very real.
How Bad is the Risk Between Thyroid Medication and Cancer?
For starters, I want to quickly distinguish between two things:
- Relative risk
- Absolute risk
An absolute risk is how likely something is to happen. The relative risk is how much the baseline for absolute risk has changed.
For example, a relative risk increase of 50% means that those on thyroid medications are 50% more likely to get cancer then they would be otherwise.
What it does not mean is that 50% of people on thyroid medication will get cancer. Let’s use another example to illustrate the point…
Let’s say your odds of winning the lottery are one in 100 million. If you buy two tickets, your relative odds of winning just went up 50%.
That does not mean that 50% of people who buy two lottery tickets win the jackpot. It means their odds of winning go from one in 100 million to one in 50 million.
The absolute odds of winning the lottery if you buy two tickets is one in 50 million. Buying a second ticket gives you a 50% relative chance of winning.
So, let’s talk a bit about how these risks stack up to one another…
How Do We Compare These Risks?
There are many factors that cause cancer, and many of them are unknown. Of the known causes, some are things we can’t possibly or easily change. You can’t change your genes.
The main controllable factors for cancer include:
- Body weight
- Tobacco use
- Alcohol use
The good news is that we think this applies the most to those who are on medication, but do not need to be on medication. And, the bad news is simply that most people who are on thyroid medication simply do not need to be on them.
Key Insight: Thyroid medications are among the top-three most-prescribed drugs world wide. They are prescribed for hypothyroidism. Yet there are different degrees of hypothyroidism. Most who are prescribed thyroid medication are not at an advanced enough degree to need medication.
Understanding Degrees of Hypothyroidism
To get a better sense of who truly needs thyroid medication, I think it would help us to get a firm grasp of the various degrees of hypothyroidism that exist…
Severe Hypothyroidism
When the thyroid gland gives out and our T4 levels drop well below range. By this point, TSH is often high (well over 20). Someone in this stage is low in thyroid hormones.
So, they almost always have symptoms. Diet, supplements, and lifestyle changes can help. And it’s true that sometimes medications can help as well.
Those who had their thyroid removed or destroyed with radiation also have severe hypothyroidism. They need medication and they benefit from it.
Mild and Moderate Hypothyroidism
Over 85% of those on thyroid medication have mild or moderate hypothyroidism. That is a problem because the body still has access to plenty of thyroid hormone.
At this stage, the thyroid has not slowed down. But, the brain is telling the thyroid to work harder. In blood tests, this is a TSH that is above optimal or above normal.
People at this stage:
- Have enough thyroid hormone already
- Make up the majority of those on treatment
- Studies have shown that treatment is harmful to them
People may have symptoms at this stage, but they are usually from autoimmunity (thyroid antibodies) and not from too little thyroid hormone. Thyroid medication does not help autoimmunity. Diet, supplements, and lifestyle changes can help.
Those with mild and moderate hypothyroidism are considered most at risk for cancer from thyroid medications. Although this is not definitive.
Who Really Needs Thyroid Medication?
Those who fall in the following buckets truly ‘need’ thyroid medication. Those who suffer from severe hypothyroidism (a very high TSH, often over 20, and T4 below range). Or, those who have had their thyroid gland removed or destroyed by radio ablation.
There are many who need thyroid medications and who get health benefits from them. Yet, there are plenty of those who can have symptoms from poor thyroid function but do not stand to benefit from thyroid medication (and, instead, simply take on more risk).
Here is a very quick table to help us distinguish the two…
- Mild – Normal labs but levels not optimal; low T3
- Moderate – TSH above range, normal T4
- Severe – TSH above range and T4 below range
- No thyroid – Thyroid gland removed or ablated with radiation
Who May Not Need Thyroid Medication?
Here is a list of reasons people are unnecessarily put on thyroid medication:
- TSH was high normal
- TSH was elevated (but T4 was not well below range)
- T3 was low
- rT3 was high
- Basal body temperature was low
- They had thyroid symptoms like weight gain, fatigue or hair loss
- T3/T4 ratio was off
- rT3/T3 ratio was off
Key Insight: Do any of those apply to you? Sometimes even more than one can apply. While your doctor might mean well, and medication even seemed to help, none of these are reasons to be put on thyroid medication and can bring too much risk.
Blood Levels and Risk
Blood levels do not completely predict cancer risk. If you are on too much medication, your risks are higher. But, the concentration of thyroid hormones in various parts of the body are not always the same as blood levels. Thyroid hormones in a pill override the body’s control.
Types of Thyroid Medication and Cancer Risk
Let’s touch very quickly on the various types of thyroid medication and how they may be related to various kinds of risks…
Does Levothyroxine Cause Cancer Because it is Synthetic?
The quick answer: No. In this context, synthetic means man-made. We define other hormones as natural if they have the same chemical form as your body’s own hormones. By this definition, all thyroid hormones are natural. They are not foreign substances.
What About Natural Thyroid, T4 and T3 or Just T3 Alone?
Most, but not all, participants studied took T4-only medications. None of the studies to date looked for differences between medications. Given what we know about the ways thyroid hormones can stimulate cancer, there is no reason to think these risks would only apply to T4.
To this point, we have clearly established that the risk is real and applies to most people on thyroid medication. Now, let’s talk about what you can do about it.
What Can You Do About It?
We have reached the point where we need to talk about action steps. If you are concerned about the risk between thyroid medication and cancer, I want to give you some solid things you can – starting today – that will help reduce that risk.
Here’s what you can do and keep in mind:
- Never take too much. Even if it makes you feel better, a high dose only ends up masking the real cause of your symptoms.
- Don’t shoot for having high levels of T3 or T4. I know that some doctors push for it, but I find many more reasons to avoid it. Cancer is just one additional risk here.
- Reduce your other risk factors, like: Avoiding nicotine and tobacco, minimizing alcohol use, getting down to a healthy body weight, or enjoying a variety of plants in your diet.
Key Insight: You should consider each of these things, as they can not only decrease your risk for various types of cancer, but can have a positive uplift on your health overall.
The Case for Deprescribing
You should also work with an advisor to see if you need thyroid medications and to try and take as little as possible. Find out if you need it.
But, what does it really mean to deprescribe? Well, it does not mean that you just throw your medications away and call it a day. Deprescribing is a process that is:
- Supervised by a medical professional
- An orderly process with clear steps
- Done over a period of several months
Even when done right, deprescribing can still cause temporary symptoms and temporary fluctuations in blood levels. Ultimately, the goal is to minimize the dose. Some will be able to come off, but any reduction is a win.
In this next step, I want to quickly dissect all of the studies in question. You’re welcome to skim or skip it, but I hope it provides even more helpful context to today’s discussion.
Breaking Down the Studies
There have been five studies that have established the connection between thyroid medications and cancer risk. Below I have a summary of each and its citation.
I’ll take you through the evidence in as much detail as possible. I have even highlighted a few points that stood out about each study, and what it contributed to our knowledge.
Study #1: Taiwan and Overall Cancer
This is the study that Jaci asked us about at the top of today’s article14. It is from Taiwan and was published in 2021. In it, 601,733 patients on thyroid medication were matched against 2.4 million control subjects who were not.
In it, thyroid medication usage correlated with a 50% increased risk of cancer.
This risk showed up after factoring in a long list of other factors that could have changed an individual’s risks. These included:
- Medications known to modify the risk of cancer (Aspirin, statins, ACE inhibitors).
- Prior diseases known to modify the risk of cancer diagnosed within the last three years (pulmonary disease, heart attack dementia, ulcer, etc.).
- Comorbidity index, which is a score of each person’s likelihood of early death based on various factors (age, solid tumor, leukemia, etc.).
Key Insight: Even after all that was taken into account, the use of thyroid medication correlated with a higher risk of cancer overall.
Nearly every type of cancer was higher among those on thyroid medication. The types of cancer that were the most highly associated included:
- Brain cancer (90% increase)
- Skin cancer (42% increase)
- Breast cancer (24% increase)
- Pancreatic (27% increase)
This was a study that looked at what had already happened to people in the past. This kind of data has its own limitations, but it cannot be ignored. For context, this is also how we learned about the link between smoking and lung cancer.
Study #2: Denmark
A Danish study from 2013 had similar findings15.
In it, 3,587 individual participants and 682 twin pairs were chosen. The individuals were on thyroid medication. In the twin pairs, one twin was and the other was not.
Overall, thyroid medication correlated with a 52% higher risk for cancer. The twin studies were done to see if the genes that caused someone to need thyroid medication might also raise the cancer risk.
If it was genetic, identical twins whose twin taking thyroid medication should have had a greater risk for cancer than fraternal twins in the same situation. But, they did not.
In fact, fraternal twins were 55% more likely to develop cancer. The risk was due to medication and was not genetic.
Study #3: Taiwan and Breast Cancer
This study was completed in 2018 and looked specifically at breast cancer risk for those on thyroid medication16. Its findings inspired the larger study from Taiwan that I mentioned at the outset. This initial study looked at 65,491 people.
It turned out that thyroid medication raised the risk of breast cancer by 22% during the first year of usage. For those on it for more than one year, the risk increased to 26%.
Study #4: Sweden
In 2020 a study was published on 253,193 people on prescription thyroid medication. Sweden has a centralized health system and was able to gather data on every person on ongoing prescription thyroid medication17.
The use of thyroid meds correlated with higher risks for every documented cancer with the exceptions of testicular cancer, male genital cancer, and connective tissue cancer.
This study also showed how the risks from thyroid medication compared to other known risk factors. You can see that thyroid medication caused a cancer risk greater than obesity or kidney disease but similar to that from diabetes or COPD.
Study #5: Taiwan
In 2017, a study was written about 3855 patients on thyroid medication18. It showed that those on medication had an 868% increased risk of thyroid cancer over non-users.
It also showed that low thyroid function was protective against breast cancer. In fact, it lowered the risk of developing breast cancer by 61%.
Thyroid Medication and Cancer Risk: Next Steps
I hope today’s discussion was an illuminating journey in the relationship between thyroid medication and cancer risk. As I mentioned before, it’s all about balance.
Thyroid hormones regulate the development of cancer cells and their growth. If we get too much, or too little, that can be a problem.
In severe hypothyroidism, there is a deficit of thyroid hormone. But, in mild and moderate hypothyroidism, there can be problems but the body still has thyroid hormones. Bringing in extra hormones rarely improves symptoms and can cause risks.
And, finally, when your TSH moves below range, there is obviously too much. But, blood levels do not always show when there is too much thyroid hormone.
When it comes to next steps, please consider changes in your thyroid medication levels (through prescribing), and also have faith in the power of your body to heal. If you take steps to minimize other risks and dial things in, you can reduce your risk effectively.
Ready to learn a bit more about your thyroid? I’d recommend taking the Thyroid Quiz today. From there, you can get a sense of where you stand and where you should go next. I hope today’s discussion was helpful for you, and I’m looking forward to checking in again soon.
1 – Uzair I.D., Grand J.C., Flamini M.I., Sanchez A.M. Molecular Actions of Thyroid Hormone on Breast Cancer Cell Migration and Invasion via Cortactin/N-WASP. Front. Endocrinol. 2019;10:139. doi: 10.3389/fendo.2019.00139.
2 – Khan S.R., Chaker L., Ruiter R., Aerts J.G.J.V., Hofman A., Dehghan A., Franco O.F., Stricker B.H.C., Peeters R.P. Thyroid function and cancer risk: The Rotterdam Study. J. Clin. Endocrinol. Metab. 2016;101:5030–5036. doi: 10.1210/jc.2016-2104.
3 – Liu Y.C., Yeh C.T., Lin K.H. Molecular Functions of Thyroid Hormone Signaling in Regulation of Cancer Progression and Anti-Apoptosis. Int. J. Mol. Sci. 2019;20:4986. doi: 10.3390/ijms20204986.
4 – Schiera G, Di Liegro CM, Di Liegro I. Involvement of Thyroid Hormones in Brain Development and Cancer. Cancers (Basel). 2021;13(11):2693. Published 2021 May 30. doi:10.3390/cancers13112693
5 – Schiera G, Di Liegro CM, Di Liegro I. Involvement of Thyroid Hormones in Brain Development and Cancer. Cancers (Basel). 2021;13(11):2693. Published 2021 May 30. doi:10.3390/cancers13112693
6 – Hercbergs A.H., Ashur-Fabian O., Garfield D. Thyroid hormones and cancer: Clinical studies of hypothyroidism in oncology. Curr. Opin. Endocrinol. Diabetes Obes. 2010;17:432–436. doi: 10.1097/MED.0b013e32833d9710.
7 – Trodello C., Higgins S., Ahadiat O., Wysong A. Hypothyroidism as a risk factor for cancer: A systematic review and implications for future studies. Cancer Sci. Res. Open Access. 2017;4:1–7. doi: 10.15226/csroa.2017.00136.
8 – Sohn SY, Seo GH, Chung JH. Risk of All-Cause Mortality in Levothyroxine-Treated Hypothyroid Patients: A Nationwide Korean Cohort Study. Front Endocrinol (Lausanne). 2021;12:680647. Published 2021 May 13. doi:10.3389/fendo.2021.680647
9 – Basen-Engquist K, Chang M. Obesity and cancer risk: recent review and evidence. Curr Oncol Rep. 2011 Feb;13(1):71-6. doi: 10.1007/s11912-010-0139-7. PMID: 21080117; PMCID: PMC3786180.
10 – Giovannucci E, Harlan DM, Archer MC, et al. Diabetes and cancer: a consensus report. Diabetes Care. 2010;33(7):1674-1685. doi:10.2337/dc10-0666
11 – Basen-Engquist K, Chang M. Obesity and cancer risk: recent review and evidence. Curr Oncol Rep. 2011 Feb;13(1):71-6. doi: 10.1007/s11912-010-0139-7. PMID: 21080117; PMCID: PMC3786180.
12 – Bagnardi V, Rota M, Botteri E, et al. Alcohol consumption and site-specific cancer risk: A comprehensive dose-response meta-analysis. British Journal of Cancer 2015; 112(3):580–593.
13 – Pesch B, Kendzia B, Gustavsson P, Jöckel KH, Johnen G, Pohlabeln H, Olsson A, Ahrens W, Gross IM, Brüske I, Wichmann HE, Merletti F, Richiardi L, Simonato L, Fortes C, Siemiatycki J, Parent ME, Consonni D, Landi MT, Caporaso N, Zaridze D, Cassidy A, Szeszenia-Dabrowska N, Rudnai P, Lissowska J, Stücker I, Fabianova E, Dumitru RS, Bencko V, Foretova L, Janout V, Rudin CM, Brennan P, Boffetta P, Straif K, Brüning T. Cigarette smoking and lung cancer–relative risk estimates for the major histological types from a pooled analysis of case-control studies. Int J Cancer. 2012 Sep 1;131(5):1210-9. doi: 10.1002/ijc.27339. Epub 2011 Dec 14. PMID: 22052329; PMCID: PMC3296911.
14 – Wu CC, Islam MM, Nguyen PA, et al. Risk of cancer in long-term levothyroxine users: Retrospective population-based study. Cancer Sci. 2021;112(6):2533-2541. doi:10.1111/cas.14908
15 – Thvilum M, Brandt F, Almind D, Christensen K, Hegedüs L, Brix TH. Excess mortality in patients diagnosed with hypothyroidism: a nationwide cohort study of singletons and twins. J Clin Endocrinol Metab. 2013;98(3):1069-1075. doi:10.1210/jc.2012-3375
16 – Wu CC, Yu YY, Yang HC, et al. Levothyroxine use and the risk of breast cancer: a nation-wide population-based case-control study. Arch Gynecol Obstet. 2018;298(2):389-396. doi:10.1007/s00404-018-4837-y
17 – Wändell P, Carlsson AC, Li X, Sundquist J, Sundquist K. Levothyroxine treatment is associated with an increased relative risk of overall and organ specific incident cancers – a cohort study of the Swedish population. Cancer Epidemiol. 2020;66:101707. doi:10.1016/j.canep.2020.101707
18 – Hung, S.-H., Chung, S.-D., & Lin, H.-C. (2017). Thyroxin Use Is Associated With Increased Risk of Thyroid Cancer in Patients With Hypothyroidism. The Journal of Clinical Pharmacology, 58(1), 29–33. doi:10.1002/jcph.972
P.S. Whenever you are ready, here is how I can help you now:
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Dr. Alan Glen Christianson (Dr. C) is a Naturopathic Endocrinologist and the author of The NY Times bestselling Adrenal Reset Diet, The Metabolism Reset Diet and The Thyroid Reset Diet.
Dr. C’s gift for figuring out what really works has helped hundreds of thousands of people reverse thyroid disease, lose weight, diabetes, and regain energy. Learn more about the surprising story that started his quest.
