Thyroid UK
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Very interesting study just released


Is Your Thyroid Medication Increasing Your Risk for Cancer?

If you have thyroid problems, you might be taking a medication to help treat the symptoms. Most of the time, when doctors give their patients a thyroid hormone, they give levothyroxine or T4. If this is what you're taking, there's some new data that suggests that the routine use of T4 for thyroid hormone replacement might be putting you at an increased risk for cancer.

In a brand new study out of The Netherlands, researchers looked to see if there was a relationship between having higher blood levels of T4 and the chances of getting cancer. This was a huge study and the results, which were published in the Journal of Endocrinological Metabolism, are astounding!

Layal Chaker, MD, of Erasmus University in Rotterdam, and his colleagues followed 10,318 men and women with a median age of 61 for an average period of 10.4 years. They wanted to see how many came down with cancer. Then he compared these results to what their blood levels of T4 were. Some of the patients were taking T4 that their doctors had prescribed and the rest were not taking any thyroid replacement.

Here's what he found.

For every unit increase of free thyroxine (FT4) there was a 42% increase in risk for any solid cancer.

Specifically, there was a 77% increased risk for breast cancer and a whopping 233% increased risk for lung cancer. According to Dr. Chaker, "To our knowledge, this is the first prospective cohort study to assess the relation between the full range of thyroid function and cancer incidence. Higher FT4 levels are significantly associated with an increased risk of any solid, lung, and breast cancer." So why in the world would having higher levels of T4 increase the risk of cancer so dramatically?

Before I can answer this question I have to give you a little primer on thyroid hormones.

There are two primary thyroid hormones – T4 and T3. The only difference between them has to do with how many iodine atoms they have. T3 has three iodines and, you guessed it, T4 has four iodines. And here's the thing. That fourth iodine molecule on T4 makes it useless. In other words, it stops T4 from being able to exert any thyroid activity. T3 is the active thyroid hormone. And virtually all of the activity that the thyroid does is through T3. So what's the point in having any T4 if it does nothing? And what's the point in doctors giving their patients T4 if it does nothing?

The action of the thyroid is to stimulate your metabolism. Only T3 will do that. But there are going to be lots of times when certain parts of the body need more of a metabolic stimulus than other parts. For example, when you eat, your digestive system needs an increase in metabolism. But when you exercise, your muscles need the increase, not your digestive system. And when you are sick, your immune system needs a metabolic push, not your muscles.

So how does the body increase the metabolism in a particular part of the body while not affecting the other parts? It does it this way. When a part of the body needs a metabolic increase, it activates an enzyme called de-iodinase which acts to remove an iodine from the T4 circulating in that area. This turns the T4 into the active hormone T3 and, bingo, you're in business. And this is where the problem lies.

Many people, especially as they get older, do not have enough of this enzyme. And when that happens, the T4 levels go up.

So it's not the actual T4 that's causing the increased risk for cancer. It's the fact that a higher T4 level in many patients indicates a deficiency of T3. And since it's T3 that determines our metabolisms, and since a lower metabolism is one of the primary causes of cancer, it's only natural that an increase in T4 levels would be a risk factor for cancer.

So are you really at an increased risk for cancer if your doctor is giving you T4?

Not necessarily. If your body is converting the T4 to T3 optimally, the answer is no. It all depends on your metabolism. But here's the problem. There are only a handful of doctors in the country actually measuring the metabolism of the patients they are giving thyroid hormone to. That means that the other doctors who do not measure metabolism have no idea whether or not they are increasing their patients'

risks or not.

So what's the answer?

In my ideal world, no patient would ever be given thyroid replacement therapy without having their metabolism measured. And the dose and kind of thyroid they get would be entirely determined by their metabolism. That's because, as you can see from this study, blood tests just give you half the story.

By Frank Shallenberger, MD


Khan SR, Chaker L, et al. Thyroid Function and Cancer Risk: The Rotterdam Study. J Clin Endocrinol Metab. 2016 Sep 20

[ Admin edited link to remove the part that was not working. Now has no session/person information. ]

24 Replies

Very interesting but worrying. Here I am with a high in range T4 but low in range T3. I've just started self medicating T3 and now I'm wondering should I reduce or stop the T4/Levothyroxine my GP prescribes? I'm obviously not converting T4 to T3. Is the T4 that most of us are prescribed doing us harm?

1 like


Adding T3 to Levothyroxine will also stimulate T4 to T3 conversion which will reduce your FT4 and raise your FT3.

My FT4 is considerably lower on T4+T3 than when I was on Levothyroxine only.



Good find. Very interesting. Another Rotterdam Study finds "There is an increased risk of AF with higher FT4 levels within the normal range, especially in younger subjects."


Hmm. See, increased levels of breast cancer for example are associated with untreated hypothyroidism. And what we can't tell from this study is how many years some of the subjects might have been left untreated, fobbed off as depressed, by their GPs and how many more were left chronically undertreated for years.

To be a bit clearer, I don't think it's the high levels of T4 causing the correlation. I think it's the hypothyroidism itself. When every cell of your body is deprived of energy - which is what happens in hypothyroidism - it's really not surprising that the cells develop faults in replication, which is what cancer is in the main.


Just what I was thinking. There's something not quite right with this study, and I do wonder at their motivation for embarking on it. And it's a bit sensationalistic, getting people worried about their levels of T4, and then saying at the end, oh! It's not actually the levels of T4 that are the problem, it's the low T3. Well, why couldn't they have said that in the first place!


The other interesting thing is that synthetic t4 is said to contain Free t4, but the ndt contains bound t4.. According to this article....

The paper only mentions free t4, Hmmm.... So could that mean that synthetic t4 is the problem?



The study found no association between levels of thyroid stimulating hormone (TSH) and any type of cancer.

My initial thought is, that makes testing for FT4 a much more sensible thing to do than test TSH. Oh!!... we already worked that out!

Deeper thoughts might follow.


Since joining TUK and being educated by this site I have continually questioned: If a healthy thyroid produces T4 and T3 why do the medics think it is OK to just give us T4 only? I have been pondering taking T3 in addition to T4 for some months. (I have very low TSH- doctors in panic mode- and also low in range T4 and T3) I think this is just the push I need to start taking T3 and hopefully feel a bit better.

Interesting that the Dutch study followed over 10,000 people over 10 years. A lot more thyroid research needs to use numbers like that instead of pathetic research on tens of people only. This is definitely what I call robust research. Let's hope they do more research into thyroid issues.


It looks like just another excuse to keep patients under-medicated to me.

The next question is, how do we improve our ability to produce deiodinase when needed? Especially those of us with deiodinase mutations.


This paper shows, yet again, the need for FT3 testing in general.

Not only might the individuals have had their states better understood, a fundamentally good thing, but this paper - written with FT3 results for the cohort available - might have read very differently.

Obviously the researchers have done what they could with the data. What extra illumination might there have been if some had the reported relatively high FT4 with either high-ish or low-ish FT3?

The lack of more widespread population FT3 data is a black hole.

I couldn't see a single mention of T3 or FT3 in the paper - did I miss it?


Yes, you missed it. Sort of....

...." So it's not the actual T4 that's causing the increased risk for cancer. It's the fact that a higher T4 level in many patients indicates a deficiency of T3. And since it's T3 that determines our metabolisms, and since a lower metabolism is one of the primary causes of cancer, it's only natural that an increase in T4 levels would be a risk factor for cancer.

So are you really at an increased risk for cancer if your doctor is giving you T4?

Not necessarily. If your body is converting the T4 to T3 optimally, the answer is no. ........"

So its not actual t4 that puts you at risk of cancer, its lack of t3, I wish they had spelled it out more clearly, in simple language that even an endocrinologist can understand.



Thank you! :-)

Right-ho - I failed to say that I was referring to the paper rather than the report of the paper!!!


I was on levo for 8 years and was eventually diagnosed with leukemia . I never felt well on T4 only and kept telling my GP so, but he only offered me antidepressants! I came off levo just after cancer diagnosis and onto NDT, which I have had to source and pay for myself. I feel let down, as do many of you I expect.


The trouble with large correlation studies is that you have to correlate the right things. Apparently they once showed that the rise in lung cancer correlated perfectly with the rise in sales of nylon socks. Sadly that only worked statistically until nylon socks fell out of favour, and so smokers still get lung cancer, even when wearing wool or cotton.

It cannot be anything but bad news if your production of the master hormone decreases. What is amazing is that we don't just die!

And to me it seems intuitively obvious that a synthetic hormone, that a) doesn't fully replace what is missing and b)is not biochemically identical to what moves around in a well person's body is not going to work as well.


The association I particularly remember is consumption of orange marmalade and pregnancy rate. In that case, they didn't even question why only half the population ever got pregnant...

It might also seem intuitively obvious that delivering thyroid hormone by mouth, stomach or gut is so far removed from what should happen that all bets are off as to what will, or will not, work effectively.

However, what is not identical between either of the synthetic thyroid hormones, once they have been absorbed, and the ones from our own thyroids (if we had one, and it worked)?

I'll go along with questioning every aspect of formulation, delivery, absorption. And every question about how the excipients affect the person, and impurities, and so on. And it is right to question what is in the blood. But what evidence do we have that there is actually any difference? Has anyone ever been able to measure Free T4 (endogenous) and Free T4 (exogenous)?


Looking at this paper is really piling on the feeling I've had recently that I don't believe anyone does well on T4 mono therapy. I'm remembering Diogenes recently commenting that on mono therapy fT4 is likely to need to be over-range to give a reasonable fT3 level. So according to this new study, anyone who doses for a good fT3 and overall health, will be ramping up other risks.

I also recently went to a thyroid cancer day run by that silly Butterfly charity. It was mostly rubbish, but the one message I went home with is that Thyroid cancer survivors have some of the worst quality of life measures of any cancer survivors. All day people were on the one hand assuming you'd be fine afterwards, and of course a few people were trotted out who had run marathons, etc. But on the other hand, it was an open secret that most people survive on frequent naps, have reduced hours at work, etc. I felt like I was seeing the stats for Levothyroxine in human form made up of a conference room of a hundred or so people.

I don't actually know where the 80% figure comes from as people that T4 mono therapy works for. But I do think lots of people will answer the question to say they're doing fine, even while they're suffering a lot. And why should they pipe up about it? We know from the forum every day that most people won't get any help, and may just be insulted. But it seems like if you ask more detailed questions about quality of life, they'll start revealing a bit more about how they're struggling.


Well believe it or not I do very well on Levo and I have no thyroid, I'm on a relatively low dose 125/150 and neither my T4 or T3 are top of the range.


Thanks Bantam12, it's good to hear some people do well! How easy did you find it to get your correct dose?


There are many many people doing well but they aren't here to say so. I don't have a problem with dose, sometimes I forget which I'm on that day and probably take the wrong one but hey ho it all works out in the end, I just don't worry about it.

It's time folks accepted that T4 works for most people and stopped assuming that we are all just suffering in silence.

PS just to note, my husband doesn't do well on T4 alone so takes both T4 and T3 so I can see both sides of the story.


But people on here are generally the ones for whom t4 does not work... If it worked we wouldnT be on a forum, we would be getting on with life. We donT assume people on t4 only are putting up with it, (though many are). I was on t4 only and thought it was a miracle wonder drug for about 10 years.... Until it stopped working. I put up with what i now know were symtoms of being under dosed..., ( i had a hysterectomy to combat the very heavy bleeding) because no one mentioned it was linked to undertreatment of hypothyroidism.

I only sought out a forum, after i had worked out for myself that i needed t3, I bought it off the internet in about 2002 and then got my doctor to prescribe it....

What was it that made you seek out a forum if you are doing well on T4?



Im not aware the forum is only for people with problems, just because I do well it doesn't mean I'm not interested in other people's issues.


There are many, many reasons for joining and remaining a member.

Statistically, I suspect (but don't know) that the majority of members are asking because they wonder if they have a thyroid issue, have just found that they have, have problems despite treatment, or are here on behalf of others. Often we see a combination. We also see members where we never know why they joined! Perhaps they don't want to disclose their reasons?

Like everything to do with thyroid, more complicated than it might at first appear. :-)


Was your reply to me helvella ? if so

I understand the many reasons people might join a forum, galathea was questioning my reason for being here.


More to anyone who read it!

But as your response had not been theaded onto galathea's, mine couldn't be threaded onto both as I would have preferred.


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