My Cart

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

Color I Color II Color III Color IV Color V Color VI

Log In or Register

fb iconLog in with Facebook