Levothyroxine: Does the Drug Increase Risk for Lung Cancer?

Italian Study Points to Increased Oxidative Stress

Many people have been asking me about an Italian study that looked at levothyroxine and lung cancer.

What the Italian study seems to be suggesting is that levothyroxine increases oxidative stress—a process that impairs the body's ability to detoxify and repair damage. Oxidative stress is a factor in disease, and in this case, they found a small increased risk of lung cancer that could be—but is not demonstrated to be—due to oxidative stress from levothyroxine.

The lungs require thyroid hormone to function properly. Hypothyroidism is associated with a variety of dysfunctions in organs, glands, and tissues. The researchers stated in their conclusions, however, that they cannot exclude the idea that the hypothyroidism itself may be a contributing factor to an increased risk for lung cancer, and not the levothyroxine used to treat it.

I asked Harvard-trained physician Richard Shames, MD, author of a number of books on thyroid disease, including the most recent Thyroid Mind Power, to comment on the study.

"Having read carefully the original Italian research article, I am not impressed.

"First of all I'm not impressed with the original Italian research paper. This was the most simple and bare-bones correlation of total amount of levothyroxine sold in Italy with the total number of women in Italy who have lung cancer. Such gross correlations do not necessarily have anything to do with "cause." The authors implied that the correlation has something to do with levothyroxine somehow causing lung cancer, but this research is far from actually showing it.

"The blogger suggests that patients should choose a more natural approach to treating low thyroid, or at least use some alternate medicine other than levothyroxine.

"With little research, we don't know whether that might also 'cause' cancer, either from the same issue of oxidative stress or from some other unproven reason.

"The research focuses on "oxidative stress." First of all, it is far from clear that the most common low-dose treatments with levothyroxine are themselves a major cause of oxidative stress. Second, it is far from clear that oxidative stress is a major cause of lung cancer. Third, it is even further from clear that avoiding levothyroxine treatment will help prevent this highly unlikely lung cancer cause.

"Basically, this cancer discussion is best put way on the back burner until more and better research is in.

"As much as I might agree with the idea of not always using levothyroxine alone, I cannotsupport the findings as a rationale for that conclusion. Moreover I believe it is a disservice to the public if "levothyroxine causes lung cancer" is used as a scare tactic, or as a reason for choosing alternative treatment for low thyroid."

I agree with Dr. Shames' concerns regarding the research. I also wanted to note several concerns about a GreenMedInfo article that touched on the subject—in particular, this quote from Sayer Ji:

"The reality is that most cases of hypothyroidism today are diagnosed in populations who are experiencing a combination of basic nutritional deficiencies and chemical exposures, or who are simply going through a temporary down-cycle in thyroid function following a natural change, such as the natural postpartum drop that occurs in women after giving birth. Even acute bouts of stress and subclinical adrenal insufficiency can cause cyclical downshifts in thyroid function."

Hashimoto's disease, which is the primary cause of hypothyroidism in the Western world, is a disease that has multiple triggers and causes, including heredity. "Nutritional deficiencies and chemical exposures"—as well as post-partum changes, stress, and adrenal dysfunction—are just a few of the many factors that are implicated in the development of Hashimoto's and hypothyroidism.

Ji also asks: "Why call a mineral deficiency-induced decrease in T4 a monolithic disease entity such as "hypothyroidism?" Why not simply call it selenium deficiency? Or, if fluoride, mercury, or any number of xenobiotic chemicals in the environment requiring selenium-dependent glutathione-mediated detoxification is causing the "low thyroid," why call chemical poisoning "hypothyroidism"? 

Thyroid patients and practitioners know that mineral deficiencies are one of many factors in hypothyroidism. Hypothyroidism cannot be referred to as simply a "selenium-deficiency disease." Adding selenium, while it may help lower antibodies in some patients, is not a treatment for many cases of hypothyroidism—the treatment is prescription thyroid hormone replacement.

As for the chemicals and toxins, while there is some evidence that they may trigger autoimmune thyroid disease in a subset of patients, removing the toxins is rarely a "cure" or "treatment" for the resulting thyroid disease.

I fully agree with Ji that the medical world is not doing due diligence in researching the role of nutritional deficiencies and toxic exposures in autoimmunity and in particular hypothyroidism, as well as the potential that levothyroxine may cause oxidative stress.

But like Dr. Shames, I believe it's premature to worry that "levothyroxine causes lung cancer"—this study does not provide enough evidence to cause alarm.

That said, it's good health practice in general, and especially perhaps for thyroid patients taking levothyroxine, to ensure they are eating an antioxidant rich diet. That means a diet heavy on vitamin-rich fruits and vegetables for the most part, which are linked to a lowered risk of cancers, including lung cancer, and are known to fight oxidative stress from all causes.

Was this page helpful?
Article Sources
  • Cornelli, Umberto et. al. "Levothyroxine and lung cancer in females: the importance of oxidative stress," Reproductive Biology and Endocrinology, 2013, 11:75 doi:10.1186/1477-7827-11-75.