Thyroid and Breast Cancer:


Jacob Schor ND, FABNO

December 12, 2012

So many women with breast cancer seem to have thyroid issues that instead of asking if they have ‘thyroid problems,’ it’s tempting to simply say, ‘tell me about your thyroid problems.’


Obviously the biggest risk factor for breast cancer and hypothyroidism is the same, ‘being a woman over 60.’ 

Yet it is more than that.  Women who have or have had breast cancer are more likely to be hypothyroid than other women.  This doesn’t make sense at first.  Let me explain.


High levels of thyroid hormones typically stimulate cancer, while low hormone levels lower risk and inhibit growth of cancer.  


A 2009 study reported that hyperthyroidism ([too much thyroid hormone and so a low TSH levels (<0.50 mU/L)], increases overall cancer risk by about a third compared to normal thyroid function (HR 1.34, 1.06-1.69, 95% CI).  This is worse in lung and prostate cancer where being hyperthyroid doubles risk  [lung cancer (HR 2.34, 1.24-4.40, 95% CI) and prostate cancer (HR 1.97, 1.04-3.76, 95% CI)].


It is even worse with breast cancer.   Hyperthyroidism really increases risk of breast cancer. In June 2010, Breast Cancer Research reported just how much.  The data came from 2,696 women tracked for t 20 years. During that time, 173 got breast cancer. When the women were ranked into four groups based by their thyroid hormone levels, the higher the levels, the greater their risk for breast cancer was. Those with the lowest thyroid hormone levels had the lowest risk.  Those with the highest levels had nearly 7 times their risk. The women in the intermediate level groups still had three and five times the risk of those women with the low normal levels.   


This works both ways. Hypothyroidism lowers a woman’s risk of getting breast cancer.


This should make sense so far but here is where it should stop making sense: women who actually have breast cancer are more likely to be hypothyroid than women without breast cancer.


There have been all sorts of theories proposed to explain this.  First, let me review the ones that have been thrown out and then tell you about the two possible theories that remain.


Initially the increased occurrence of hypothyroid disease in breast cancer patients was considered to result from cancer treatment; either radiation damage, chemotherapy injury, or the stress of surgery, or even just the stress diagnosis. Each of these ideas have been disproven.


Radiation treatments made the most sense but in 2008, it was disproven.  Researchers from MD Anderson compared 38,225 women with breast cancer against 111,944 controls and found the five year incidence of hypothyroidism was identical (14%) whether the patients had received radiation or not.  While radiation treatments did not increase rates of hypothyroidism, breast cancer certainly did.  The breast cancer women had a 21% higher risk of being hypothyroid compared to the control group.


In a 2009 Norwegian paper the breast cancer patients also had higher rates of hypothyroidism, triple the rate compared to the control group (18% vs. 6%, p < 0.001).  


A 2006 Italian study tested thyroid function even before diagnosis or surgery; a third of the women who were eventually diagnosed with breast cancer already had elevated thyroid antibody levels.



We are left with the fact that while hypothyroidism protects a woman from getting breast cancer, more women who have breast cancer are hypothyroid. It should be the other way around.



Let us digress for a moment.  Could this be the body’s response to breast cancer?  For example it is common to see a pseudo iron deficiency with acute infection or with some chronic disease.  The body seems to hide the iron away as ferritin when suitably provoked, causing what looks like iron deficiency anemia.  This response seems to slow the infection and perhaps even the cancer.   Maybe thyroid levels drop in response to cancer, in a similar manner. 

Maybe the hypothyroid breast cancer patient might be better off left alone, that is their thyroid disease not treated.


Don’t ponder this too long as there are possible explanations and rationales why we should treat hypothyroidism in these women.


In the first place, there is no evidence that this association between breast cancer and thyroid disease is cause and effect.  It makes better sense to think that the thyroid disorders associated with breast cancer and the cancer itself are related to a common predisposing factor.


Thyroid hormone supplementation may actually help fight breast cancer. According to a recent study by Sar et al, thyroid hormone induces apoptosis in breast cancer cells.  


It is worth noting for those readers who are curious about these things that the first reported use of thyroid hormone to treat breast cancer was by George Thomas Beatson in 1896.   


There are two other possible explanations for this hypothyroidism.  The first and possibly strongest explanation is that nutritional deficiencies, in particular iodine and possibly selenium deficiency may be to blame.  The second theory getting attention is that the breast cancer triggers an autoimmune reaction leading to hypothyroidism.


Iodine deficiency is a plausible explanation.  Iodine is the greatest common denominator linking both breast and thyroid health. Peter Smyth wrote in a 2003 paper that the only, “…  area in which thyroid and breast functions overlap is in the uptake and utilization of dietary iodide.”   Thyroid hormone is made from iodine. Iodine is also important in the breast. Our dear colleague Lyn Patrick ND recently wrote me that, “The breast is designed as an elegant iodine delivery device for a growing infant, after that it [iodine] becomes a residual antioxidant.    While link between iodine and breast health is less well known, using iodine to treat fibrocystic breast disease is familiar to most alternative practitioners.


Breast cancer incidence in Japan is only 6.6 per 100,000, while in the United States it is 22 per 100,000.  (In the United Kingdom it is even higher at 27 per 100,000.) In Japan dietary seaweed provides women with 25 times more dietary iodine per day than is found in the American diet.  This high iodine intake may in part account for the low risk of breast cancer among Japanese women.  As iodine is considered an, “antiproliferative agent contributing to the integrity of normal mammary gland,” this may be why.  Animal and human studies, suggest that iodine suppresses breast cancer development to the degree that some researchers suggest iodine should be used for breast cancer therapy.


Funahashi et al have published a series of animal experiments suggesting an anti cancer role of iodine.   Rats pretreated rats with iodine, using either Lugol’s iodine or seaweed were protected from developing breast cancer after they were exposed to a powerful carcinogen normally used to trigger breast cancer.   In another study, Funahashi’s group found that an iodine rich solution made from seaweed and water suppressed carcinogenesis in rats and in vitro induced apoptosis more effectively than the chemotherapy drug fluorouracil. 

Researchers are justifiably wondering whether, “…. seaweed may be applicable for prevention of breast cancer.”


Another link between low thyroid and breast cancer may be selenium deficiency.  The thyroid gland is loaded with selenium; it contains more selenium by weight than any other organ.  Pretty much every known selenoprotein found in the human body is found in the thyroid gland. These selenium proteins protect thyroid cells from damage caused by hydrogen peroxide that is reeased while making thyroid hormone.  


Selenium is useful for thyroid disease; selenium may decrease thyroid antibody levels in autoimmune thyroid disease.  Taken during pregnancy, selenium decreases incidence of postpartum thyroiditis and hypothyroidism.  Selenium may even normalize function in Grave’s disease.


Selenium also protects against breast cancer.  Selenium alters genes to make the body less susceptible to getting cancer. Selenium compounds also act as aromatase inhibitors. Doing so would obviously lower breast cancer risk, especially in post menopausal women. A 2011 study reported an inverse association with selenium levels and breast cancer, that is, the lower the selenium, the greater the risk for cancer. A 2004 review of the clinical trials of micronutrients being investigated for breast cancer prevention, “… indicates that selenium is one of the most promising agents.


Even though the U.S. population has the lowest rate of iodine deficiency in the world, it is still about 11%. This is because about 90% of us use iodized salt. The highest rate of iodine deficiency, by the way, is in Europe where only one quarter of households use iodized salt and 52% of the population is considered deficient. Iodine levels are falling in the United States, median urinary levels are half what they were in the 1970s.  This is no doubt due to our well intentioned efforts to eat a healthier; people try to avoid salt in general, iodized salt in particular, along with wanting natural breads without dough conditioners, avoided red food coloring and trying to stay away from dairy foods. All of these provide dietary iodine.


While this iodine/selenium theory sounds good, it does not explain everything. Here’s the another curious piece to the puzzle: women with breast cancer, who also have autoimmune thyroid disease, have a significantly better cancer prognosis. In particular, TPO antibodies are associated with an improvement in both disease-free and overall outcome in breast cancer patients and   “… the magnitude of this prognostic effect was of a similar order of magnitude to well established prognostic indices for breast cancer such as axillary nodal status or tumour size.” 


The deficiency theory doesn’t account for this.  Peter Smyth put forward another hypothesis when in 2003 he asked whether, “… the perceived breast cancer–thyroid disease relationship is thyroid or iodine related or, in the case of thyroid antibodies, is the consequence of an immune response to the carcinoma.”


So here is a second possible explanation to explain the association between hypothyroidism and breast cancer.  The immune system may accidentally attack the thyroid gland after being provoked by the cancer. 


Something akin to this happens with immunotherapy treatments using interleukin-2 and interferon α.  These treatments may increase thyroid antibody levels and this increase is predictive of long-term survival in renal cancer and melanoma. Perhaps patients with breast cancer experience an immune response to the breast tumor that is also directed against the thyroid gland.  Both breast and thyroid tissue share the same sodium-iodide symporter and the immune system may simply target an antigen the two have in common.


Few things in biology are simple. Both explanations might be valid.  Some patients may be iodine deficient.  Others may have autoimmune thyroid disease triggered by their cancer.  Perhaps we should be testing breast cancer patients both for iodine deficiency and for thyroid antibodies?


While these theories remain unproven, they appear reasonable and perhaps will take the confusion out of a perplexing situation.






Interesting links for more information on iodine:


Tina Kaczor and Alan Christianson’s Profile on Iodine published in the April 5 edition of the Natural Medicine Journal.


Lyn Patrick’s review in June 2008 Alternative Medicine Review:


Alan Gaby’s 2005 editorial and the resultant debate in this journal:


Peter Smyth’s 2003 review article, “The thyroid, iodine and breast cancer.”

Mayo Clinic Website:


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