Study: Millions Treated for Hypothyroidism At Risk for Lung Cancer

A new study has uncovered a link between the synthetic thyroid hormone (trade name Synthroid) used to treat millions diagnosed with hypothyroidism and lung cancer, bringing to the forefront the harmful role that overdiagnosis and overtreatment plays in nutritional deficiency and chemical exposure related 'diseases.'

A new Italian study published in the journal Reproductive Biology and Endocrinology titled, "Levothyroxine and lung cancer in females: the importance of oxidative stress," has raised a concerning possibility: levothyroxine (T4), one of the world's most commonly prescribed forms of hormone replacement, may be raising the risk of lung cancer in millions of men and women being treated for low thyroid function (hypothyroidism).

The authors of the study pointed out that levothyroxine (T4) treatment can lead to medication-induced (iatrogenic) thyroid over-activity (hyperthyroidism) and oxidative stress that can lead to a significant patient discomfort. They also noted that oxidative stress is one of the causes of chronic diseases and cancer.

The study looked at the prevalence of breast, colorectal, gastric and lung cancer in 18 Italian Regions during 2010 and correlated this data with sales of LT4 in 2009, focusing on women aged 30-84. (They noted that this age range corresponds to more than 80% of the consumers of the drug and to about 99% of all malignant cancers). They then correlated drug sales with cancers, eliminating the contribution of age and smoking for lung cancer risk.  They found no significant correlation between T4 sales and breast, colorectal and gastric cancers, but did find a significant correlation for lung cancer (p < 0.05) corrected for smoking and age.

In discussing their findings, the authors noted that they could not exclude the possibility that the condition of hypothyroidism could favor the development of lung cancer. However, they pointed out that the opposite was recently described, and that "...hypothyroidism reduces the aggressiveness of some cancers because of the presence of thyroid hormone receptors on cancer cells, and spontaneous hypothyroidism may delay onset or reduce aggressiveness of cancers."[1]

Other research indicating that increased thyroid activity (hyperthyroidism) contributes to lung cancer pathogenesis was presented, including:

●      Recently T4 has been reported as one of the several endogenous factors capable of supporting proliferation of lung cancer cells.[2]

●      The observation that patients with small cell carcinoma of the lung often present symptoms suggestive of hyperthyroidism (i.e. weight loss, anorexia) was made many years ago together with an over production of both T4 and T3.[3]

●      An old clinical observation on the relationship between lung cancer and thyroid function reported that patients were characterized by a low concentration of T3 and an increased T4/T3 ratio due to a decrease of 5'-monodeiodination (DI).[4]

●      More recently DI activity in lung cancer was found to be lower than in peripheral lung tissue.[5]

The authors also cite animal experimental research which has shown that T4 increases the oxidative stress[6] and spontaneous pulmonary metastases in mice.[7] They also elaborate on a mechanism of T4-induced lung tissue toxicity:

"[I]n rats lung the deiodination of LT4 is the lowest compared to all the other tissues.[8] This means that the amount of LT4 reaching the lungs following an external supplementation cannot to be transformed into LT3 as in the other tissues, and make lungs very vulnerable to possible toxic effects of LT4.

The authors delved deeper into the reason why oxidative stress taking place during T4 treatment is particularly linked to lung cancer only:

The hypothesis could be that in lungs the increase of hypoxia-induced factor (HIF-1) which is determined by T4 can make oxygen much more available, increasing locally the oxidative stress together with a dangerous angiogenesis stimulation.[9] [10] [11]  Although we should never forget that LT4 is a life-saving thyroid hormone replacement, and that one should not exclude that the pathological reason that leads to the prescription of LT4 could favor the lung cancer development also.

The authors expanded on their findings further:

"However, the impression we get from our experience in epidemiological studies monitoring[12] is that this drug should be prescribed more cautiously. Considering that the population in Italy is about 60 million people and the sales of LT4 in the country in 2010 were 17.69 million boxes (+ 6% Vs 2009), hypothyroidism, which is the main reason for the prescription, should be a real national concern (almost 0.7 boxes/women/year). Most of the time, doctors tell patients treated with LT4 that any side effects will be temporary and almost ineluctable, and are usually dealt with through dose reduction. Assessment of oxidative stress and its balance has not been taken into consideration up to now as it should be."

Finally, they pointed out that T4, which is a synthetic form of thyroid hormone, might be avoided if T3 or the desiccated pig-derived form known as Armour Thyroid was used to control the oxidative-stress producing hydroperoxides.


Biomedicine today is awash in controversy associated with the overdiagnosis and overtreatment of asymptomatic populations. With the recent announcement by a National Cancer Institute commissioned expert panel that a variety of so-called 'indolent' neoplasms such as ductal carcinoma in situ (DCIS) and high-grade prostatic intraepithelial neoplasia (HGPIN), formerly understood to be forms of breast and prostate cancers, respectively, should be renamed to reflect their intrinsic benignity, a veritable sea change is occurring within the conventional medical establishment. Millions of people who were not only wrongly diagnosed with 'cancers' and subsequently aggressively treated for them with chemotherapy, radiation and surgery, are now being told that they would have been better left undiagnosed and untreated. Or, simply look at what the young-adult, T-score based definition of osteoporosis and osteopenia has done to convert millions of healthy middle-age and older women into drug-treatable patients, or, what the lipid hypothesis of cardiovascular disease causation has done to create a 30-billion dollar a year market in statin drugs, to fully appreciate the harms of overdiagnosis and overtreatment.

The increasingly global medical-industrial complex has begun to co-opt all aspects of the natural human bodily life cycle, and all the organ systems within it, as potential targets of medical surveillance and intervention, not excluding the human thyroid. According to data from the National Health and Nutrition Examination Survey, years 1988-1994, [13] while 0.3% of the American population exhibits overt symptoms of hypothyroidism, 14 times more  (4.3%) have so-called 'subclinical' hypothyroidism only detected via blood work. This relatively symptom-free reservoir of 'hypothyroid' cases provides a veritable gold mine of potential office visits, billable services and drug prescriptions.

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. At, we have indexed plenty of peer-reviewed research on over 30 natural substances, including nutrients and minerals, which may help to resolve low thyroid function, and conversely, over 30 unnatural substances, e.g. rocket fuel, pesticides, fluoride, which induce low thyroid function. View the data on our Hypothyroidism Research page.

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