The synthetic thyroid hormone levothyroxine has been linked to increased breast cancer risk for a number of years; but now new research shows Levothyroxine is linked to a 50% increase of any cancer and even bone loss.
A short summary of this REVIEW:
Levothyroxine is prescribed because of 'hypothyroidism' (low thyroid hormone levels) which may simply be caused by low plasma iodine and/or low selenium - did your doctor first check for either of these before assigning you the drug?
The synthetic hormone is also prescribed in about 10-15 per cent of cases with thyroid damage caused by Hashimoto's (nodules), or lymphocytic thyroiditis (goitre). Both of these are a result of poor gut microbiome function.
Levothyroxine, marketed as Synthyroid, in some countries is the second most common drug prescribed to older adults in the USA with 23 million people taking it, representing 7 per cent of the total population.
Worryingly, researchers from Johns Hopkins have found that a significant number of adults who had had hypothyroidism at one point in their lives, had just been 'left on it' and didn't actually need it anymore!
To produce adequate levels of natural thyroid hormone you first need adequate levels of plasma iodine (consuming sea food or sea kelp four times per week should be sufficient). You also need a little selenium (200 mcg) twice a week to ensure the release of the natural hormone.
Research shows that Levothyroxine is linked with an increased risk of breast cancer.
Two new studies have shown that Levothyroxine is linked with a 50 per cent increased risk of many cancers.
Researchers in the johns Hopkins study showed that high thyroid levels (from unnecessary supplemental synthetic Levothyroxine) could also cause bone loss (and therefore osteopenia and osteoporosis) in older adults.
In a study with patients with goitre, researchers found that iodine supplementation helped just as well as supplementing with synthetic thyroid hormone.
Is Levothyroxine potentially damaging to the body?
This article is the fourth update of an article that originated in 2012 (writes Chris Woollams, former Oxford University Biochemist and a founder of CANCERactive).
The use of Levothyroxine had previously been linked to an increased risk of breast cancer and ovarian cancer. There is a thyroid-breast cancer link. There is also a thyroid hormone-oestrogen link. Recent research has linked the synthetic thyroid hormone to a 50 per cent all cancer risk - particularly, pancreatic, skin, brain, as well as breast and ovarian. High thyroid hormone levels are linked to higher rates of cancer. Johns Hopkins have now linked high thyroid hormone levels to loss of bone density.
'Hypothyroidism' or low thyroid hormone levels, leads to symptoms such as low energy, fatigue and weight gain, is linked to lower levels of breast cancer. The main cause of hypothyroidism is low iodine with a little due to low selenium, Always check your iodine levels first, as it is possible that they are the true cause of your fatigue and weight gain, Sea food and sea kelp/sea moss are good sources of iodine.
Iodine helps oxygenate your cells, controlling correct metabolism. It reduces levels of cancer and cancer stem cells, anaemia and levels of virus and pathogens. One study showed that 88 per cent of people were deficient in iodine when diagnosed with cancer.
You will also find yourself prescribed Levothyroxine if you have a thyroid damaging disease, for example, Hashimoto's or lymphocytic thyroiditis. Symptoms of Hashimoto's resemble those of IBS. Don't just live with gut problems - fix them before thyroid damage occurs; Heal your gut!
Finally, any drug - chemo, antibiotics, Proton Pump Inhibitors, statins etc. - will lower your magnesium levels. Levothyroxine is no exception. Low magnesium? A poor immune system, increased risk of cancer, a weaker liver, weaker bones. Magnesium is involved in over 300 reactions and 800 pathways in a healthy body.
Levothyroxine - the cancer link
i) Breast cancer?
The origins of this article lay solely in breast cancer. We first wrote this article more than 12 years ago. A link between diseases of the thyroid gland and breast cancer was first noted in the early 1970s, since when there have been many expert articles. Some early articles linked people taking thyroid hormones with a higher risk of breast cancer back in 1976. In one article, risk rose from 9.2% to 33% when taking synthetic hormone supplements (1).
In 2018, research from Taipei Medical School claimed their new study was the first to suggest levothyroxine increased the risk of breast cancer! They were not the first!! When comparing 65,491 women with breast cancer to 261,964 controls they noted a 'significant increase in breast cancer risk' where women were taking levothyroxine. This risk increased the longer a woman took the drug, and with age (2).
b) Levothyroxine and any cancer?
A large-scale study of 601,733 cases and 2,406,932 controls showed Levothyroxine users to have a 50% higher risk of cancer at any site. Levothyroxine use was significantly associated with an increased risk of cancer - particularly brain, skin, pancreatic, and female breast cancers. The authors concluded that while 'Levothyroxine remains a highly effective therapy for hypothyroidism; physicians should carefully consider levothyroxine therapy and monitor patients' condition to avoid negative outcomes' (3)
In 2020 research which had followed the Swedish population; both men and women had increased cancer risk if they took Levothyroxin. For men, increased risks were found for cancers of the thyroid gland and other endocrine glands. For women, increased risks were found for cancers of the breast, endometrium, cervix, stomach, colon, liver, pancreas, urinary bladder, skin, and leukemia (14)
c) A Thyroid - Breast axis?
There seems anyway to be a breast cancer - thyroid cancer link. Rates of thyroid cancer are increased in women with a history of breast cancer, and women with a history of Thyroid cancer have more breast cancer according to a 2016 study. (4).
Ordinarily, you would expect people with sluggish dispositions and lower metabolic rates to have less cancer. In 2005 a team of researchers at the MD Anderson Cancer Center in Texas showed that women with lowered thyroid activity had less breast cancer by 61 per cent. Women with high thyroid activity developed more. And women with breast cancer were 57 per cent less likely to have hypothyroidism (that is where a person produces less thyroid hormone).
A meta-analysis said much the same in 2020. The Thyroid that is linked to low hormone production - hypothyroidism - is linked with less breast cancer. So something else is causing the problem (11).
d) There may well be a gut issue.
For example, Turken et al (5): Women with breast cancer have higher levels of anti-thyroid peroxidase antibodies and the incidences of both autoimmune and non-autoimmune thyroid diseases is higher in women with breast cancer.
Auto-immune diseases are now known to start in the gut. There seem to be links with each of Chronic Fatigue Syndrome (6), Fibromyalgia (7) and particularly Hashimoto’s.
Hashimoto's disease, or Hashimoto's Thyroiditis is known to be linked to dysbiosis, and infections such as Helicobacter pylori, Epstein Barr, Lyme disease and more. We have an article on it (8) - you really should read it. The gut issues and infections, cause inflammation and the production of antibodies, and these antibodies can attack the thyroid causing a similar inflammation there. Hence Hashimoto's is thought of as an auto-immune disease, where the body is attacked by its own immune system.
Hashimoto’s has also been shown to be linked to Small Intestinal Bacterial Overgrowth, or SIBO (9).
The patient now has two problems. If they don't address the gut issues, the Hashimoto's thyroiditis can get worse. If a person develops thyroid nodules, the person is more likely for them to be cancerous if they have Hashimoto's. Not surprisingly, Hashimoto's is linked to Thyroid cancer. Another study, also not surprisingly linked Hashimoto's to colorectal cancer (10).
A poorly functioning benign thyroid causing Hypothyroidism, reduces the risk of breast cancer (11).
It would seem however, where there is damage to the thyroid (say, from Hashimoto's), or there is cancer present (nodules etc), or the patient is given levothyroxine for the problem, then this can be linked to breast cancer. Maybe there's another risk factor unrelated to these things..
e) There may be a low iodine and selenium issue
What do Thyroid hormones do? They are very important in many ways all to do with your metabolism. This is why good levels prevent fatigue, but high levels can cause higher metabolism and this can lead to cancer.
The thyroid is a store of selenium. Selenium is needed to produce the Thyroid hormone T-4. Sources of selenium are oatmeal, lentils, chicken, turkey, eggs, cashew nuts, some Brazil nuts.
Iodine is an ingredient in the thyroid hormone. Conveying this into cells is an important role of the thyroid. Low thyroid hormone and/or low plasma iodine have the same fatigue causing effects. However, Iodine is essential for good cellular metabolism and for oxygenating cells. We know low iodine is linked with many cancers.
Iodine is in seafood - fish, prawns, sea kelp and sea moss. People consume less of these nowadays. Iodine was added to salt - iodised table salt. People rarely use this now. Recent research on cancer showed 88% of patients severely deficient in iodine. We cover this problem in our revised article ... Go to: 'iodine and cancer.'
Very low iodine, poor metabolism, increased risk of cancer.
When we first published this article, we were written to by two separate American Doctors who commented thus:
'If someone comes to us with sluggishness, tiredness, low energy, we would first look to gut problems. If we suspected a thyroid problem, first we would test for low iodine. We find this in about two-thirds of patients. Without iodine you simply don’t make enough thyroid hormone and the metabolism slows.
If iodine is not the issue, as a last resort we supplement with thyroxine. Once a person starts taking it, they are committed for life. You don’t start lightly. In fact, we prescribe different levels for different body weights and even change the dose by season, depending upon whether it is hot or cold weather.'
None of the British women on thyroxine we come into contact with on Personal Prescriptions is ever given an iodine test.
e) Levothyroxine and ovarian cancer
Thyroid hormone and thyroid problems are not just linked to breast cancer. Roswell Park Comprehensive Cancer Center report a link with Ovarian cancer found over 11 studies.
f) Can you come off Levothyroxine?
The answer varies. Certainly not if you had nodules or goitre and an operation. But if that's not the case and you never had your iodine or selenium levels checked before being prescribed the synthetic drug, reducing the dose and even coming off the drug is possible. Given that 'a significant number of people on Levothyroxine don't now need it' some people may have corrected their iodine and selenium levels already!
The start point is 'Can you eat more sea fish?' We suggest you start be eating at least 4 helpings of sea fish a week, or supplementing with a sea kelp pill. Next, take 200 mcg of Selenium a week. After a month or so, you could have your iodine levels checked and lower your dose of the synthetic hormone.
We have had a lot of patients who lowered their dose slowly but progressively and eventually came off the drug.
Of course, you should tell your doctor what you are doing. To repeat, you cannot do this if your low thyroid hormone was caused by damage.
Levothyroxine and Bone loss
Elena Ghotbi, M.D., postdoctoral research fellow at Johns Hopkins University School of Medicine in Baltimore, Maryland led a team to review Levothyroxine (Synthyroid) use and found a significant proportion of people taking the drug no longer had hypothyroidism.
In 2024 research (15) the Johns Hopkins team found that long-term use of synthetic thyroid hormone, levothyroxine, was associated with greater loss of total body bone mass and bone density—even in participants whose TSH levels were within the normal range—over a median follow-up period of 6.3 years.
The key questions users of Levothyroxine need to ask themselves are,
"Do I still need Levothyroxine?"
And "If my iodine levels are low, would correcting them reduce my need to take Levothyroxine?"
And "If I have damage (nodules and goitre), is Levothyroxine important or would supplementing with iodine help?"
The Thyroid, hypothyroidism and hyperthyroidism
The thyroid is an organ in the front of the neck and is shaped rather like a butterfly. The thyroid glands principal hormone is thyroxine, or thyroxin, and this has a metabolic-rate-increasing effect on almost all of the tissues of the body.
a) Hyperthyroidism
Hyperthyroism has been linked repeatedly to higher levels of cancer. The University of Maryland has linked high levels of the hormone to excitable, hyperactive children and ADHD.
b) Hypothyroidism
Symptoms - a puffy face, tiredness, fatigue, and low energy, muscle cramps, hair loss, constipation, depression, memory loss and decreased libido.
Since the metabolic rate declines, it is likely that the patient will also gain weight more easily.
Hypothyroidism in extremis can lead to changed (decreased) ovulation patterns in women and even loss of fertility. Equally, a number of pregnant women suffer from the disease but do not notice it, as fatigue is all too common anyway in pregnancy. One in seven women, and one in twenty men develop low thyroid hormone production as they age. Often they don’t even know it. A simple blood test can be used for diagnosis.
The thyroid, in fact, makes two hormones - triiodothyronine (called T3) and thyroxine (T4). They are tyrosine-based hormones and both are partly composed of iodine. Tyrosine itself is a non-essential amino acid derived from the amino acid phenylalanine. It is essential in brain function because it is involved in neurotransmitter production - dopamine, epinephrine and norepinephrine. Tyrosine-kinase has now been linked with cellular energy systems, and cancer regulation.
How hormones thyroxine and oestrogen interact
Your hormones are all linked in a finely balanced web. The technical name for this is homeostasis. Put one out and the rest will soon follow. Hormones interact, that's what they are supposed to do.
Is there also another issue? Maybe synthetic hormones are not good for us. HRT has a lot of research behind it. A study from Oxford University showed that long term use led to a three-fold increase in breast cancer risk, for example. Maybe it's as simple as that. Hormones which can work in parts per trillion they are so effective, notice a synthetic compound when it crosses their path!
Bizarrely, some scientists have argued that the answer is to block the excess thyroid hormone using another synthetic drug! Dr Massimo Cristofanilli, leading the MD Anderson research team stated that: Thyroxine and oestrogen shared many similar pathways in the process of breast cancer. If drugs like Tamoxifen can be used to block the oestrogen pathway (via receptor sites on breast cells) then it might be possible to design drugs to block the thyroxine receptors.
A hormone made in the pituitary gland directly stimulates the thyroid to produce thyroxine. (This hormone, unsurprisingly, is called thyroid stimulating hormone or TSH). Dietary problems may affect production levels of TSH but the issue may be far more complex. Neuropeptides made in response to external factors like stress, anxiety, fear - will arrive in the hypothalamus which directs the pituitary to make hormones. And these affect your thyroid and it controls your metabolism.
Mental state may also be a crucial factor in this Hypothalamus-pituitary-thyroid-breast cancer conundrum and certainly more significant than your doctor's training in this area.
A typical treatment for a woman with low thyroid hormone production might be supplementation using 80 mcg of synthetic thyroxine.
And what prompted this article in the first place was that I was helping three ladies with a Personal Prescriptions and each had breast cancer, each having been prescribed thyroxine at exactly the same dosage level (despite different ages, sizes, weights) for more than five years.
There has been research on exactly this issue. A team of researchers from the Mayo Clinic looked into breast cancer incidence one or more years after women were given synthetic thyroxine and concluded that women taking the supplement were well below normal rates of breast cancer: (47 vs 52.9). However, they compared the thyroxine-supplemented women with normal women and showed the rates were about 10 per cent less. Wouldn’t it have been more accurate to compare the supplement users to other hypothyroid sufferers who were not using supplementation and looked for any increase? Anyway, from the MD Anderson study we would expect a 61 per cent lower figure; but in the Mayo study, the supplemented group turned in a 10 per cent lowered figure. Was the difference due to the synthetic hormone?
Perhaps the oldest study is the most relevant. The Ferdinand Sauerbruch Hospital in Wuppertal, Germany showed that women who took thyroxine had twice the risk of breast cancer. In the research amongst a group of women having screening mammograms - research that everyone seems to have ignored for 30 years - the incidence of breast cancer was twice as high in a group taking synthetic thyroxine for hypothyroidism. (12.1% in supplement group; 6.2% in controls). Worse, the incidence of breast cancer was higher still, where the women had taken synthetic thyroxine for more than 15 years (19.5%)
Synthetic thyroid hormone treatment or simply fix the iodine, which is better?
Few people know that the Hoxsey Therapy had potassium iodide as one of its ingredients. You can check iodine levels easily (a 24 hour urine sample would do it; or a Lugol's oil patch).
There is clear research from the USA that synthetic thyroxine therapy in the presence of iodine deficiency increases the risk of breast cancer and probably thyroid cancer as well. Endocrinologist, Dr. Guy Abraham (formerly of the U.C.L.A. Department of Endocrinology), is an expert who is convinced that everyone needs to be on iodine therapy first until their iodine stores have been properly restored.
Only if that doesn’t work, you should consider synthetic hormone. In a large multicentre German Study on endemic goitre, the researchers showed that (13) the group taking iodine fared better than the group taking the hormone supplement. This followed a 1993 showing that iodine or Levothyroxine performed equally well in goitre patients..
Maybe your first visit for help if you are sluggish with no energy should be to a naturopath who understands gut problems and iodine deficiency.
Radioactive Iodine Treatment linked to cancer, especially breast cancer
A final word!
People who have radioactive iodine treatment, for example for a thyroid issue such as hyperthyroidism, end up with higher levels of radiation in their tissues and increased levels of cancer, particularly breast cancer. This conclusion has come from 2019 research from the National Cancer Institute.
Go to: Overview on breast cancer, symptoms, causes and alternative treatments
* * * * * * * * * *
References:
1. Breast cancer. Relationship to thyroid supplements for hypothyroidism; C C Kapdi, J N Wolfe;JAMA. 1976 Sep 6;236(10):1124-7.
2. Levothyroxine use and the risk of breast cancer: a nation-wide population-based case-control study; Chieh-Chen Wu et al; Arch Gynecol Obstet, 2018 Aug;298(2):389-396
3. Risk of cancer in long-term levothyroxine users: Retrospective population-based study; Chieh Chen Wu et al; Cancer Sci
. 2021 Jun;112(6):2533-2541
4. The Breast-Thyroid Cancer Link: A Systematic Review and Meta-Analysis; Sarah M. Nielsen et al; Cancer Epidemiolo; Vol 25 issue 2, 1 Feb 2016.
5. Breast cancer in association with thyroid disorders; Orhan Turken et al; Breast Cancer Res. 2003; 5(5): R110–R113.
6. Chronic Fatigue syndrome - an explanation - Chronic Fatigue Syndrome
7. Fibromyalgia - causes and treatment - Fibromyalgia.
8. Hashimoto's, causes,and the thyroid - https://chriswoollamshealthwatch.com/your-illness/gut-health/hashimotos-disease/
9. SIBO - How experts get SIBO wrong
10. Cancer risk in patients with Hashimoto's thyroiditis: a nationwide cohort study; Y-K Chen et al; Br J Cancer. 2013 Oct 29; 109(9): 2496–2501.
11. High prevalence of breast cancer in patients with benign thyroid diseases; I Muller et al; J Endocrinol Invest. 2011 May;34(5):349-52.
12. Does hypothyroidism increase the risk of breast cancer: evidence from a meta-analysis; Wang, B., Lu, Z., Huang, Y. et al. BMC Cancer 20, 733 (2020).
13. Treatment of endemic goitre due to iodine deficiency with iodine, levothyroxine or both: results of a multicentre trial; G. HINTZE, D. EMRICH, J. KÖBBERLING; December 1989 https://doi.org/10.1111/j.1365-2362.1989.tb00270.x
14. Levothyroxine treatment is associated with an increased relative risk of overall and organ specific incident cancers – a cohort study of the Swedish population; Per Wundell et al; Cancer Epidemiology Volume 66, June 2020, 101707
15. Common Thyroid Medicine Linked to Bone Loss; RSNA November 25, 2024;
CANCERactive - the appliance of Science