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Levothyroxine and Breast Cancer

Levothyroxine and Breast Cancer

The link between Levothyroxine and Breast Cancer comes from the idea that people with a high metabolic rate as patients using levothyroxine have increased production of oxygen free radicals or ROS (reactive oxygen species).

Reactive oxygen species can damage the DNA and result in DNA mutations that could increase the chances of breast cancer.

In addition, chronic conditions like hypothyroidism and hyperthyroidism may be associated with increased production of inflammatory mediators that can also result in DNA damage.

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What is Levothyroxine?

Levothyroxine is used to treat an underactive thyroid gland (hypothyroidism). Thyroid hormones are produced by the thyroid gland and aid in regulating growth and energy levels.

Levothyroxine of T4 is one of the most commonly used medications and is among the WHO’s list of essential and life-saving medicines.

Untreated hypothyroidism may increase one’s chances of:

  • Atherosclerosis
  • Hypertension
  • Heart diseases
  • Dementia
  • Impaired neuronal conduction (neuropathy)
  • Infections
  • Obesity and Weight gain
  • Obstructive sleep apnea
  • Impaired hearing

Whether Levothyroxine or Hypothyroidism is associated with breast cancer or not is unclear. To understand the connection between Levothyroxine and Breast Cancer, more studies are needed.

Data that is available is presented here.

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Is there a Link Between Breast Cancer and Levothyroxine?

A study from 2018 [ref]

This particular study analyzed the use of levothyroxine and its connection with breast cancer. According to this research, the link between levothyroxine and an elevated risk of breast cancer was evident.

However, In order to evaluate the impact of levothyroxine usage on the risk of breast cancer. They also suggested that it is necessary to conduct a bigger, more prolonged, prospective, randomized-controlled experiment.

A study from 2020 [ref]

Only this particular study suggested that the use of levothyroxine is associated with a lower chance of breast cancer, exactly 11% lower.

However, the mechanism through which levothyroxine lowered the risks is unknown. The limitations of this study were that it was unable to justify whether the lowered risk was due to the use of levothyroxine or due to hypothyroidism.

A study from 2021 [ref]

Since levothyroxine is a drug that is frequently administered to treat an underactive thyroid, its usage and the likelihood of developing cancer is mostly underestimated.

The findings of this study demonstrated a substantial link between the usage of levothyroxine and an elevated risk of cancer, specifically brain, skin, pancreatic, and female breast cancer.

Levothyroxine is still a very effective treatment for hypothyroidism, therefore doctors should carefully evaluate it and closely monitor their patients’ conditions to prevent adverse effects.

Additional research is required to verify these results and assess the underlying biological causes.

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Is there a link between hypothyroidism and breast cancer?

Since levothyroxine is a medication that is used to treat hypothyroidism it is crucial to see if there is a connection between breast cancer and an underactive thyroid gland.

When the thyroid gland does not generate enough thyroid hormone, hypothyroidism, or an underactive thyroid, develops.

Weight gain, exhaustion, constipation, melancholy, dry hair, and a sluggish heartbeat are all signs of an underactive thyroid. Similar to hyperthyroidism, it affects women more frequently than males.

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According to Harvard Health Publishing

​​Researchers examined a large cohort of Danish women. Subjects were diagnosed with thyroid disorders between 1978 and 2013.

In order to ascertain if having an overactive or underactive thyroid impacts a woman’s risk of breast cancer.

More than 60,000 and more than 80,000 of the research subjects, respectively, had thyroid conditions that resulted in under- or overactivity.

They observed the patients for five to seven years and discovered that those with an overactive thyroid had a somewhat elevated risk of breast cancer – to be exact, an 11% greater risk.

On the other hand, the risk of breast cancer was reduced by 6% in women with an underactive thyroid.

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According to a study from 2019 [ref]

Another study analyzed the link between an underactive thyroid gland and breast cancer. According to this prospective cohort analysis, hypothyroidism that was present at the time of diagnosis or that developed incidentally during follow-up is not linked to a recurrence of breast cancer or a higher risk of death from any cause.

From a clinical standpoint, this is comforting for hypothyroidism patients. And their doctors as well, who may emphasize that hypothyroidism is unlikely to have a negative effect on the clinical course of breast cancer or its survival.

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Final Verdict

Hypothyroidism, if left untreated can result in significant morbidities and even death. It can cause obesity, weight gain, hypertension, heart diseases, memory impairment, nerve damage, and fluid in the lungs, abdomen, and around the heart.

Hypothyroidism should never be left untreated. However, overdosing on Levothyroxine and the use of Levothyroxine for weight loss or increased metabolic rate, work performance, and memory should be avoided.

Since most studies mentioned above were observational studies, data linking the association between Levothyroxin and Breast Cancer is insufficient.

People using Levothyroxine should not discontinue their treatment or reduce their doses as untreated hypothyroidism can make a person very ill.

The risk of breast cancer with Levothyroxine may be a thousand times lower than the risks of developing atherosclerosis and heart diseases due to hypothyroidism.

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What do you think?

Written by Dr. Ahmed

I am Dr. Ahmed (MBBS; FCPS Medicine), an Internist and a practicing physician. I am in the medical field for over fifteen years working in one of the busiest hospitals and writing medical posts for over 5 years.

I love my family, my profession, my blog, nature, hiking, and simple life. Read more about me, my family, and my qualifications

Here is a link to My Facebook Page. You can also contact me by email at or at My Twitter Account
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