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A breast cancer diagnosis is one of the most common types of cancer globally. According to the World Health Organization (WHO), there were 2.3 million diagnosed patients with breast cancer in 2020. Thankfully, treatments have made remarkable progress over the years, leading to more breast cancer survivors living healthy lives after their treatment. As breast cancer survivors celebrate their victories over the disease, we must be aware of the potential long-term health complications that can arise post-treatment. One primary concern is the increased risk of developing thyroid disorders. Let’s examine how various treatments can be a risk factor for hypothyroidism.

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To understand the link between breast cancer treatment and the increased risk of hypothyroidism in breast cancer survival, let’s begin by looking at hypothyroidism and how it can impact the body.

Hypothyroidism – an underactive thyroid – occurs when the thyroid gland fails to produce sufficient thyroid hormones. These hormones regulate the body’s metabolism, energy production, and overall well-being. A diagnosis of hypothyroidism is made with blood tests that measure key hormones, including TSH (thyroid-stimulating hormone), free T4 (thyroxine), free T3 (triiodothyronine), and thyroid peroxidase (TPO) antibodies.

There are three main types of hypothyroidism:

• Overt primary hypothyroidism: characterized by a high TSH and low free T4 concentration.
• Subclinical hypothyroidism: characterized by a high TSH and normal free T4 concentration.
• Central hypothyroidism: characterized by a low T4 concentration and a TSH concentration that may not be accurate (TSH might be anywhere between low and elevated). Central hypothyroidism is typically caused by hypothalamic disease or pituitary disease.

Patients with hypothyroidism can experience a variety of symptoms that vary between individuals. Some symptoms may not even be noticeable early in the thyroid disease process. The most common symptoms include:

• Fatigue
• Weight gain
• Cold intolerance
• Hair loss
• Depression
• Constipation
• Muscle weakness
• Memory problems
• Abnormal menstrual cycles

It is crucial to remember that if left untreated, hypothyroidism can lead to other health concerns such as high cholesterol, kidney disease, heart disease, and fertility issues.

Your Guide To The Genetics Of Thyroid Function

Knowing your risk for thyroid disease can help you stay in front of complications. Learn in this guide how genes impact your risk for hypothyroidism.

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Previous studies and researchers have looked into how breast cancer treatment can lead to hypothyroidism in women. Let’s explore specific treatments to find out why.

Radiation therapy

One of the primary treatments for breast cancer is radiation therapy, which involves using targeted high-energy X-rays to destroy cancer cells. Unfortunately, the radiation can also affect the neighboring thyroid gland, causing damage that may lead to hypothyroidism after treatment. A recent research article found that women with breast cancer who received radiation therapy had a significantly higher risk of developing thyroid diseases, such as hypothyroidism, than those who did not. The patients at the highest risk of hypothyroidism received a more aggressive treatment with radiation targeting the supraclavicular lymph nodes.

Chemotherapy

Chemotherapy is another prevalent type of treatment for patients with breast cancer. While chemotherapy primarily aims to target cancer cells, one of the risks of breast cancer therapy is that it can inadvertently affect the thyroid gland, disrupting normal function and increasing the risk for hypothyroidism. A 2011study recommended monitoring thyroid function in breast cancer patients closely during and after chemotherapy treatments to facilitate early detection and treatment of hypothyroidism and improvement in quality of life.

Hormone therapy

Hormone therapy is often used to treat hormone receptor-positive breast cancer (breast cancer influenced by estrogen and progesterone). Medications like tamoxifen and aromatase inhibitors are commonly prescribed and have been found to be effective in preventing cancer recurrence. However, a subsequent risk is that it may also affect thyroid function, increasing the risk of hypothyroidism. For example, this 2023 article reported a higher incidence of hypothyroidism in patients with breast cancer treated with tamoxifen.

Immune checkpoint inhibitors (ICIs)

Newer cancer treatments called immune checkpoint inhibitors (ICIs) have significantly impacted how we treat cancer, including women with breast cancer. However, these treatments can sometimes cause problems with the immune system, with thyroid disorders being one of ICI treatment's most common side effects. Breast cancer patients starting an ICI must learn about the signs and symptoms of thyroid disorders and keep an eye out for them both during and after completion of treatment and upon entering into survivorship.

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Due to the increased risk of hypothyroidism in breast cancer survivors, it’s important to check thyroid function regularly with a complete thyroid test panel. This can help with the early detection of any thyroid abnormalities. Early detection can lead to appropriate treatment with thyroid hormone replacement medication, which can help ease symptoms and restore normal thyroid function.

Breast cancer survivors deserve recognition and celebration for their strength and resilience in overcoming this challenging disease. When we increase awareness of potential long-term health consequences of breast cancer treatment -- including increased risk of hypothyroidism and more -- we can help minimize the impact and ensure survivors’ continued well-being and quality of life.

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Given the relationship between breast cancer treatment and hypothyroidism, Paloma Health encourages survivors to get routine thyroid screenings regularly.

Paloma Health’s comprehensive at-home thyroid testing kit gives you a complete picture of your thyroid function. The kit tests for four common thyroid markers – TSH, free T4, free T3, and TPO antibodies – and only requires a painless finger prick blood sample. You’ll quickly get your confidential results and a personalized analysis online.

Paloma Health’s support for your thyroid health doesn’t stop with test results. You can also schedule a telemedicine appointment with one of our team of top thyroid specialists. Our knowledgeable practitioners can further help you understand your results and develop a plan for optimal treatment of your hypothyroidism.

Free T3

Free T3 is a hormone produced by the thyroid gland. Levels of this fluctuate when individuals have an under or overactive thyroid gland.

TSH

Thyroid-stimulating hormone, TSH, is the hormone responsible for controlling hormone production by the thyroid gland. The hormone TSH is considered the most sensitive marker for screening for thyroid diseases and conditions. Our thyroid panel is ideal for TSH testing at home and will tell you how your levels compare to normal TSH levels.

TPO Antibodies

Thyroid peroxidase antibodies are antibodies that can bind to thyroid enzymes, suppressing thyroid function. They are elevated in a condition called Hashimoto's disease, which is the most common type of hypothyroidism in the USA.

Free T4

Free T4 is the predominant hormone produced by the thyroid gland. Levels fluctuate when individuals have an under or overactive thyroid gland. Testing your free T4 with this thyroid function test lets you see if your thyroid hormone production is at a normal level.

Reverse T3

Add-on

RT3 is a metabolite of T4. Typically, when T4 loses an atom of iodine—a process known as monodeiodination—it becomes (T3), the active thyroid hormone.The body also converts T4 into rT3, which is an inactive form of T3 that is incapable of the metabolic activity that is normally carried out by T3.

Vitamin D

Add-on

Some observational studies have found low blood levels of vitamin D in patients with hypothyroidism and Hashimoto’s as well as hyperthyroidism due to Gravesʼ disease. It is not clear from these studies if low vitamin D is a cause, a consequence or an innocent bystander in the development of these common thyroid conditions.

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Can breast cancer treatment cause hypothyroidism?

Yes. Radiation therapy, chemotherapy, hormone therapy such as tamoxifen, and immune checkpoint inhibitors have all been linked to an increased risk of hypothyroidism in breast cancer patients and survivors.

Why does radiation therapy for breast cancer increase the risk of hypothyroidism?

Radiation aimed at breast cancer can also affect the nearby thyroid gland. Research has found that patients who received radiation targeting the supraclavicular lymph nodes had the highest risk of developing hypothyroidism.

Do immune checkpoint inhibitors affect the thyroid?

Yes. Thyroid disorders are one of the most common side effects of immune checkpoint inhibitor treatment, so patients starting this therapy should watch for signs and symptoms of thyroid problems during and after treatment.

How often should breast cancer survivors get their thyroid checked?

Because of the increased risk of hypothyroidism, breast cancer survivors are encouraged to get routine thyroid screenings with a complete thyroid test panel to catch any abnormalities early.

How is hypothyroidism in breast cancer survivors treated?

Once diagnosed through blood tests measuring TSH, free T4, free T3, and TPO antibodies, hypothyroidism is typically treated with thyroid hormone replacement medication, which can ease symptoms and restore normal thyroid function.