Effects of Green Tea on the Thyroid

Green tea is considered one of the healthiest beverages in the world.

The amazing health benefits it provides make green tea a common ingredient in dietary supplements too. People drink the tea or use supplements that contain it for a wide range of reasons.

Some people drink tea to support their health while others to avoid coffee.

Ask any tea lover and they’ll tell you green tea is on the very top of their favorite tea list. This hot beverage improves a number of functions in our body, but how does it affect your thyroid?

Let’s see.

Brief overview of green tea

Green tea has been dried without fermenting and the entire process has minimum oxidation.

It is made from the leaf Camelia Sinensis, an evergreen shrub that is also used for white tea, black tea, and other types of tea.

The liquor after brewing the beverage is green which explains its green (and sometimes green yellowish color).

The tea originated in China, but today it is used across the globe. Some records show that the use of tea leaves originated in the southwest area of China more than 3000 years ago.

That said, the real history of green tea starts in the 8th century with the discovery of a method that steams the leaves to inhibit their oxidation. Four hundred years later, in the 12th century, the new method of fixing the leaves was introduced[i].

The reason why green tea has been used for thousands of years is its incredible health potential which stems from abundant nutritional content that includes proteins (15-20% dry weight), amino acids, calcium, chromium, magnesium, manganese, copper, iron, selenium, zinc, molybdenum, phosphorus, sodium, potassium, linoleic acid (LA), alpha linoleic acid (ALA), vitamins B, C, E[ii], and many other healthy compounds.

Green tea is about 30% polyphenols by weight including large amounts of catechin called epigallocatechin gallate (EGCG). Catechins are active ingredients in green tea that function as antioxidants and provide many other health effects.

They are responsible for a majority of health benefits of green tea.

Thyroid and green tea

Even though a multitude of studies investigated properties and effects of green tea, not many of them focused on its relationship with thyroid.

Is green tea good or bad for the function of the butterfly-shaped gland? We aimed to find out.

Combats tissue injury

Green tea is a rich source of powerful antioxidants that protect our body from free radicals and oxidative stress. One study aimed to investigate the protective effects of green tea in the possible toxic effect of Tributyltin on thyroid follicular cells (cells in the thyroid gland that are responsible for production and secretion of T3 and T4 hormones).

Tributyltin is the main ingredient of biocides used to control a broad spectrum of organisms.

Common uses of Tributyltin include wood treatment and preservation, antifouling of boats, antifungal actions in textiles and industrial water systems, just to name a few. This particular compound is also one of the most widespread contaminants that accumulate in our food chain.

Tributyltin disrupts the endocrine system and exhibits anti-thyroid properties.

Scientists from Egypt found that Tributyltin toxicity induces oxidative stress that disrupts the function of the thyroid, but the administration of green tea extract was successful in combating tissue injury that occurred due to toxicity[iii].

In other words, the study revealed that green tea protects tissue injury caused by oxidative stress. Of course, it owes these benefits to potent antioxidants.

Helps manage thyroid-associated ophthalmopathy

Thyroid-associated ophthalmopathy (TAO) is an autoimmune disorder characterized by enlargement of extraocular muscles and increase in fatty or connective tissue volume.

The condition is usually associated with hyperthyroidism and Graves’ disease, but it can also affect euthyroid persons and individuals with hypothyroidism[iv].

Lee J.Y. et al carried out a study which found that EGCG suppresses the IL-1β-induced expression of IL-8 through inhibition of NF-kB, p38, and ERK pathways.

What are all these symbols?

To elaborate:

  • IL-1β – key mediator of inflammatory response
  • IL-8 – released from several cell types during inflammation
  • NF-kB – pro-inflammatory signaling pathway
  • p38 – activated by inflammatory cytokines
  • ERK – signaling cascade that plays a crucial role in various cell regulatory processes

Scientists concluded the study explaining that EGCG from green tea could be used a pharmacological agent for treatment of TAO[v].

Suppression of thyroid hormones and anti-thyroid activity

Even though green tea exhibits protective effects under some circumstances, it can also have a negative effect on the butterfly-shaped gland.

Chandra A.K. et al published a study wherein they reported that green tea catechins at the concentrations of 0.10, 0.20, and 0.30 mg/ml lead to decrease in the activities of thyroid peroxidase (levels of antibodies) and 5’-deiodinase I (catalyzes the inactivation of thyroid hormone).

Also, green tea’s catechins led to a substantial decrease in T3 and T4 levels accompanied by a significant elevation of serum TSH.

These findings indicate that green tea flavonoids possess an anti-thyroid activity[vi].

The suppression of thyroid hormones leads to hypothyroidism.

In persons with this common thyroid disorder, symptoms could aggravate the hormonal imbalance and symptoms associated with it.

Sakamoto Y. et al discovered that dietary administration of green tea catechins at high doses induced goiters in rats. Scientists elaborated that goitrogenic effects are a result of anti-thyroid activity of green tea[vii].

Why does green tea have anti-thyroid effects?

The answer is in fluoride. Green tea has high levels of fluoride which is why it can disturb the function of thyroid gland.

This happens because EGCG absorbs fluoride from the environment and releases it into your body. Fluoride can displace iodine, an important nutrient essential for thyroid health. Impaired absorption of iodine doesn’t allow thyroid to function properly and it contributes to the disrupted production of its hormones.

Basically, the oldest leaves contain the highest levels of fluoride.

Why does fluoride displace iodine in the first place?

Fluoride and iodine are both halogens. The fluoride, the negative ion of the element fluorine easily displaces iodine in the body because it is much lighter and therefore more reactive. In fact the activity of any one of the halogens is inversely proportion to its atomic weight.

In other words, one halogen can displace another one of a higher atomic weight but cannot displace one of lower weight thereby, results fluoride- thyroid-iodine antagonism which in turn lead to interference with iodine uptake.

Interestingly, fluoride competes for the receptor sites on the thyroid which respond to TSH. What happens next? The hormone reaches a patient’s thyroid gland and fewer hormones are manufactured[viii].

Thyroid cancer

Thyroid cancer occurs in the cells of the thyroid gland and although it’s not in the group of the most common cancers in the United States, the rates are increasing. According to the estimates published by the American Cancer Society, about 53,990 new cases of thyroid cancer will be diagnosed in 2018. Of these, 40,900 will affect women and 13,090 men. This year, it is expected that 2060 people will die from thyroid cancer. Women will account for 1100 thyroid cancer deaths while men 960[ix].

Although green tea can protect our body from different forms of cancer, evidence shows it can increase the risk of thyroid cancer. Michikawa T. et al conducted a study whose purpose was to analyze the influence of green tea on thyroid cancer.

For the purpose of the study, scientists analyzed data from a prospective cohort of 100,507 persons (48,802 men and 51,705 women). Participants were between 40 and 69 years old. They were asked to complete a questionnaire about their green tea and coffee consumption.

Findings showed that premenopausal women who drank 5 or more cups of green tea were more likely to develop thyroid cancer. Surprisingly, postmenopausal women who consumed 5 or more cups of green tea had a lower risk of this severe disease. Scientists found no link between green tea intake and thyroid cancer risk in men[x].

Should I avoid green tea?

It’s not uncommon to come across websites that claim you should avoid green tea if you have some thyroid disorder or have to take thyroid hormone replacement. Studies that were carried out on this subject usually involved high doses and they primarily focused on rats.

In order to determine whether thyroid patients should avoid green tea, it is essential to conduct more studies on this topic.

At this point, there is no evidence that one cup, a moderate intake of green tea, could harm your thyroid and affect its function.

One would have to drink high doses of green tea every day to disrupt thyroid function.

Medical treatment guidelines do not prohibit you from consuming green tea if you’re taking thyroid medication. T

he best thing to do is to consult your doctor. Based on your condition and severity of thyroid disorder, the physician will inform you whether it’s safe to drink green tea or not.

Green tea benefits

Green tea isn’t considered the healthiest beverage in the world for no reason. Let’s take a closer look at some of its most potent health benefits:

  • Improves brain function – green tea contains an amino acid called l-theanine which crosses the blood-brain barrier[xi]. The amino acid increases the activity of inhibitory neurotransmitter GABA which has anti-anxiety effects and it improves dopamine levels
  • Weight loss – green tea is on the ingredient list of a vast majority of weight loss and fat burning supplements. That’s because it speeds up metabolism, increases fat burning and allows you to slim down successfully. Evidence shows green tea catechins prevent overweight and obesity[xii]. This is particularly important if you bear in mind that thyroid disorders, such as hypothyroidism, can lead to weight gain
  • Lower diabetes risk – green tea improves insulin sensitivity and reduces blood sugar levels[xiii]
  • Heart health – green tea increases antioxidant capacity of the blood, thus protecting LDL particles from oxidation[xiv] – one of the most common pathways that lead to heart disease
  • Immunomodulating effects – evidence supports the use of EGCG as a potential therapeutic agent in preventing and ameliorating T cell-mediated immune diseases[xv]


Green tea has been used in traditional Chinese medicine for centuries. Today, this tea is available all over the world and it is considered as one of the healthiest beverages. Indeed, green tea can boost weight loss, support heart health, lower the risk of diabetes, decrease cancer risk, and so much more.

The relationship between green tea and thyroid is complex and poorly studied.

Current evidence shows conflicting results. In some cases, green tea exhibits protective effects, but it can also exhibit goitrogenic effects and suppress production of thyroid hormones. Green tea is an abundant source of fluoride which displaces iodine.

However, this doesn’t mean you should avoid green tea.

More research on humans is necessary to elucidate the effects of this tea on our thyroid. Moderate consumption is considered generally safe, but you should consult your doctor anyway.


[i] Chinese tea history part I – Green tea history, Teavivre.com https://www.teavivre.com/info/green-tea-history.html

[ii] Chacko SM, Thambi PT, Kuttan R, Nishigaki I. Beneficial effects of green tea: A literature review. Chinese Medicine. 2010;5:13. doi:10.1186/1749-8546-5-13. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2855614/

[iii] Badr El Dine FMM, Nabil IM, Dwedar FI. The effect of Tributyltin on thyroid follicular cells of adult male albino rats and possible protective role of green tea: a toxicological, histological, and biochemical study. Egyptian Journal of Forensic Sciences 2017:7(1):7. Doi: 10.1186/s41935-017-0012-z https://www.ncbi.nlm.nih.gov/pubmed/28781899

[iv] Thyroid ophthalmopathy, American Academy of Ophthalmology  http://eyewiki.aao.org/Thyroid_Ophthalmopathy

[v] Lee JY, Paik JS, Yun M, et al. The effect of (-)-epigallocatechin-3-gallate on IL-1β induced IL-8 expression in orbital fibroblast from patients with thyroid-associated ophthalmopathy. PLoS One 2016 Feb 5;11(2):e0148645. Doi: 10.1371/journal.pone.0148645 https://www.ncbi.nlm.nih.gov/pubmed/26848751

[vi] Chandra AK, De N. Catechin induced modulation in the activities of thyroid hormone synthesizing enzymes leading to hypothyroidism. Molecular and Cellular Biochemistry 2013 Feb;374(1-2):37-48. Doi: 10.1007/s11010-012-1503-8 https://www.ncbi.nlm.nih.gov/pubmed/23117228

[vii] Sakamoto Y, Mikuriya H, Tayama K, et al. Goitrogenic effects of green tea extract catechins by dietary administration in rats. Archives of Toxicology 2001 Dec;75(10):591-6 https://www.ncbi.nlm.nih.gov/pubmed/11808919

[viii] Singh N, Verma KG, Verma P, Sidhu GK, Sachdeva S. A comparative study of fluoride ingestion levels, serum thyroid hormone & TSH level derangements, dental fluorosis status among school children from endemic and non-endemic fluorosis areas. SpringerPlus. 2014;3:7. doi:10.1186/2193-1801-3-7. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3890436/

[ix] Key statistics for thyroid cancer, American Cancer Society https://www.cancer.org/cancer/thyroid-cancer/about/key-statistics.html

[x] Michikawa T, Inoue M, Shimazu T, et al. Green tea and coffee consumption and its association with thyroid cancer risk: a population-based cohort study in Japan. Cancer Causes and Control 2011 Jul;22(7):985-93. Doi: 10.1007/s10552-011-9771-2 https://www.ncbi.nlm.nih.gov/pubmed/21562752

[xi] Nobre AC, Rao A, Owen GN. L-theanine, a natural constituent in tea, and its effect on mental state. Asia Pacific Journal of Clinical Nutrition 2008;17 Suppl 1:167-8 https://www.ncbi.nlm.nih.gov/pubmed/18296328

[xii] Diepvens K, Westerterp KR, Westerterp-Plantenga MS. Obesity and thermogenesis related to the consumption of caffeine , ephedrine, capsaicin, and green tea. American Journal of Physiology 2007 Jan;292(1):R77-85. Doi. 10.1152/ajpregu.0032.2005 https://www.ncbi.nlm.nih.gov/pubmed/16840650

[xiii] Liu K, Zhou R, Wang B, et al. Effect of green tea on glucose control and insulin sensitivity: a meta-analysis of 17 randomized controlled trials. American Journal of Clinical Nutrition 2013 Aug;98(2):340-8. Doi: 10.3945/ajcn.112.052746 https://www.ncbi.nlm.nih.gov/pubmed/23803878

[xiv] Yang TTC, Koo MWL. Inhibitory effect of Chinese green tea on endothelial cell-induced LDL oxidation. Atherosclerosis 2000 Jan;148(1):67-73. Doi: 10.1016/s0021-9150(99)00239-7 http://www.sciencedirect.com/science/article/pii/S0021915099002397

[xv] Pae M, Wu D. Immunomodulating effects of epigallocatechin-3-gallate from green tea: mechanisms and applications. Food and Function 2013 Sep;4(9):1287-303. Doi: 10.1039/c3fo60076a https://www.ncbi.nlm.nih.gov/pubmed/23835657

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