Quercetin Side Effects for Thyroid

Can quercetin affect thyroid?


Quercetin is an antioxidant found in many fruits and vegetables and belongs to a group of plant pigments called flavonoids.

More than 6,000 types of phytonutrients are found in this group.

This bioflavonoid is well-known for many things, including its antioxidant properties, anti-cancer effects, and anti-allergy properties, but also the fact that it interferes with some hormone metabolism (especially thyroid and estrogen hormones) and affects the activity of different enzymes.

It is usually concentrated in the skins and outer areas of vegetables and fruits, and it can be damaged by the cooking process, particularly at high temperatures.

Therefore, try to avoid grilling your veggies because they will have a lower amount of flavonoids than raw or steamed.

Side effects of quercetin Quercetin Side Effects for Thyroid

Important note – these side effects start to arise when individuals intake large doses of this antioxidant in a supplement form.

Thyroid hormone

The thyroid gland is the most important endocrine gland in the neck area, and its role is to produce the thyroid hormone.

This essential hormone has an effect on almost all tissues of the body.

According to a 2014 study, ingesting this flavonoid as a supplement can reduce the activity of thyroperoxidase, an important enzyme that is part of the production of the thyroid hormone. In addition, it can reduce the activity of the hepatic deiodinase, a liver enzyme important in the activation of the thyroid hormone.


More than 80% of individuals taking supplements have an allergy, that tends to manifest with the same symptoms as a food allergy, such as nasal complications (which can lead to a sinus infection), fatigue, skin rashes, difficulty breathing, headaches, asthma, anaphylactic shock, or swelling of the lips, mouth, tongue, or throat.


It inhibits an important enzyme (CYP3A4) found in the intestine and in the liver and is involved in the metabolism of many prescription medications, like fluoroquinolones, an antibiotic used in the treatment of certain bacterial infections.

Kidney damage

People suffering from kidney disease should not take this supplement, particularly doses greater than 1 gram per day, because it may interfere with kidney health.

Other side effects include – upset stomach, migraines, excessive sweating, tingling of the hands, and nausea. More importantly, pregnant and breastfeeding women should avoid the supplement.

Top foods with the highest content of quercetin Quercetin Side Effects for Thyroid

This antioxidant is commonly found in deeply colored vegetables and fruits.

Note – organically produced fruits and vegetables contain more flavonoids.

Fruits – apples, berries (cranberries, chokeberry, lingonberries, rowanberries, crowberries, blueberries, raspberries, bilberries), grapes, cherries, tomatoes, peppers (all colors), plums.

Vegetables – red onions, broccoli, spinach, radish, asparagus, cabbage, kale, sweet potato.

It is also found in whole grains (especially buckwheat), black and green tea, beans (red and green lentils, red kidney beans, mung beans, chickpeas, carob, cocoa), herbs (sage, parsley, dill, fennel, St. John’s wort, American elder, Ginkgo Biloba).

Health benefits

Fights free radicals

Free radicals can build up in cells and are derived either from external sources such as exposure to cigarette smoking, X-rays, air pollutants, ozone, and industrial compounds or from normal metabolic processes in the human body.

Free radicals adversely alter proteins, lipids, and DNA and trigger many human diseases. Considered one of the most abundant and powerful antioxidants in the human diet, this bioflavonoid plays an essential part in fighting free radical damage and inflammation.

Cancer prevention

Cancer has a major impact on society across the world, and in the U.S. It is considered that approximately 39.6% of women and men will be diagnosed with cancer at some point during their lifetimes.

This antioxidant acts by freezing replicating cancer cells, thus trapping them at a non-productive phase of the cell reproduction cycle. Additionally, it may suppress the activity of the mutant P53 gene, a characteristic in the majority of human cancers.

Cardiovascular health

Cardiovascular disease is the main global cause of death (around 30% of all deaths). In 2009 researchers at the Institute of Human Nutrition and Food Science, in Germany, have concluded that this bioflavonoid protects against bad cholesterol oxidation.

Therefore, this may be extremely beneficial because oxidation causes LDL cholesterol to stick to artery walls which can lead to hypertension and other serious health problems.


Diabetes is a chronic disease that appears when your blood sugar (blood glucose) is higher than normal.

For example, in the U.S., about 8.3% of the population has diabetes – that’s 25.8 million patients.

This antioxidant helps to reduce symptoms of diabetes patients, according to a 2013 study, which established that consuming foods high in this flavonoid improves plasma insulin levels and reduces blood sugar levels in streptozotocin-induced diabetic rats.

This implies that this antioxidant is a good method to prevent diabetic vascular complications in both insulin resistance (type 2 diabetes) and deficiency (type 1 diabetes).

ALSO READ: Foods High In Lectins – Complete List

Contact dermatitis

Contact dermatitis causes the skin to become blistered, red, dry, and cracked.

Symptoms can affect any part of the body, but most usually, they affect the face and hands.

A study in 2012 at the Department of Molecular Physiology and Pharmacology, Boston, USA, concluded that it is effective at inhibiting redness caused by contact dermatitis.

Weight loss

Obesity and overweight are defined as excessive or abnormal fat accumulation that may have adverse effects on health.

According to a recent study, obesity is responsible for 500,000 cancer deaths worldwide per year.

Combining this flavonoid with resveratrol (a potent polyphenol, especially found in the skins of red grapes) is a powerful way to induce apoptosis (cell death) of adipocytes (fat cells) and to stop adipogenesis.

This would lead to fat reduction. Moreover, it increases levels of adiponectin (a hormone that lowers your appetite and increases the rate at which your body breaks down fat).

Dosage recommendations

This supplement is usually available in capsule and pill form and is commonly packaged with bromelain (an enzyme mainly found in pineapples) which increases its bioavailability.

Therapeutic dosages can range from 250 to 500mg 3 times/day.

Bottom line

Your best bet is to just eat a wide variety of fruits and vegetables and consider intaking the supplementation form only after a consult with your doctor.

Image source – https://pixabay.com/photos/groceries-fruit-vegan-soy-food-1343141/

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