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Diet can influence thyroid function
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Hypothyroidism is a condition commonly associated with weight gain, but no established eating plan has been shown to treat underactive thyroid.
However, several micronutrients are known to play a role in improving thyroid function, according to Angela M. Leung, MD, assistant professor of medicine at the UCLA David Geffen School of Medicine and an endocrinologist at both UCLA and the VA Greater Los Angeles Healthcare System.
“Since certain micronutrients are recognized to influence thyroid function, the makeup of a person’s diet can indeed be important when evaluating thyroid disease,” Leung told Endocrine Today. “In addition, specific components in foods can either increase or decrease the ability of these micronutrients, and thus consuming them can also indirectly affect thyroid hormone levels.”
Additionally, future studies will be needed to determine the potential role of certain foods, such as dietary soy products and cruciferous vegetables, on thyroid function.
Leung discussed micronutrients linked to thyroid function, foods that contain these micronutrients, and foods that have been theorized to promote healthy thyroid function. She also emphasized the importance of moderation and a balanced diet for individuals at all levels of thyroid function.
Which nutrients influence thyroid function?
Leung: The micronutrient most understood in relation to thyroid health is iodine, since it is transported into the thyroid follicular cell as a requirement in the thyroid hormone synthesis pathway. The U.S. recommended dietary allowance (RDA) for iodine is 150 µg per day in adults, and 220 µg per day and 290 µg per day in pregnant and lactating women, respectively. Dietary sources of iodine include iodized salt, seafood (including seaweed and fish), and some breads and grains.
Selenium is a micronutrient that may be important for thyroid hormone metabolism. The U.S. RDA of selenium in men and nonpregnant, nonlactating women is 55 µg. Foods containing the highest amounts of dietary selenium are seafood and organ meats, but more common sources in the U.S. diet would be breads, grains, meat, poultry, fish and eggs. Brazil nuts are also rich in selenium, and a single nut provides up to 140 µg. However, the thyroid contains the highest amount of selenium in the body, so thyroid function is unlikely to be adversely affected even in situations of selenium deficiency. Thus, selenium supplementation is not standardly recommended for the purposes of maintaining thyroid health, with the exception of individuals with mild active Graves’ eye disease. In these patients, 200 µg of daily selenium is recommended as a 6-month trial.
Unfortunately, the potential roles of iron, zinc, copper and magnesium in thyroid hormone synthesis and metabolism are much less understood. Some studies have reported associations between serum levels of these micronutrients with thyroid hormone levels, but the data have been inconsistent. Checking the biological levels of these micronutrients is not generally recommended in the evaluation of thyroid disease.
Which foods, if any, are best avoided if one has hypothyroidism, hyperthyroidism, or even normal thyroid function?
Leung: Adequate iodine intake is well-recognized to be important to ensure the normal production of thyroid hormones. Since cruciferous vegetables contain compounds that can inhibit the availability of iodine for the thyroid gland, eating them in high amounts has been theorized to result in hypothyroidism. Examples of cruciferous vegetables include broccoli, cabbage, Brussels sprouts, kale, turnips, cauliflower, collard greens and bok choy.
However, the impact of eating cruciferous vegetables on thyroid hormone production has not been rigorously studied, and it is generally unknown what quantity of these vegetables one would have to ingest to induce hypothyroidism. In one extreme case report, one elderly woman who ate 1.5 kg of raw bok choy daily for several months presented to her local ED in myxedema coma, which was thought to result from the cruciferous vegetable ingestion.
From the available evidence, it is important to encourage a well-rounded diet, which includes vegetables for their overall health benefits, but not in excessive quantities. This pertains to both the general population and also to individuals with known hypothyroidism.
Dietary soy products, including soy milk, tofu, soy sauce, tempeh and miso, contain isoflavones in variable quantities, depending in part on whether the product has been fermented. Through the inhibitory action of isoflavone on thyroid peroxidase, an important component in the thyroid hormone synthesis pathway, it is possible that soy consumption may also potentially result in hypothyroidism. Many studies have not demonstrated adverse effects of soy consumption, but generally speaking, this is a topic that requires much greater study. It is important to ensure adequate iodine nutrition in those who consume a high amount of soy products.
Although other dietary factors, such as coffee, tea and alcohol, appear to have no effect on thyroid cancer risk, the relationships between consumption of these items and thyroid health has also not been well-studied. It is important to note that coffee does decrease the absorption of oral levothyroxine in individuals being treated for hypothyroidism, and thus this medication should be taken separately before drinking coffee.
Finally, the potential benefit of vitamin D as a preventive or therapeutic agent for various thyroid diseases remains unclear and requires further research.
Are there dietary supplements that affect thyroid health, either positively or negatively?
Leung: Recommendations as guided by the U.S. RDA should be followed for all micronutrients. In particular, consuming excess iodine can result in hypothyroidism or hyperthyroidism through well-established physiologic mechanisms and should be avoided. This is important since some dietary supplements may contain very high levels of iodine, including those that are purely iodine-containing and/or kelp-derived. It is not recommended to take any supplements that contain greater than 500 µg of iodine per daily serving, and clinicians should always ask patients if they are taking any such supplements. – by Jennifer Byrne
For more information:
Angela M. Leung, MD, can be reached at amleung@mednet.ucla.edu.
Disclosure: Leung reports no relevant financial disclosures.