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Selenium and Thyroid Health

Selenium and Thyroid Health

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A trace mineral for major benefit

Sitting at the front of the neck, like a butterfly spreading its wings over the throat, the thyroid is one of the most exposed glands of the endocrine system. Controlling growth, development, and metabolism, the thyroid requires proper nutrition to function and produce hormones appropriately. The changes in our dieting practices over the decades, coupled with environmental exposures and a loss of nutrient density in the soil in which our food grows, have resulted in a dramatic increase in the incidence of thyroid disorders. Thankfully, thyroid health may be optimized in several cases through nutritional support.

In addition to iodine, which is necessary for the formation of thyroid hormone, one nutrient of particular importance for thyroid health is the trace mineral selenium.[1]

Selenium supports thyroid function through a few different mechanisms:

Antioxidant support

Glutathione peroxidase is an antioxidant highly active in the thyroid gland. This enzyme protects the thyroid, among other tissues, from oxidative damage and requires selenium in order to function properly.[2] Selenium deficiency has thus been correlated with reduced thyroid glutathione peroxidase activity,[3] and selenium supplementation has been shown to increase glutathione peroxidase activity. This may help protect the thyroid from the potentially toxic effects of iodine overload,[4] and may even mitigate the risk of thyroid cancer.[5]

T4 to T3 conversion

Selenium is also essential for the proper functioning of the enzyme that turns thyroid hormone into its active form.[6]

Selenium is also essential for the proper functioning of the enzyme that turns thyroid hormone into its active form.

T4, the less active form of thyroid hormone, contains four iodine molecules; T3, the biologically active form of the hormone, contains three. The thyroid gland produces much more T4 than T3 – about four times as much. As the body’s metabolic demands increase, so too do the cells’ need for T3. Under normal circumstances, an enzyme removes an iodine molecule from T4, converting it into T3. This enzyme, known as 5′-deiodinase (pronounced “five prime deiodinase”), requires selenium in order to function properly.[7] Low selenium levels have thus been associated with reduced conversion of T4 to T3 and a lower T3:T4 ratio,[8] which causes symptoms of hypothyroidism like fatigue, dry skin, brittle hair and nails, depression, difficulty losing weight, menstrual irregularities, and high cholesterol.

Although 5′-deiodinase requires selenium to work properly, another de-iodinase enzyme – this one called 5-deiodinase (no prime) – does not. 5-deiodinase converts T4 into reverse T3 (rT3), a hormone that, unlike T3, has no biological activity and thus does not support metabolism.[9] In addition to selenium deficiency, rT3 levels can also rise in the presence of high insulin states (such as in diabetes/metabolic syndrome) and high cortisol levels (as seen with chronic stress).[10]

Decreased antibody production and iodine protection

Selenium supplementation has been shown to dampen the autoantibody response (the process by which the immune system attacks one’s own cells) seen in autoimmune thyroid diseases like Hashimoto’s[11],[12] and postpartum thyroiditis.[13]

Selenium supplementation has been shown to dampen the autoantibody response (the process by which the immune system attacks one’s own cells) seen in autoimmune thyroid diseases like Hashimoto’s and postpartum thyroiditis.

Selenium may also protect the thyroid gland from damage by exposure to high doses of iodine. Selenium has been shown to reduce the incidence of iodine-induced autoimmune thyroiditis and decrease levels of thyroglobulin (Tg) antibodies, the types of antibodies typically elevated in autoimmune hypothyroidism.[14],[15]

In cases of iodine deficiency, selenium supplementation may too be of value, as deficiencies of selenium and iodine commonly co-exist.[16],[17] In fact, selenium deficiency may exacerbate the detrimental effects of iodine deficiency such as stunted physical growth and compromised brain development.[18]

In a study done with elders affected by deficiencies of iodine and selenium, however, selenium supplementation was found to improve glutathione peroxidase activity, but it did not seem to affect thyroid function.[19] These findings suggest that normalizing iodine levels before supplementing with selenium may yield better outcomes with respect to thyroid function.[20]

Sources of selenium

Good dietary sources of selenium include organ meats and seafood. In general there is a wide variation in the selenium content of grains, fruits, nuts, and vegetables, but some plants – like garlic, Brazil nuts, and several Brassica vegetables (the “broccoli family”) – accumulate selenium more than other foods. For this reason, these plant-based foods are known as “selenium accumulators.”[21]

The nutritional content of plants, nuts, and grains, however, varies largely depending on the selenium content of the soil in which they grow. For example, a Brazil nut grown in selenium-poor soil contains an average of 10 mcg (micrograms) – one tenth of the 100 mcg of selenium found in a Brazil nut grown in selenium-rich soil.[22] For this reason, choosing carefully sourced selenium supplements may be of value in supporting optimal thyroid health.

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