A 2017 summary of the research literature supports the idea that optimal selenium levels are needed for antioxidant protection against harmful free radicals in the thyroid gland and for the normal metabolism of thyroid hormones [Ventura 2017].
A 2018 research literature review shows that selenium supplementation can reduce anti-thyroperoxidase antibody levels and can improve thyroid ultrasound features. In addition, selenium supplementation is associated with improved symptoms and improved quality of life in patients with Graves orbitopathy [Santos 2018].
Note: Anti-thyroperoxidase antibody levels that are too high (>500 IU/ml) are associated with an increased risk of hypothyroidism in autoimmune thyroiditis, which is the most common form of thyroid disorder [Ehlers 2016].
Note: Graves orbitopathy (aka Graves ophthalmopathy) is the pathology of the bony cavity and tissues around the eye. It can occur in up to 50% of patients with Graves’ disease, which is an autoimmune disease that is the most common cause of hyperthyroidism.
Low Selenium Status and Autoimmune Thyroid Diseases
Autoimmune thyroid diseases affect ca. 2% of the world population, women more than men. Santos et al  state that Graves’ Disease (hyperthyroidism) and Hashimoto Thyroiditis (hypothyroidism) are the most common forms of autoimmune thyroid diseases. Hashimoto’s disease is approximately ten times more common than Graves’ disease.
Low plasma and serum selenium concentrations (below 70 micrograms per liter) have been reported in patients with autoimmune thyroid diseases. Selenium supplementation is thought to modify inflammatory and immune responses in patients with autoimmune thyroiditis [Santos 2018].
Selenium and Selenoproteins and Autoimmune Thyroiditis
Selenium is a key component of thyroid hormone metabolism. The thyroid has the highest concentration of selenium of all tissues (0.2-2.0 micrograms per gram) [Rayman 2012; Ventura 2017].
Selenium is rarely found in its elemental form in the body. Instead, as a component of the 21st amino acid, selenocysteine, it is incorporated into 25 known selenoproteins. The biological functions of selenium take effect primarily through the actions of these selenoproteins.
Most of the selenoproteins are expressed in the thyroid gland [Santos 2018]. Of particular note are the seleno-enzymes:
- iodothyronine deiodinase type 1 and type 2 = catalysts for the conversion of the inactive precursor thyroxine (T4) to the active thyroid hormone triiodothyronine (T3)
- thioredoxin reductases = antioxidants protecting the thyroid gland against oxidative damage and regulating redox status in the thyroid gland
- glutathione peroxidase 1 = antioxidant protection against oxidative stress
- glutathione peroxidase 3 = anti-inflammatory action
- selenoprotein P = selenium transport and antioxidant defense
Ongoing Selenium and Thyroid Disorder Clinical Studies
At this writing (March 2020), there are two ongoing randomized controlled clinical studies designed to answer questions about the efficacy of selenium supplementation in thyroid disorder.
CATALYST Study = “The Chronic Autoimmune Thyroiditis Quality of Life Selenium Trial”
472 autoimmune thyroiditis patients who are being treated with levothyroxine are given 200 micrograms of selenium-enriched yeast or placebo daily for 12 months.
The primary objective of the study is to determine the extent of the effect of selenium supplementation on the patients’ quality of life. Secondary objectives are to evaluate the effect of the selenium supplementation on levothyroxine dosage, on serum T3/T4 ratio, on serum anti-thyroperoxidase antibody levels, and on immunological and oxidative stress bio-markers [Winther 2014].
GRASS Study = “The Graves’ Disease Selenium Supplementation Trial”
492 Graves’ disease patients with Graves’ hyperthyroidism who are being treated with anti thyroid drugs are given 200 micrograms of selenium-enriched yeast or placebo daily for 24 – 30 months. The purpose of the study is to see whether the selenium supplementation leads to a decrease in anti thyroid drug failures, to faster remission of the disease, and to improved quality of life [Ventura 2017].
Conclusion: Selenium Status and Thyroid Function
The maintenance of adequate selenium status from food and supplements is necessary for overall health and to prevent thyroid disorders [Ventura 2017].
Several studies have shown a significant reduction in mortality risk associated with serum selenium status of around 135 micrograms per liter [Hurst 2010].
The graph of the relationship between selenium status and health and disease may be a U-shaped graph with a somewhat broad bottom of the U ranging from 100 to 170 micrograms per liter [Rayman 2012, figure 3].
Selenium supplementation may be useful in patients with Hashimoto’s disease (hypothyroidism) [Ventura 2017].
Selenium supplementation seems to be beneficial in patients with mild to moderate Graves’ orbitopathy [Ventura 2017].
Ongoing trials (CATALYST study and GRASS study) will evaluate the benefits of using selenium-enriched yeast supplements with thyroid disorder patients [Ventura 2017].
Selenium-enriched yeast supplements have been shown to reduce the levels of bio-markers of oxidative stress; selenomethionine supplements of the same magnitude did not. This finding suggests that selenium-containing compounds other than selenomethionine may account for the reduction in oxidative stress [Richie 2014].
Ehlers M, Jordan AL, Feldkamp J, Fritzen R, Quadbeck B, Haase M, Allelein S, Schmid C & Schott M. (2016). Anti-Thyroperoxidase antibody levels > 500 IU/ml indicate a moderately increased risk for developing hypothyroidism in autoimmune thyroiditis. Horm Metab Res.; 48(10):623-629. [PubMed]
Hurst R, Armah CN, Dainty JR & Hart DJ. (2010). Establishing optimal selenium status: results of a randomized, double-blind, placebo-controlled trial. Am J Clin Nutr; 91: 923-31. [PubMed]
Rayman MP. (2012). Selenium and human health. Lancet; 379(9822): 1256-68. [PubMed]
Richie JP, Das A, Calcagnotto AM, Sinha R & El-Bayoumy K. (2014). Comparative effects of two different forms of selenium on oxidative stress biomarkers in healthy men: A randomized clinical trial. Cancer Prev Res (Phila); 7(8): 796-804. [PubMed]
Santos LR, Neves C, Melo M & Soares P. (2018). Selenium and selenoproteins in immune mediated thyroid disorders. Diagnostics (Basel); 8: 70-82. [PubMed]
Ventura M, Melo M & Carrilho F. (2017). Selenium and thyroid disease: from pathophysiology to treatment. Int J Endocrinol; Article ID 1297658. [PubMed]
Winther KH, Watt T, Bjorner JB & Cramon P. (2014). The chronic autoimmune thyroiditis quality of life selenium trial (CATALYST): study protocol for a randomized controlled trial. Trials; 15: 115. [PubMed]
The information provided in this review article is not intended as medical advice and should not be used as such.
31 March 2020