The thyroid gland is the organ in the body that contains the most selenium per gram of tissue [Wang 2023].
In a 2023 review article, Wang et al summarize the reasons why adequate selenium intake and status are necessary for good thyroid health.
Humans cannot synthesize selenium: the daily intake of selenium depends on the contents of the individual’s diet.
- The selenium content of food depends on the selenium content of the soil, which varies extensively from region to region of the world. Much of Europe has selenium-poor soil; much of the United States has soil considerably richer in selenium.
- Selenium is a micronutrient that makes possible the body’s synthesis of some 25 identified selenoproteins containing the amino acid selenocysteine.
- The best known selenoproteins – such as the glutathione peroxidases, the thioredoxin reductases, and the iodothyronine deiodinases – are expressed in the thyroid gland, where they contribute to thyroid hormone metabolism and to antioxidant defense.
- A selenium deficiency will increase the risk of several kinds of thyroid diseases.
Selenium Supplementation and Thyroid Diseases
Wang et al  report the following outcomes from clinical trials of selenium supplementation. They advise that we need more clinical evidence for the efficacy of selenium treatment of thyroid disorders.
- Selenium supplementation slows the progression of Graves’ orbitopathy and improves the quality of the patients’ lives.
- Selenium supplementation is associated with the decreased levels of anti-thyroid peroxidase antibodies and with improved thyroid ultrasound structure in patients with Hashimoto’s thyroiditis.
- Selenium supplementation has shown variable anticancer activity in patients with thyroid cancer.
Strong Association Between Selenium and Thyroid Disease
Selenium and Graves’ disease
Wang et al  reviewed the data from 11 clinical trials. Nine clinical trials showed that selenium supplementation results in faster achievement of normal thyroid function in patients with hyperthyroidism. Two clinical trials did not show the beneficial effect of selenium supplementation. The difference in outcomes may be related to differences in the form of the selenium supplementation, the dose, the duration of supplementation, and the nutritional status of the study participants.
Wang et al  report that many studies confirm that selenium deficiency is a risk factor for Graves’ disease. Relatively low selenium status seems to be an independent risk factor for Graves’ disease.
Selenium and Hashimoto’s thyroiditis
Hashimoto’s thyroiditis develops when the immune system produces autoantibodies that attack the thyroid gland, resulting in insufficient thyroid hormone secretion. Diminished immune function is associated with selenium deficiency [Wang 2023].
Wang et al  report that Hashimoto’s thyroiditis is the leading cause of primary hypothyroidism in areas where iodine is abundant. The prevalence of hypothyroidism varies in different regions. The prevalence of overt hypothyroidism in the general population ranges from 0.2% to 5.3% in Europe and from 0.3% to 3.7% in the United States. Women are ten times more likely to be diagnosed with hypothyroidism than men. The prevalence increases to 20% for women 75 years old or older [Wang 2023].
Cross-sectional studies confirm the existence of low selenium status in Hashimoto’s thyroiditis patients. A 2022 clinical study has confirmed that selenium supplementation on the order of 100 micrograms per day improves thyroid function and the quality of life of Hashimoto’s thyroiditis patients [Kryczyk-Kozioł 2022].
Wang et al  conclude that selenium deficiency seems to be a risk factor for Hashimoto’s thyroiditis. However, the available evidence is not sufficient to justify the use of selenium supplementation as part of the treatment of Hashimoto’s thyroiditis even though selenium supplementation does provide antioxidant protection and does improve immune function. When selenium supplementation of Hashimoto’s thyroiditis patients is necessary, the supplementation must be based on considerations of the patient’s baseline selenium status, gender, weight, and medical condition.
Selenium and thyroid tumors
In recent decades, the incidence of thyroid cancer has been increasing. Selenium deficiency is a factor in the development of thyroid tumors and in the progression of thyroid cancer. Some study findings indicate that low levels of serum selenium are associated with higher stages of thyroid cancer [Wang 2023].
Thus far, there are no medical recommendations for the addition of
selenium supplementation to the treatment of thyroid cancer. However, selenium’s anticancer properties have attracted the attention of researchers. Researchers need to investigate which formulations and doses of selenium supplements inhibit the development and/or the progression of cancer [Wang 2023].
Assessment of Selenium Status
Commonly, health professionals assess selenium status in the blood, the hair, the toenails, and the urine.
Blood Selenium Status
Wang et al  note that serum selenium status is not necessarily a good biomarker for tissue selenium status. Furthermore, adequate serum selenium status is not necessarily an indicator of adequate thyroid gland selenium status.
In the blood, it is important to assess the concentration of selenoprotein P, the primary transportation and storage protein for selenium. In cases of insufficient selenium intakes, selenoprotein P levels will decrease first. Selenium supplementation will first increase the level of antioxidant selenoproteins Glutathione peroxidase and then the level of selenoprotein P to normal levels [Wang 2023].
Rayman et al have estimated that the optimal blood selenium status is ca. 125 mcg/L [Winther 2020, figure 3]. In an earlier publication, Rayman has indicated that sub-optimal blood selenium status below 100 mcg/L is associated with a higher risk of mortality [Rayman 2012, figure 3].
Assessment of Selenium in the Urine
Wang et al  report that about 50-60% of ingested selenium is excreted in urine. Measuring the selenium in urine gives an estimate of selenium intake over a short period of time. It should be noted that different selenium formulations have different retention rates. One selenium yeast preparation has a documented 74% retention rate [Bugel 2008].
Assessment of Selenium in the Hair
Selenium content in hair represents selenium status over a period of weeks and even months. A study conducted in China has shown the following reference values for selenium content in hair:
Less than 0.20 mg/kg = selenium deficiency
0.20-0.25 mg/kg = marginal selenium deficiency
0.25-0.50 mg/kg = medium selenium status
Greater than 0.50 mg/kg = high selenium status
However, hair selenium status may vary according to ethnicity, gender, and region [Wang 2023].
Assessment of Selenium in Toenails
Researchers sometimes use toenail selenium concentrations as a reliable biomarker of long-term selenium exposure [Filippini 2017].
- Selenium performs an essential role in maintaining normal thyroid function.
- Selenium deficiency is a risk factor for many thyroid disorders.
- Selenium supplementation needs to be investigated in the context of thyroid disorders. At present, we await publication of the outcomes of the CATALYST study and the GRASS study [both studies described in Ventura 2017].
- Possibly of interest is Coenzyme Q10 supplementation as selenium and Coenzyme Q10 have a biological inter-relationship [Aaseth 2023].
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The information presented in this review article is not intended as medical advice and should not be used as such.
30 September 2023