Iodine is an essential component of the thyroid hormones. After iodine, selenium is arguably the micronutrient most important to the thyroid gland. Proportionally, there is more selenium in the thyroid gland than there is selenium in any other organ in the body. There are good reasons for the presence of selenium in the thyroid gland. Iodine and selenium are both required for thyroid hormone synthesis and function.
Our bodies do not make selenium. We must get the selenium that we need from our food and from supplements. The selenium that we absorb is incorporated into the amino acid selenocysteine. Selenocysteine, then, is a necessary component of some 25 selenoproteins that are needed for various biological functions.
Selenoproteins and thyroid function
Among the selenoproteins important to the optimal functioning of the thyroid gland are the following types of selenoproteins:
- Iodothyronine deiodinases (abbreviated DIO) = enzymes that are involved in the conversion of the biologically passive thyroid hormone T4 to the biologically active thyroid hormone T3, which is important for the maintenance of normal cell metabolism
- Glutathione peroxidases (abbreviated GPx) = anti-oxidant enzymes that protect the thyroid against oxidative damage (damage caused by harmful free radicals)
- Thioredoxin reductases (abbreviated TRx) = enzymes that promote healthy cell growth and structure
Why the thyroid is important
The thyroid gland is that little butterfly-shaped organ that sits at the base of the neck. It releases the hormones that regulate our body’s metabolism. These thyroid hormones, the T3 and the T4 thyroid hormones, are released directly into the blood and flow through the blood to almost all body cells.
The thyroid hormones regulate several bio-chemical processes in the cells:
- Body temperature
- Body weight gain or loss
- Breathing rate
- Cholesterol levels
- Heart rate
- Menstrual cycles
- Nervous system
Selenium supplementation and thyroid disorders
Studies of the health effects of selenium supplementation on thyroid function can be classified in three distinct types of studies:
- Hypothyroidism (underactive thyroid) in general and Hashimoto’s thyroiditis in particular
- Thyroid nodules (fluid lumps in the throat)
- Hyperthyroidism (overactive thyroid) in general and Graves’ disease in particular
Selenium and the underactive thyroid
The underactive thyroid is often caused by the auto-immune disease Hashimoto’s thyroiditis. It is typically treated with levothyroxine, which is the synthetic drug form of thyroid hormone thyroxine (T4). Researchers have done several studies to see whether levothyroxine in combination with selenium supplementation is more effective than levothyroxine alone [Ventura].
Basically, the results to date show that selenium supplementation of patients with underactive thyroid, especially patients with low plasma selenium status, may be beneficial even if the patients are taking levothyroxine [Ventura]; however, the evidence is considered not yet complete enough for definitive clinical decision making [van Zuuren].
Nota bene: The studies have been done with an inorganic form of selenium – sodium selenite – or with a synthetic form of selenium – selenomethionine. Daily dosages have ranged from 60 to 200 micrograms [Ventura]. The point is, the studies have not been done with the newer, more effective high-selenium yeast supplements.
Selenium supplementation in the CATALYST study
We are waiting for the results of the CATALYST trial (Chronic Autoimmune Thyroiditis Quality of Life Selenium Trial), expected to be completed in 2019. The CATALYST trial is a randomized, double-blinded, placebo-controlled study of 472 participants that has the following study outcome measures:
- Primary outcome: Thyroid-related quality of life
- Secondary outcome: Thyroid peroxidase antibody concentration
- Secondary outcome: Levothyroxine (LT4) dosage
Study participants assigned to the intervention group receive 200 micrograms of selenium-enriched yeast in the form of two oral tablets once a day for 12 months in addition to their normal medications. Study participants assigned to the control group receive two placebo tablets that are, identical in appearance, taste and smell, once a day for 12 months in addition to their normal medications.
The CATALYST researchers will follow the study participants for an additional 6 months after the end of the intervention period. The Danish firm Pharma Nord Aps supplies the high-selenium yeast supplement SelenoPrecise® and the placebo supplement [Winther].
Selenium supplementation and thyroid nodules
A few small studies have been done assessing the relationship between low selenium status and thyroid volume. Generally, the studies suggest that there is an inverse relationship between plasma selenium levels and thyroid volume as measured by ultrasound [Ventura]. More research data are needed.
One large Chinese study (6152 patients) suggests greater prevalence of all forms of thyroid disorders (hypothyroidism, autoimmune thyroiditis, enlarged thyroid) in the low-selenium regions of China as compared to the selenium-adequate regions [Wu].
Selenium supplementation and the overactive thyroid
Graves’ disease is an autoimmune thyroid disease that causes the thyroid gland to grow and produce more thyroid hormone than the body needs. Graves’ ophthalmopathy is the name given to the eye problems associated with Graves’ disease.
The results from three small studies suggest that selenium supplementation in addition to treatment with methimazole is more effective at restoring Graves’ disease patients to normal thyroid function than is methimazole treatment alone [Ventura].
One randomized controlled trial has shown that supplementation with 200 micrograms of selenium daily was associated with improved quality of life, fewer eye problems, and delayed progression of the disease in patients with Graves’ orbitopathy [Marcocci].
Selenium supplementation in the GRASS study
Like the CATALYST study described above, the GRASS study (GRAves’ disease Selenium Supplementation trial) is ongoing. The study researchers are investigating whether selenium supplementation in conjunction with standard treatment of patients with Graves’ hyperthyroidism produces less treatment failure and faster remission than placebo supplementation together with standard treatment [Watt].
The daily dosage in the GRASS study is 2 times 100 micrograms of a high-selenium yeast preparation. The treatment period runs from 24-30 months for each participant.
The researchers will collect and analyze data to determine the proportion of patients in each group, the selenium group and the placebo group, who have treatment failure outcomes in the GRASS study. The following events are specifically listed as treatment failure outcomes:
- The patient receives anti-thyroid medication at any level during the last 12 months (± 1 month) of the study period.
- The participant patient has thyroid hyperfunction (TSH < 0.1) during the last 12 months (± 1 month) of the study period.
- The patient must be referred to therapy (radioactive iodine or thyroid surgery) at some point during the study period.
The hope is that the group that receives selenium supplements will have significantly fewer treatment failure outcomes.
Summary: selenium and thyroid disorders
- We need more studies and more data relating selenium supplementation and thyroid disorders; however, the study results that we have are generally encouraging [Toulis; Fan].
- We need results from studies using high-selenium yeast preparations as the treatment.
Fan, Y., Xu, S., Zhang, H., Cao, W., Wang, K., Chen, G., & … Liu, C. (2014). Selenium supplementation for autoimmune thyroiditis: a systematic review and meta-analysis. International Journal of Endocrinology, 2014904573. doi:10.1155/2014/904573.
Marcocci, C., Kahaly, G. J., Krassas, G. E., Bartalena, L., Prummel, M., Stahl, M., & Wiersinga, W. (2011). Selenium and the course of mild Graves’ orbitopathy. The New England Journal Of Medicine, 364(20), 1920-1931.
Toulis, K. A., Anastasilakis, A. D., Tzellos, T. G., Goulis, D. G., & Kouvelas, D. (2010). Selenium supplementation in the treatment of Hashimoto’s thyroiditis: a systematic review and a meta-analysis. Thyroid: Official Journal of The American Thyroid Association, 20(10), 1163-1173.
van Zuuren, E. J., Albusta, A. Y., Fedorowicz, Z., Carter, B., & Pijl, H. (2014). Selenium Supplementation for Hashimoto’s Thyroiditis: Summary of a Cochrane Systematic Review. European Thyroid Journal, 3(1), 25-31.
Ventura, M., Melo, M. & Carrilho, F. (2017). Selenium and thyroid disease: from pathophysiology to treatment. International Journal of Endocrinology. doi: 10.1155/2017/1297658.
Watt, T., Cramon, P., Bjorner, J. B., Bonnema, S. J., Feldt-Rasmussen, U., Gluud, C., & Rasmussen, A. K. (2013). Selenium supplementation for patients with Graves’ hyperthyroidism (the GRASS trial): study protocol for a randomized controlled trial. Trials, 14119. doi:10.1186/1745-6215-14-119
Winther, K. H., Watt, T., Bjørner, J. B., Cramon, P., Feldt-Rasmussen, U., Gluud, C., & Bonnema, S. J. (2014). The chronic autoimmune thyroiditis quality of life selenium trial (CATALYST): study protocol for a randomized controlled trial. Trials, 15115. doi:10.1186/1745-6215-15-115.
Wu, Q., Rayman, M. P., Lv, H., Schomburg, L., Cui, B., Gao, C., & … Shi, B. (2015). Low Population Selenium Status Is Associated with Increased Prevalence of Thyroid Disease. The Journal of Clinical Endocrinology And Metabolism, 100(11), 4037-4047.
The information presented in this article is not intended as medical advice and should not be used as such.