*The article was written by Dr. Maria Fernanda Barca, doctor of endocrinology and member of the Thyroid group at the Hospital das Clínicas, Faculty of Medicine, University of São Paulo (USP), and published on The Conversation Brasil platform.
In recent years, selenium has started to be seen frequently on social media, on supplement shelves, and in doctors’ offices. We talk about it as an ally for immunity, the thyroid, fertility and even the prevention of chronic diseases.
Part of this interest has a scientific basis: selenium is an essential micronutrient and participates in fundamental processes in the body. But another part is born from hasty extrapolations of still limited results, which allows us to understand why the subject divides opinions.
The debate is intensifying, particularly in the area of thyroid diseases, a set of pathologies that are much more common than one might imagine. Estimates from the World Health Organization (WHO) indicate that around 750 million people worldwide live with a disease related to this endocrine gland. In Brazil, calculations from the Brazilian Society of Endocrinology and Metabolology (SBEM) suggest that up to 60% of the population will develop a thyroid disorder during their lifetime.
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Located in the neck region, the thyroid produces the hormones T3 (triiodothyronine) and T4 (thyroxine), which regulate metabolism, influence fertility and participate in the functioning of vital organs, such as the heart and brain. Among the most common changes are hypothyroidism (insufficient production of hormones) and hyperthyroidism (excessive production).
The risk of having low selenium levels in the body
In the 1990s, researchers identified that selenium integrates enzymes essential for the activation of thyroid hormones. From then on, the mineral began to be studied as a possible ally in the management of thyroid diseases, particularly those of autoimmune origin.
According to observational studies, low selenium levels may be associated with increased risks of benign thyroid disease. Additionally, international epidemiological research links mineral deficiency to an increased risk of autoimmune thyroiditis (when the immune system attacks the thyroid), such as Hashimoto’s thyroiditis (chronic inflammation usually leading to hypothyroidism) and Graves’ disease (autoimmune associated with hyperthyroidism).
In patients with serious illnesses, studies indicate that selenium supplementation, when used as an adjunct to conventional treatment, may be associated with faster remission of symptoms and improved quality of life.
It is precisely at this stage that the debate must be conducted with more care and scientific rigor. The fact that selenium is an essential micronutrient does not authorize its indiscriminate prescription. Some of the dissemination on the qualities of the mineral goes beyond what the available data allows us to say with certainty.
Fame and exaggerated myths
One of the most recurring myths is that selenium boosts immunity and largely prevents disease. In practice, this is necessary for the proper functioning of the immune system, but there is no evidence that supplementation beyond physiological needs provides additional benefits to people without a deficiency.
Another frequently circulated argument is that selenium has a protective effect against cancer and cardiovascular diseases. Large clinical trials have not consistently confirmed this hypothesis. Scientific literature indicates that indiscriminate supplementation does not reduce the risk of these diseases and, in some contexts, may be associated with adverse effects.
However, there are well-established consensuses. Selenium is an essential micronutrient, but the body requires it in small amounts. It is also important to draw attention to the fact that the margin between the appropriate dose and the potentially toxic dose is narrow. In a large part of the population, especially in countries like Brazil, the usual diet already provides enough selenium, without the need for supplementation.
Use by pregnant women and special situations
When the focus shifts to pregnant women, interpretation of evidence requires even more caution. An international consensus does not routinely recommend selenium supplementation for pregnant women with Hashimoto’s thyroiditis (autoimmune thyroid disease), citing inconsistency of results and possible risks associated with high doses.
At the same time, studies and reviews published between 2024 and 2025 suggest that low levels of selenium in the maternal body are associated with adverse pregnancy outcomes, including greater autoimmune activity and an increased risk of postpartum thyroiditis (inflammation of the thyroid after childbirth). These studies also indicate that the response to supplementation is not uniform. Effects vary depending on baseline selenium level, disease activity, and clinical monitoring.
A review analyzing 21 studies involving 1,610 patients with Hashimoto’s thyroiditis concluded that adequate and controlled replacement was associated with a reduction in autoantibodies, a decrease in TSH levels, and a slight improvement in quality of life.
It should be emphasized that selenium does not treat thyroid diseases in isolation. In case of deficiency, supplementation can serve as support to control inflammation and autoimmunity, but always in combination with the indicated drug treatment.
When excess becomes a problem
Selenium is obtained mainly from food, through the consumption of meat, seafood, eggs and grains. The recommended daily dose for adults is 55 micrograms per day, reaching 60 micrograms for pregnant women and 70 micrograms for breastfeeding women. The maximum safe limit is 400 micrograms per day. Brazil nuts contain large amounts of this mineral: one to two units per day may be enough to meet the daily recommendation.
The potential benefits associated with selenium do not outweigh the risks of indiscriminate use. The scientific literature shows that the proximity between an adequate dose and a toxic dose deserves attention. High consumption can cause hair loss, brittle nails, fatigue, nausea, diarrhea and skin rashes. In more severe cases, neurological changes and kidney or heart failure have been recorded.
Given this scenario, supplementation should only be considered after clinical and laboratory evaluation, especially in individuals with autoimmune thyroid diseases, infertility, or other specific conditions.
Available evidence suggests that selenium may play a complementary role in the management of certain thyroid diseases, including autoimmune diseases. However, the body of available evidence indicates that the use of selenium should remain limited to specific situations, with a well-defined dose and indication. The warning remains: more selenium does not mean better health.
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