Selenium and the thyroid gland: why the trace element is important even before birth — and how not to overdo it

In the Nutrients review, Italian pediatricians and nutritionists have compiled the main facts about selenium, a microelement without which the thyroid gland cannot function normally. The authors show that selenium is critical from the fetal period to adolescence: it is part of enzymes that activate thyroid hormones (T4 → T3) and protect tissue from oxidative stress. Deficiency is associated with the risk of developmental disorders and hormonal metabolism failures, and excess is associated with toxic effects. Conclusion: we underestimate the role of selenium in prenatal and childhood support and need clear, safe strategies - from nutrition for expectant mothers to screening for risk groups.

Background

• The thyroid gland depends not only on iodine, but also on selenium. Selenium is a part of deiodinases (DIO1/2/3) - selenoproteins that activate and inactivate thyroid hormones (T4 ↔ T3) and thus maintain local and systemic hormonal balance. It is also needed for antioxidant protection of the gland tissue (glutathione peroxidase, thioredoxin reductase).
• Periods of increased vulnerability include pregnancy, early life, and adolescence. During pregnancy, maternal selenium status is often reduced, and deficiency is associated with the risk of postpartum thyroiditis; in some RCTs, supplementation reduced inflammatory activity and the incidence of hypothyroidism after delivery, although current reviews highlight the limited and heterogeneous nature of the evidence.
• Selenium deficiency is underestimated in pediatric endocrinology. According to the review, the risk of decreased status is higher in preterm infants, children with malabsorption/restrictive diets, and those with a diet high in ultra-processed foods; however, selenium is important for normal thyroid development from fetus to adolescence.
• The geography of nutrition matters. The selenium content of food depends greatly on the soil and food chain: in "poor" regions, deficiency is more common; in "rich" regions, excess supplements can lead to excess.
• There is an “upper limit of safety”. EFSA (2023) has set a UL of 255 μg/day for adults (including pregnant/nursing women); for children, the UL is derived proportionally to body weight. Chronic excess (selenoses) manifests itself in particular by hair loss and brittle nails, as well as gastrointestinal symptoms and neurological complaints. This highlights the U-shaped relationship of “too little/too much”.
• Practical conclusion from the evidence base: Priority is given to dietary sources (fish/seafood, eggs, meat, dairy, whole grains) and targeted status monitoring in risk groups; routine high-dose supplementation during pregnancy without indications is not recommended.

What kind of work is this?

This is a narrative review (not a clinical trial). The team systematically reviewed the literature and identified 68 publications for in-depth analysis to describe the role of selenium in thyroid development and function from fetus to adolescence, and discuss deficiency, sources, bioavailability, and safe doses. The authors call for selenium status to be included in obstetric and pediatric guidelines.

Why does the thyroid need selenium?

• Hormone switch. Selenium is the key to the work of deiodinases (DIO1/2/3): these selenoproteins convert relatively “passive” thyroxine (T4) into active triiodothyronine (T3) and, conversely, inactivate excess hormones. Without adequate selenium, the T4/T3 balance suffers.
• Antioxidant protection of the gland. Other selenium-dependent enzymes - glutathione peroxidase and thioredoxin reductase - neutralize peroxides that inevitably form during the synthesis of hormones, protecting thyroid tissue from damage.
• Pregnancy and early life. During pregnancy, the need for selenium increases, while the mother's level often falls (hemodilution, increased expenditure on the fetus). Adequate intake is associated with a lower risk of postpartum thyroiditis and more stable autoantibodies; during breastfeeding, selenium levels in milk are influenced, for example, by fish consumption.

Where is it most often lacking?

• Geography. Selenium content in foods is highly dependent on soil/sea: in certain regions of the world (parts of China, Africa) deficiency is a public health issue.
• Children with restrictive diets/diseases and. The risk of decreased selenium status is higher in phenylketonuria (protein restrictions), IBD (malabsorption), in premature infants on parenteral nutrition and in diets with a high proportion of ultra-processed foods (lower micronutrient density).
• Dietary patterns: In childhood studies, ultra-processed foods were associated with a higher risk of inadequate selenium intake.

How much is needed: guidelines and approximate norms

European and international authorities use different approaches: EFSA uses biomarkers (selenoprotein P, GPx activity), WHO uses levels that prevent deficiency in different countries. National standards vary. For example, in Italy (LARN), the recommended levels for children are:
1-3 years - 15 μg/day; 4-6 - 25 μg; 7-10 - 40 μg; 11-14 - 50 μg; 15-17 - 55 μg/day (adult level). These figures illustrate the age-related increase in need; in your country, follow the local recommendations.

It is also important to know the upper safety limit: EFSA set the UL of 255 μg/day for adults (including pregnant/nursing women) in 2023. Exceedances are more common in people who regularly take high-dose supplements or abuse Brazil nuts. An early sign of overdose is hair loss/brittle nails. For children, the UL is calculated based on body weight (allometrically).

Sources and bioavailability

• Food first. Selenium comes from seafood, fish, eggs, meat, whole grains, dairy products; concentrations vary by region. Organic forms (e.g. seleno-methionine, "selenium-enriched yeast") are generally better absorbed/retained than inorganic forms (selenate/selenite).
• What influences absorption. Bioavailability depends on the chemical form and matrix of the food; accompanying vitamins A/D/E, sufficient protein and fat also play a role.

What to do in practice (pregnancy → teenagers)

• Pregnancy. Monitor maternal status, especially in the third trimester when fetal reserves are being built and maternal selenium levels naturally decline. Several studies have suggested threshold levels of serum selenium in the second and third trimesters (targets ~0.90 and 0.78 μmol/L), below which the risk of adverse outcomes is higher. Discuss local practice and target values with your physician.
• Breastfeeding: Regular fish in the mother's diet (taking into account mercury recommendations for pregnant/nursing women) as a natural way to support selenium in milk.
• Children and teenagers. Priority is given to regular food with sufficient protein and whole grains; we minimize the share of ultra-processed products. Special attention is given to children with restrictive diets and chronic gastrointestinal diseases; the issue of supplements is decided individually with a pediatrician.

Beware: More Doesn't Mean Better

The review highlights the U-shaped problem: deficiency is harmful, but excess is dangerous. EFSA directly states that the safe limit can be exceeded by the sum of: food + dietary supplements (+ Brazil nuts). Therefore, the authors' strategy is education, nutrition, targeted screening of risk groups; supplements - according to indications, with an understanding of the upper levels.

Viewing Limitations

This is a summary of the evidence (many observational data, fewer RCTs in children). The authors do not provide a universal “prescription for supplements”; they provide a framework for policy and clinical practice and highlight the need for randomized trials in pregnant women and children.

Source: Calcaterra V. et al. Thyroid Health and Selenium: The Critical Role of Adequate Intake from Fetal Development to Adolescence. Nutrients 17(14):2362, 2025. https://doi.org/10.3390/nu17142362