Selenium for Thyroid Function — Thyroid Hormones
Essential trace mineral critical for thyroid hormone metabolism. Cofactor for deiodinase enzymes and glutathione peroxidase in the thyroid gland.
Overview
Selenium is an essential trace mineral with critical roles in thyroid hormone metabolism. The thyroid gland contains the highest selenium concentration per gram of tissue of any organ in the body. Selenium is incorporated into selenoproteins including the iodothyronine deiodinases (DIO1, DIO2, DIO3) — the enzymes responsible for converting T4 to active T3 and for T3 inactivation — and glutathione peroxidases (GPx), which protect the thyroid from oxidative damage caused by hydrogen peroxide generated during thyroid hormone synthesis. Selenium supplementation has been most studied in autoimmune thyroiditis (Hashimoto's). A meta-analysis of 16 RCTs found that selenomethionine 200 mcg daily significantly reduced thyroid peroxidase antibodies (TPOAb) by 40-50% over 3-12 months in patients with Hashimoto's thyroiditis. Whether this antibody reduction translates to meaningful clinical improvement (reduced progression to hypothyroidism, improved quality of life) remains debated. Selenium status varies geographically — populations in selenium-poor soil regions (parts of Europe, China, Africa) are at higher risk of deficiency. The recommended dietary allowance (RDA) is 55 mcg/day, with a tolerable upper intake level of 400 mcg/day. Chronic selenium excess (>400 mcg/day) causes selenosis — characterized by garlic breath, brittle nails, hair loss, GI upset, and neurological symptoms.
Indications
- Essential nutrient: Thyroid hormone metabolism and antioxidant protection
- Evidence-supported: Hashimoto's thyroiditis (TPOAb reduction)
- Evidence-supported: Graves' disease and thyroid eye disease (some studies)
- Supplement: General thyroid support in selenium-deficient populations
Mechanism of Action
Selenium is incorporated as selenocysteine into the active site of deiodinases DIO1, DIO2, and DIO3, which control T4-to-T3 conversion and T3 inactivation
Dosing
| Compound | Dose | Frequency | Notes |
|---|---|---|---|
| Selenomethionine | 200 mcg | Once daily | Most studied dose for Hashimoto's thyroiditis; organic selenium form |
| Selenium (any form) | 55-100 mcg | Once daily | General thyroid support / maintenance dose |
| Brazil nuts | 1-2 nuts | Daily | Natural source; highly variable selenium content (50-200 mcg per nut) |
Evidence Grade
GRADE B
Safety & Contraindications
- Toxicity (selenosis) at chronic intake >400 mcg/day: garlic breath, hair loss, brittle nails, GI upset, fatigue, neurological effects
- Narrow therapeutic window between adequacy and toxicity
- Selenomethionine form is better absorbed and stored than sodium selenite
- Brazil nuts are extremely high in selenium — 1-2 nuts may provide 100-200 mcg
- Check selenium levels before supplementing if diet is adequate
- Potential increased risk of type 2 diabetes with high selenium intake (observational data)