Thyroid Health Guide
Learn how selenium and iodine work together for thyroid health, their safe upper limits, and who should be cautious when supplementing both nutrients.
Yes, selenium and iodine can and often should be taken together — they work cooperatively for thyroid health. Iodine is essential for thyroid hormone production, while selenium is required for the enzymes that convert T4 to active T3 and protect the thyroid from oxidative damage. However, both have narrow safe ranges: the UL for selenium is 400 mcg/day and iodine is 1,100 mcg/day. Combining supplements without checking total intake can push you past these limits.
| Nutrient | RDA | UL (per day) | Role in thyroid |
|---|---|---|---|
| Selenium | 55 mcg | 400 mcg | T4 → T3 conversion; antioxidant protection |
| Iodine | 150 mcg | 1,100 mcg | Essential for T3 and T4 hormone synthesis |
| Selenium (pregnancy) | 60 mcg | 400 mcg | Higher needs during pregnancy |
| Iodine (pregnancy) | 220 mcg | 1,100 mcg | Critical for fetal brain development |
Source: FDA Dietary Reference Intakes, WHO/UNICEF iodine guidelines, NIH Office of Dietary Supplements.
Selenium supports the T4-to-T3 conversion in the thyroid. The deiodinase enzymes that activate thyroid hormone are selenium-dependent. Without adequate selenium, iodine supplementation alone may not optimize thyroid function. This is why they work best together.
Brazil nuts are extremely high in selenium. A single Brazil nut contains approximately 90 mcg of selenium — nearly double the RDA. Eating 2–3 Brazil nuts daily plus a selenium-containing multivitamin can easily push you toward the 400 mcg UL.
Kelp and seaweed supplements can easily exceed the iodine UL. Some kelp supplements contain 500–2,000 mcg of iodine per serving — up to nearly double the 1,100 mcg UL. If you take a thyroid support formula, check the iodine content carefully.
Individuals with Hashimoto's thyroiditis, Graves' disease, or other thyroid conditions should be particularly cautious. Selenium supplementation (200 mcg/day) has shown benefit in reducing thyroid antibodies in Hashimoto's, but iodine excess can trigger or worsen autoimmune thyroid disease. Always work with an endocrinologist.
Iodine needs increase to 220 mcg/day during pregnancy and 290 mcg/day during lactation — critical for fetal and infant brain development. However, prenatal vitamins already contain iodine. Adding kelp or thyroid supplements on top can exceed the UL. Selenium needs also increase (RDA: 60 mcg in pregnancy).
Levothyroxine (Synthroid) and other thyroid medications are dose-calibrated to your current iodine intake. Adding iodine or selenium supplements can alter thyroid hormone levels and affect your medication dosage. Any supplementation should be discussed with your endocrinologist.
Selenium and iodine can stack up across your multivitamin, thyroid formula, and seaweed supplements. Audit your full stack to check total intake against safe limits.
Audit your supplement stackDisclaimer: NutriAudit is a decision-support tool designed to help you review your supplement stack for potential duplicate, conflicting, or excessive ingredients. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making changes to your supplement routine, especially if you are pregnant, nursing, taking medications, or have a medical condition.
Based on reference standards from FDA, EFSA, TGA, and MHLW.
Last updated: April 7, 2026 · Data sourced from FDA Dietary Reference Intakes, EFSA Scientific Opinions, and NIH Office of Dietary Supplements.