Selenium supports thyroid hormone conversion
Selenium is essential for deiodinase enzymes that convert T4 to active T3. Adequate selenium may improve thyroid medication effectiveness, especially in autoimmune thyroiditis (Hashimoto's).
200 mcg selenium may support thyroid function. Don't exceed this dose.
Can take separately from thyroid medication
Selenium supports thyroid hormone conversion. 200 mcg selenium may support thyroid function. Don't exceed this dose.
Can take separately from thyroid medication
This interaction is rated “Info” by Formulate. Selenium is essential for deiodinase enzymes that convert T4 to active T3. Adequate selenium may improve thyroid medication effectiveness, especially in autoimmune thyroiditis (Hashimoto's).
Studies from our registry that mention selenium or thyroid medication. Each links to the primary source and the other Formulate pages citing it.
Mentions thyroid medication · 2026
Memorial Sloan Kettering Integrative Medicine Monograph: Ashwagandha
Primary literature · Monograph
Documents thyrotoxicosis case reports associated with ashwagandha use. States ashwagandha may increase thyroxine levels. Also notes CYP3A4 induction (moderate) and sedative-additive effects. Does not give a hard avoid-with-levothyroxine warning but flags thyroid monitoring.
Mentions selenium · 2009
Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT)
Lippman SM, Klein EA, Goodman PJ, et al. · JAMA
Large primary-prevention RCT (n=35,533) found selenium 200µg/day did not prevent prostate cancer — and later follow-up raised concerns about increased type 2 diabetes risk at supplemental doses in selenium-replete populations.
Mentions selenium · 2000
The importance of selenium to human health
Rayman MP · Lancet
Landmark review of selenium's role in thyroid hormone metabolism, immune function, and antioxidant defense — with clinical relevance for deficient populations but not universally beneficial supplementation.
Other interactions to know
Ashwagandha can raise thyroid hormone levels and stack additively with levothyroxine, risking thyrotoxicosis.
Calcium blocks thyroid medication absorption
Iron reduces levothyroxine absorption
Iodine supplementation can destabilize thyroid
Magnesium may reduce thyroid med absorption
Soy may interfere with thyroid medication
Check your full stack in the free interaction checker.
The free checker lets you add any combination of supplements and medications at once. The free web app goes further — every stack change surfaces interaction alerts automatically.
Medical disclaimer. This page is for educational purposes only and does not replace advice from a qualified healthcare provider.
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