Iron reduces levothyroxine absorption
Iron forms insoluble complexes with levothyroxine, reducing absorption significantly. Can cause treatment failure and persistent hypothyroid symptoms.
Take thyroid medication 4+ hours before iron supplements.
Thyroid med in morning, iron in evening
Iron reduces levothyroxine absorption. Take thyroid medication 4+ hours before iron supplements.
Thyroid med in morning, iron in evening
This interaction is rated “Warning” by Formulate. Iron forms insoluble complexes with levothyroxine, reducing absorption significantly. Can cause treatment failure and persistent hypothyroid symptoms.
Studies from our registry that mention iron 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 iron · 2017
Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women
Stoffel NU, Cercamondi CI, Brittenham G, et al. · Blood
Landmark absorption study showing iron supplements taken daily (or twice-daily) trigger hepcidin elevation that blocks subsequent absorption — supporting alternate-day dosing as more efficient for iron repletion.
Mentions iron · 2010
Iron bioavailability and dietary reference values
Hurrell R, Egli I · American Journal of Clinical Nutrition
Reviews enhancers (vitamin C, meat) and inhibitors (phytate, polyphenols, calcium) of non-heme iron absorption, providing the mechanistic basis for pairing iron with vitamin C and separating it from tea/coffee.
Mentions iron · 2000
Prediction of dietary iron absorption: an algorithm for calculating absorption and bioavailability
Hallberg and Hulthén · American Journal of Clinical Nutrition
Foundational model for non-heme iron absorption factoring vitamin C, phytates, tannins, and calcium interactions.
Other interactions to know
Calcium significantly reduces iron absorption
Ashwagandha can raise thyroid hormone levels and stack additively with levothyroxine, risking thyrotoxicosis.
Calcium blocks thyroid medication absorption
Iodine supplementation can destabilize thyroid
Fluoroquinolones and tetracyclines chelate iron
Zinc and iron compete for absorption
Antacids significantly reduce iron absorption
Magnesium may reduce thyroid med absorption
Check your full stack in the free interaction checker.
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Medical disclaimer. This page is for educational purposes only and does not replace advice from a qualified healthcare provider.
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