Iron Interactions

Iron reduces levothyroxine absorption by up to 65%

Calcium in dairy products significantly reduces iron absorption by up to 50%.

Iron forms an insoluble chelate complex with doxycycline in the gastrointestinal tract, dramatically reducing absorption of the antibiotic. Studies show ferrous sulfate co-administration can lower doxycycline serum levels by 80% or more, potentially leading to treatment failure.

Alcohol can increase iron absorption to potentially harmful levels and damage the liver.

Caffeine (coffee, tea) can reduce iron absorption by up to 60%.

Calcium inhibits iron absorption by up to 60%

Iron and zinc compete for absorption when taken together in high doses

Polyphenols (tannins) in black tea bind non-heme iron in the gut lumen to form insoluble iron-tannate complexes, blocking absorption. Inhibition of non-heme iron uptake from a meal can reach 79-94% when black tea is consumed with food.

Iron salts (ferrous sulfate, fumarate, gluconate, bisglycinate) can chelate thyroid hormone in the gut and reduce absorption of liothyronine, similar to the established interaction with levothyroxine. Concurrent dosing can cause inadequate T3 effect and elevated TSH.

Green tea catechins, especially epigallocatechin-3-gallate (EGCG), chelate non-heme iron in the gut and form insoluble complexes that reduce intestinal absorption. The effect is dose-related and most pronounced when tea is consumed with iron-containing meals or supplements.

Peppermint tea is rich in polyphenols (particularly rosmarinic acid) and tannins that bind non-heme iron in the gut, forming insoluble complexes that cannot be absorbed. Controlled studies show peppermint tea can reduce non-heme iron absorption from a meal by up to 84%.

Omeprazole reduces absorption of nonheme (plant and supplemental) iron by raising gastric pH, which prevents the reduction of ferric (Fe3+) to absorbable ferrous (Fe2+) iron. Recent research also shows PPIs upregulate hepcidin and downregulate duodenal ferroportin, directly blocking iron export from enterocytes.

Oolong tea sits between green and black tea in polyphenol oxidation and contains substantial tannins and catechins that bind non-heme iron in the gut to form insoluble iron-polyphenol complexes. Drinking oolong tea with meals reduces absorption of dietary and supplemental non-heme iron.

Whey protein contains calcium and bioactive peptides that can chelate iron in the gut and reduce its absorption. Studies in iron-fortified casein-whey drinks show calcium added with whey reduces iron absorption by approximately 18 to 27 percent.

Coffee contains chlorogenic acid and other polyphenols with galloyl groups that chelate non-heme iron in the gut lumen, forming insoluble complexes. A cup of coffee taken with a meal can reduce non-heme iron absorption by roughly 39% to 60%.

Recent iron supplementation does not meaningfully affect ferritin measurement itself (ferritin is a storage protein, not free iron in serum), but it does spike serum iron and transferrin saturation values measured in the same panel, leading to misleading interpretation of overall iron status. A ferritin result drawn shortly after an iron tablet can also be misinterpreted alongside falsely elevated serum iron, suggesting iron stores are repleted when they are not.

Copper is required to synthesize ceruloplasmin, a ferroxidase that oxidizes ferrous iron to ferric iron so it can bind transferrin and be transported to bone marrow for hemoglobin production. Without adequate copper, dietary or supplemental iron cannot be mobilized effectively and copper-deficiency anemia develops even when iron stores are normal.

Vitamin A and beta-carotene improve absorption of non-heme iron from plant foods by forming soluble complexes with iron that protect it from binding to phytates and polyphenols in the gut. In a controlled human study, vitamin A roughly doubled iron absorption from rice and increased absorption from wheat and corn.

Vitamin C significantly enhances non-heme iron absorption (up to 6x)