Iron Supplements and Levothyroxine: A Clinically Important Interaction

Levothyroxine (sold as Synthroid, Tirosint, and generics) is one of the most commonly prescribed medications in the United States — the standard treatment for hypothyroidism. Iron supplements are also extremely common, particularly among people with anemia, pregnancy, or heavy menstrual periods. The overlap is frequent enough that their interaction is well worth understanding.

Iron forms insoluble complexes — chelates — with levothyroxine molecules when both are present in the gastrointestinal tract at the same time. The resulting complex is poorly absorbed, so less of the thyroid medication reaches the bloodstream. Studies have measured reductions in levothyroxine bioavailability when taken with ferrous sulfate, with some reporting decreases in the range of 30–40%. The practical consequence is that thyroid hormone levels may fall below the intended therapeutic range. This interaction is recognized in levothyroxine prescribing information and noted in the NIH ODS iron fact sheet. It applies to ferrous sulfate (the most common supplement form), ferrous gluconate, and ferric salts. Dietary iron from food is unlikely to cause the same degree of interference because it arrives in the gut differently than a supplement bolus.

The interaction is rated moderate rather than high because it is generally manageable — it does not create an acute dangerous situation. The concern is chronic under-medication of thyroid disease, which has its own health consequences, particularly for people where TSH control matters. Timing is the practical solution: levothyroxine is typically taken on an empty stomach in the morning, with iron and other supplements taken separately by several hours.

Source: Iron — Health Professional Fact Sheet (NIH Office of Dietary Supplements)