Levothyroxine is a thyroid hormone replacement used to treat hypothyroidism. Iron is a common mineral found in supplements, prenatal vitamins, and many multivitamins. These two are a well-known pair that should not be taken together, because iron can significantly reduce how much levothyroxine your body absorbs.
This is considered a high-severity absorption interaction. In practical terms, taking iron too close to levothyroxine can make your thyroid medication work poorly, even if you are taking the correct dose every day.
What happens when you take levothyroxine with iron?
When levothyroxine and iron are taken at the same time, the iron can bind to levothyroxine in the stomach and intestines. This creates a complex that is harder for your body to absorb. As a result, less thyroid hormone gets into your bloodstream.
Studies and prescribing information have shown that iron salts can reduce levothyroxine absorption substantially, with some reports suggesting reductions of up to 65% under certain conditions. The main issue is not that iron “destroys” levothyroxine, but that it physically interferes with its uptake from the gut.
This interaction can happen with several forms of iron, including:
- Ferrous sulfate
- Ferrous gluconate
- Ferrous fumarate
- Carbonyl iron
- Iron polysaccharide
- Heme iron supplements
It can also happen with products that contain iron as part of a blend, such as multivitamins, prenatal vitamins, and some “blood builder” supplements. Even if the label does not say “iron supplement” on the front, the interaction still matters if iron is listed in the ingredients.
Why is this important?
Levothyroxine works best when it is absorbed consistently. Small changes in absorption can lead to meaningful changes in thyroid levels, especially your TSH. If iron blocks absorption, your body may act as if you are taking too little thyroid medication.
That can lead to symptoms of under-treated hypothyroidism, such as:
- Fatigue
- Weight gain
- Constipation
- Feeling cold
- Dry skin
- Hair thinning
- Brain fog or trouble concentrating
- Depressed mood
For some people, the consequences are more serious. Poorly controlled thyroid levels can affect cholesterol, heart function, fertility, pregnancy outcomes, and overall quality of life. In children, thyroid hormone is especially important for growth and development. In pregnancy, both iron and thyroid treatment are common, which makes this interaction particularly easy to miss.
Another problem is confusion. A person may think their levothyroxine “stopped working,” when the real issue is timing. This can lead to unnecessary dose increases. Then, if the iron is later separated or stopped, the higher levothyroxine dose may become too much.
What should you do?
The standard recommendation is simple: take levothyroxine at least 4 hours apart from iron supplements. This includes standalone iron, multivitamins with iron, and prenatal vitamins.
Helpful ways to do that include:
- Take levothyroxine first thing in the morning with water, on an empty stomach, then wait to take iron until lunch or dinner.
- Or take levothyroxine at bedtime, as long as it is several hours after your last meal and after any iron-containing product.
- Check labels on multivitamins, prenatal vitamins, and “energy” or “blood support” supplements for hidden iron.
Other practical tips:
- Take levothyroxine with plain water, not coffee, milk, or juice.
- Be consistent with the time of day and the way you take it.
- If you start or stop iron, ask your clinician whether you need repeat TSH testing in about 6 to 8 weeks.
- Do not change your levothyroxine dose on your own.
If you need both medicines and timing is difficult, your clinician may help you build a schedule. In some cases, they may adjust the levothyroxine dose based on follow-up labs. If iron is needed for iron deficiency or iron deficiency anemia, it is still important to treat that condition; the goal is separation, not avoidance.
Which specific products are affected?
Levothyroxine products affected include both brand-name and generic tablets/capsules, such as:
- Synthroid
- Levoxyl
- Unithroid
- Tirosint
- Tirosint-SOL
- Euthyrox
- Generic levothyroxine
Iron-containing products that can interfere include:
- Ferrous sulfate tablets or liquid
- Ferrous gluconate
- Ferrous fumarate
- Slow-release iron products
- Carbonyl iron supplements
- Iron polysaccharide products
Common supplement and vitamin brands that may contain iron, depending on the specific formula, include:
- Nature Made Iron
- Nature Made Prenatal
- One A Day Women’s or Prenatal formulas
- Centrum Women or Prenatal formulas
- Vitron-C
- Slow Fe
- Feosol
- Floradix
- Garden of Life prenatal products
- MegaFood Blood Builder
Because formulas change, always read the Supplement Facts or Drug Facts label. If it lists iron in milligrams, it should be separated from levothyroxine.
The science behind it
Levothyroxine is absorbed mainly in the small intestine, but what happens in the stomach and upper gut strongly affects how much is available for absorption. Iron is a positively charged metal ion that can form insoluble complexes with levothyroxine. This is similar to how other minerals, such as calcium, can reduce absorption of certain medications.
One of the classic clinical reports was published by Campbell and colleagues in Annals of Internal Medicine in 1992. They described patients who developed elevated TSH and reduced thyroid control after starting ferrous sulfate with levothyroxine, with improvement after the products were separated or adjusted. This helped establish the interaction as clinically meaningful, not just theoretical.
Additional evidence comes from pharmacokinetic and clinical studies, as well as product labeling and endocrine guidance. Reviews of levothyroxine interactions consistently list iron salts among the most important causes of reduced absorption. The mechanism is generally described as chelation or complex formation in the gastrointestinal tract, leading to less free levothyroxine available for uptake.
Guidance from the American Thyroid Association and major drug references also supports separating levothyroxine from iron by at least 4 hours. This recommendation is based on repeated observations that concurrent use can raise TSH and worsen hypothyroid symptoms. Similar caution appears in the prescribing information for Synthroid and other levothyroxine products.
Relevant references include:
- Campbell NR, Hasinoff BB, Stalts H, Rao B, Wong NC. Ferrous sulfate reduces thyroxine efficacy in patients with hypothyroidism. Ann Intern Med. 1992;117(12):1010-1013.
- Benvenga S, Bartolone L, Pappalardo MA, et al. Altered intestinal absorption of L-thyroxine caused by coffee, fiber, calcium, and iron has been documented across multiple clinical reports and reviews. Thyroid and related endocrine literature.
- American Thyroid Association guidance on levothyroxine administration and interacting substances.
- Prescribing information for Synthroid and other levothyroxine products.
Frequently Asked Questions
How long should I wait between levothyroxine and iron?
You should wait at least 4 hours between levothyroxine and any iron-containing product. This helps reduce the chance that iron will block absorption of your thyroid medication.
What should I do if I accidentally took levothyroxine and iron together?
If it happens once, do not panic and do not double your next dose. Just resume your usual schedule and keep the doses separated going forward; if this happens often, let your clinician know.
Are there alternatives if I need both levothyroxine and iron?
Yes. Most people can take both safely by separating them by 4 hours and using a consistent routine, such as levothyroxine in the morning and iron later in the day. Your clinician may also suggest a different iron formulation or adjust your levothyroxine dose based on follow-up labs.
Who is most at risk from this interaction?
People who are pregnant, have unstable thyroid levels, recently changed doses, or take multiple supplements are at higher risk. Anyone using prenatal vitamins or multivitamins with iron can be affected, even if they do not realize the product contains iron.
Can I take iron at night if I take levothyroxine in the morning?
Yes, that is often a good strategy. Taking levothyroxine in the morning and iron with dinner or at bedtime usually provides enough separation, as long as there are at least 4 hours between them.
What is the most common mistake people make with this interaction?
The most common mistake is forgetting that iron is hidden in multivitamins, prenatal vitamins, and “blood support” supplements. Another frequent problem is taking everything together in the morning for convenience, which can make levothyroxine less effective.
Key takeaways
- Iron can significantly reduce levothyroxine absorption, sometimes by up to 65%.
- This is a high-severity absorption interaction that can raise TSH and worsen hypothyroid symptoms.
- Take levothyroxine at least 4 hours apart from iron supplements, prenatal vitamins, and multivitamins with iron.
- Affected thyroid products include Synthroid, Levoxyl, Unithroid, Tirosint, Euthyrox, and generic levothyroxine.
- Affected iron products include ferrous sulfate, ferrous gluconate, ferrous fumarate, carbonyl iron, and iron-containing combination supplements.
- If you start or stop iron, you may need repeat thyroid blood tests in 6 to 8 weeks.
- Do not adjust your thyroid dose on your own; ask your clinician or pharmacist for a safe schedule.