Levothyroxine and supplements: 6 sourced interactions
Thyroid hormone, also sold as Synthroid, Levoxyl, Unithroid and Euthyrox. Of these, 4 studied in people and 2 named by an FDA label or NIH page. Plus 1 predicted from how each works (not established). Plus 2 food and drink lines. Each links its source and the date it was checked.
From the Interaction Map Sources checked Oct 25, 2026
A combination missing here is not on our list; that does not mean it is safe together.
Levothyroxine with supplements
Most serious first. Open a row for the mechanism, the source's words and the date checked.
Educational only. Check with your pharmacist or prescriber before changing anything you take.
Moderate
5BiotinStudiedBiotin can make thyroid blood tests wrong
moderate · Studied in people (PubMed) · evidence B · Lab test interference
Excess biotin in the blood interferes with tests that use biotin–streptavidin binding.
To raise with your pharmacist or prescriberTell your prescriber and the lab about biotin (including hair, skin and nail products) before thyroid tests, and ask how long to pause it first.
- PMID 28973622 · Li D, JAMA 2017 · Nonrandomised crossover trial (6 adults)
- PMID 32567529 · Moerman A, Acta Clin Belg 2022 · Narrative review
- Checked Oct 10, 2026
CalciumStudiedCalcium taken with levothyroxine lowers its absorption
moderate · Studied in people (PubMed) · evidence B · Absorption in the gut
In lab tests, thyroxine stuck to calcium carbonate at stomach-like acidity, which likely reduces how much is absorbed.
To raise with your pharmacist or prescriberDo not swallow them together. Ask your prescriber or pharmacist how far apart to take them, and whether to recheck TSH after starting or stopping calcium.
- PMID 10838651 · Singh N, JAMA 2000 · Prospective cohort study (20 patients)
- Checked Oct 10, 2026
IronStudiedIron taken with levothyroxine makes it less effective
moderate · Studied in people (PubMed) · evidence B · Absorption in the gut
Mixed in the lab, iron and thyroxine formed a poorly soluble complex, the likely cause.
To raise with your pharmacist or prescriberDo not swallow them together. Ask your prescriber how to space them, and about a TSH check after starting iron.
- PMID 1443969 · Campbell NR, Ann Intern Med 1992 · Uncontrolled clinical trial (14 patients)
- Checked Oct 10, 2026
ChromiumLabel or NIHAn NIH page names this pair (absorption in the gut).
moderate · Named by an FDA label or NIH page · Absorption in the gut
In the source’s own words; rated moderate from that wording, not from a study of the pair.
“A small study found that taking chromium picolinate supplements at the same time as levothyroxine (used to treat hypothyroidism) decreases levothyroxine absorption over 6 hours [78].”
- NIH Office of Dietary Supplements · Office of Dietary Supplements · Chromium, Interactions with Medications · Levothyroxine
- Checked Oct 25, 2026
MagnesiumLabel or NIHThe FDA label names this pair (absorption in the gut).
moderate · Named by an FDA label or NIH page · Absorption in the gut
In the source’s own words; rated moderate from that wording, not from a study of the pair.
“Sucralfate, antacids and proton pump inhibitors may cause hypochlorhydria, affect intragastric pH, and reduce levothyroxine absorption.”
- FDA drug label (DailyMed) · SYNTHROID (levothyroxine sodium) tablets, for oral use (AbbVie), section 7.1
- Checked Oct 23, 2026
Minor
2AshwagandhaStudiedAshwagandha may shift thyroid levels
minor · Studied in people (PubMed) · evidence B · Cause not established
The trial measured hormone levels; it did not establish how ashwagandha affects them.
To raise with your pharmacist or prescriberTell your prescriber before starting ashwagandha, and ask whether to recheck thyroid levels (inferred).
- PMID 28829155 · Sharma AK, J Altern Complement Med 2018 · Randomised placebo-controlled trial (50 adults)
- Checked Oct 10, 2026
ZincPredictedTaken together, Zinc may cut how much Levothyroxine is absorbed; spacing them apart is usually discussed.
minor · Predicted from how each works, not established · Absorption in the gut
Predicted from what each is known to do, not studied as a pair and not established.
Taken together, Zinc may cut how much Levothyroxine is absorbed; spacing them apart is usually discussed.
- NIH Office of Dietary Supplements · Zinc: binds medicines in the gut (Office of Dietary Supplements · Zinc: Fact Sheet for Health Professionals, Interactions with Medications > Antibiotics)
- FDA drug label (DailyMed) · Levothyroxine: absorption affected by other products (SYNTHROID (levothyroxine sodium) tablets, for oral use (AbbVie), section 2.1)
- Checked Oct 23, 2026
Levothyroxine with food and drink
Food and drink
2CoffeeStudiedCoffee with levothyroxine reduces how much is absorbed
moderate · Studied in people (PubMed) · evidence B · Absorption in the gut
Coffee interferes with thyroxine absorption in the gut.
To raise with your pharmacist or prescriberTake levothyroxine with water. In the study, waiting 60 minutes before coffee avoided the effect.
- PMID 18341376 · Benvenga S, Thyroid 2008 · Case series with controlled absorption tests; PubMed type: Case Reports
- Checked Oct 10, 2026
GrapefruitLabel or NIHThe FDA label names this pair (absorption in the gut).
moderate · Named by an FDA label or NIH page · Absorption in the gut
In the source’s own words; rated moderate from that wording, not from a study of the pair.
“Grapefruit juice may delay the absorption of levothyroxine and reduce its bioavailability.”
- FDA drug label (DailyMed) · SYNTHROID (levothyroxine sodium) tablets, for oral use (AbbVie), section 7.9
- Checked Oct 23, 2026
How these lines are sourced
Three kinds of line
- Studied in people (PubMed): PubMed records that were opened and read, graded A (several controlled studies, or a finding in the dosing guidance), B (at least one controlled human study) or C (case reports, observational data or mechanism only).
- Named by an FDA label or NIH page: quoted word for word from that page. Its severity comes from the source’s wording, not from a study of the pair.
- Predicted from how each works, not established: built from sourced facts about each substance, such as the FDA’s table of enzyme roles. Never rated major.
Interaction MapCheck Levothyroxine with everything else you take, together
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