Quick answer
Ask your pharmacist or doctor before combining. Official sources document 10 supplements that can matter with levothyroxine: calcium, iron, magnesium, chromium, inulin fiber, vitamin C, biotin, B complex, multivitamins and ashwagandha. The levothyroxine label and NIH describe lower absorption, wrong thyroid test results or changed thyroid hormone levels.1,2,3
Key facts
- The levothyroxine label lists calcium carbonate and ferrous sulfate among products that may bind the medicine and reduce how much is absorbed.1
- The same label warns that biotin supplements can give wrong results on thyroid hormone tests.2
- NIH says ashwagandha might interact with thyroid hormone medicines, because it raised thyroid hormone levels in studies.3
- Vitamin C is the reverse case: NIH says it might raise how much levothyroxine is absorbed.4
- MedlinePlus also names niacin, antacids and proton pump inhibitors among nonprescription products that may interact with levothyroxine.5
What is levothyroxine, and why do supplements matter with it?
MedlinePlus describes levothyroxine as a thyroid hormone that replaces the hormone the body normally makes. It is prescribed for an underactive thyroid and, alongside surgery and radioactive iodine, for thyroid cancer. The same page asks everyone who takes it to tell their doctor and pharmacist about every vitamin, nutritional supplement and herbal product they use.5
The 10 entries on this page fall into four groups: minerals that can bind the tablet in the gut, a fiber that may do the same, vitamins that can change absorption or the blood tests used to set the dose, and one herb that may change thyroid hormone levels directly.1,2,3,6
Which minerals can lower how much levothyroxine is absorbed?
Calcium and iron carry the clearest wording, because the levothyroxine label names calcium carbonate and ferrous sulfate and says they may bind the medicine and reduce its absorption.1,7
Ask your pharmacist how far apart to take them
Calcium carbonate can bind levothyroxine in the gut and reduce how much is absorbed. The levothyroxine label lists calcium carbonate among products to keep apart from the medicine, and a small study found calcium citrate and acetate had a similar effect; ask your pharmacist how to space them.1,8,9,10,11
On the medicine's label
The small study behind the calcium entry used 8 healthy adults and measured a drop in absorption of about 20% to 25% over the following 6 hours with calcium as carbonate, citrate or acetate.11
Ask your pharmacist how far apart to take them
Iron supplements can reduce how much levothyroxine the body absorbs and make it less effective. The levothyroxine label lists ferrous sulfate among products that may bind the medicine and says to take them apart; ask your pharmacist how to space them.7,12,13,14
On the medicine's label
Both of NIH's iron fact sheets, the one for consumers and the one for health professionals, name levothyroxine among the medicines iron can interact with.12,13
Ask your pharmacist how far apart to take them
Calcium and iron can reduce how much levothyroxine is absorbed. The levothyroxine label tells patients that iron and calcium supplements can decrease its absorption, and MedlinePlus names calcium carbonate and iron supplements, and also niacin, among products that may interact with it. Many multivitamins contain calcium, and some contain iron.15,16,17,18
On the medicine's label
A multivitamin is where many people take calcium and iron without noticing. Read its Supplement Facts panel for both minerals, and ask your pharmacist what the label means for the product you use.17,18
Ask your pharmacist how far apart to take them
The levothyroxine label lists antacids, such as those made with aluminum and magnesium hydroxides, among products that may reduce levothyroxine absorption by lowering stomach acid. In a small 2025 study, magnesium aspartate taken at the same time lowered levothyroxine absorption by 12%, and magnesium citrate by 7%, a drop too small to rule out chance.19,20,21
On the medicine's label
The 2025 trial behind the magnesium entry gave a single levothyroxine dose to 15 healthy adults. Its authors still advised taking levothyroxine apart from magnesium products.21
Ask your pharmacist how far apart to take them
In a small study of seven volunteers, chromium picolinate taken at the same time as levothyroxine lowered how much levothyroxine was absorbed over 6 hours. NIH lists this interaction and says it might keep you from getting the full effect of the medicine.22,23,24
Reported in studies
The chromium entry rests on one small study of seven volunteers, and NIH's consumer sheet lists levothyroxine among the medicines chromium may interact with.22,24
Does fiber such as inulin affect levothyroxine?
The levothyroxine label names soybean flour, cottonseed meal, walnuts and dietary fiber as things that may bind the medicine and lower how much is absorbed. It does not name inulin, but FDA allows inulin to be counted as dietary fiber on Supplement Facts labels, which is why the entry below exists.6,25
Ask your pharmacist or doctor first
The levothyroxine label says dietary fiber may bind levothyroxine and lower how much is absorbed. Inulin counts as dietary fiber on US labels, although the levothyroxine label does not name it.6,25
On the medicine's label
No study on this page tested inulin with levothyroxine. The entry applies the label's general fiber wording, so ask your pharmacist about any fiber product you use, whether it is a powder, a bar or a drink mix.6
Can vitamin C change how much levothyroxine you absorb?
Vitamin C is the one entry that points toward more absorption, not less. NIH's fact sheet on immune function says vitamin C taken with levothyroxine might raise how much of the medicine is absorbed.4
Ask your pharmacist or doctor first
Vitamin C taken at the same time as levothyroxine might increase how much of the medicine is absorbed.4
Reported in studies
The entry says only that it might. It carries the ask-first label, not the spacing label used for minerals, so the useful step is to tell your prescriber that you take it.
Can biotin or a B complex change thyroid blood test results?
These two entries are about testing, not absorption. The levothyroxine label says biotin interferes with thyroid hormone tests built on biotin and streptavidin and may give wrong results, and NIH says intakes above the recommended amount can make some lab tests read falsely high or low.2,26
Ask your pharmacist or doctor first
Biotin can distort the thyroid blood tests used to check and adjust a levothyroxine dose. The levothyroxine label says biotin supplements interfere with thyroid hormone tests built on biotin and streptavidin, which may give wrong results, and tells patients to stop biotin and biotin-containing supplements for a time before thyroid testing.2,26
On the medicine's label
Ask your pharmacist or doctor first
B complex supplements can contain biotin. The levothyroxine label says biotin interferes with thyroid hormone tests built on biotin and streptavidin, which may give wrong results, and tells patients to stop biotin and any biotin-containing supplement for a time before thyroid function testing.27,28
On the medicine's label
Because B complex products can contain biotin, the label's request that prescribers ask about biotin applies to them as well. Mention your B complex before any thyroid blood test.27,28
Which herbal supplements should you avoid with levothyroxine?
Ashwagandha is the only herbal supplement among the sources checked. NIH's fact sheet for professionals and NCCIH both say it might interact with thyroid hormone medicines, and NIH links it to thyroid changes.3,29
Ask your pharmacist or doctor first
NIH says ashwagandha might interact with thyroid hormone medicines, such as levothyroxine. The concern comes from studies in which ashwagandha raised thyroid hormone levels, and from case reports of an overactive thyroid in people taking it.3,29,30,31
Expected from how the two work, not shown in studies
Ashwagandha may change thyroid hormone levels. In a trial of 50 people with a mildly underactive thyroid, 600 mg a day for 8 weeks lowered TSH and raised T3 and T4, and three women developed an overactive thyroid while taking it, which settled when they stopped.3,32
The sources checked do not address other herbal supplements. That is not a finding either way, so ask your pharmacist.
Which supplements should you avoid with thyroid problems, nodules or thyroid cancer?
Searches about an underactive thyroid, thyroid problems or thyroid cancer lead here because MedlinePlus lists those as reasons levothyroxine is prescribed. This page sorts supplements by what the sources say they do to the medicine, not by the thyroid condition.5
The sources checked do not address thyroid nodules, and they say nothing about supplements for a person who does not take levothyroxine. Questions about your own thyroid condition belong with your doctor.
What do 'on the label', 'in studies' and 'expected' mean on this page?
| Supplement | How it is known | Label shown |
|---|---|---|
| Calcium | On the levothyroxine label | Ask your pharmacist how far apart |
| Iron | On the levothyroxine label | Ask your pharmacist how far apart |
| Multivitamin | On the levothyroxine label, through its calcium and iron | Ask your pharmacist how far apart |
| Magnesium | On the levothyroxine label, through antacid ingredients | Ask your pharmacist how far apart |
| Chromium | In studies | Ask your pharmacist how far apart |
| Inulin | On the levothyroxine label, through its wording on fiber | Ask your pharmacist or doctor first |
| Biotin | On the levothyroxine label | Ask your pharmacist or doctor first |
| B complex | On the levothyroxine label, through its biotin | Ask your pharmacist or doctor first |
| Vitamin C | In studies | Ask your pharmacist or doctor first |
| Ashwagandha | Expected from how the two work | Ask your pharmacist or doctor first |
Seven entries rest on the label, two on studies and one is expected. Five carry the spacing label and five the ask-first label. No entry here carries the avoid label, so the honest answer to the question in the title is mostly about spacing and about telling your prescriber before thyroid tests.
What to ask your pharmacist about levothyroxine and supplements
- Do my multivitamin, antacid or other products contain calcium, iron or magnesium?
- How far apart should I take levothyroxine from each of them?
- Do any of my supplements contain biotin, and should I mention that before my next thyroid blood test?
- Do I use a fiber product or an acid reducer that could matter too?5
- If I start, stop or change a supplement, should my thyroid levels be checked sooner?31
Questions people ask
What supplements interact with levothyroxine?
The sources checked document 10: calcium, iron, magnesium, chromium, inulin fiber, vitamin C, biotin, B complex, multivitamins and ashwagandha. They describe lower absorption, wrong thyroid test results or changed thyroid hormone levels. MedlinePlus also names niacin, antacids and proton pump inhibitors.1,2,5
What vitamins should you not take with levothyroxine?
The vitamins in the sources checked are vitamin C, biotin and B complex, and multivitamins matter through their calcium and iron. None is labeled avoid: the entries ask for spacing or for notice before thyroid tests, so ask your pharmacist about each one.2,4,15
What herbal supplements interact with levothyroxine?
Ashwagandha is the herbal supplement the sources checked name: NIH says it might interact with thyroid hormone medicines such as levothyroxine. The sources do not address other herbal products. That is not a finding either way, so ask your pharmacist.3,29
What supplements should you avoid with hypothyroidism or thyroid problems?
Levothyroxine is prescribed for an underactive thyroid, so this page lists what is documented for that medicine, not for the condition. The sources checked do not rank supplements by thyroid condition, so ask your doctor about your own situation.5
What supplements should you avoid with thyroid medication?
This page covers levothyroxine only. Other thyroid medicines have their own labels, which the sources checked here do not cover, so ask your pharmacist about any other thyroid medicine you take.1
What supplements should you avoid with thyroid cancer or thyroid nodules?
MedlinePlus says levothyroxine is also used alongside surgery and radioactive iodine for thyroid cancer. The sources checked do not address thyroid nodules or give supplement advice for either condition, so ask your doctor.5
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Sources
- Levothyroxine sodium tablets: prescribing information (Alvogen). DailyMed, U.S. National Library of Medicine. Effective September 15, 2026. Opened 2026-10-07.
- Levothyroxine sodium tablets: prescribing information (Cardinal Health 107, LLC). DailyMed, U.S. National Library of Medicine. Published September 24, 2026. Opened 2026-10-07.
- National Institutes of Health, Office of Dietary Supplements. Ashwagandha: Is it helpful for stress, anxiety, or sleep? Fact Sheet for Health Professionals. Updated May 2, 2025. Opened 2026-10-07.
- National Institutes of Health, Office of Dietary Supplements. Dietary Supplements for Immune Function and Infectious Diseases: Fact Sheet for Health Professionals. Updated March 10, 2025. Opened 2026-10-07.
- MedlinePlus, U.S. National Library of Medicine. Levothyroxine: MedlinePlus Drug Information. Last revised February 15, 2019. Opened 2026-10-11.
- Levothyroxine sodium tablets: prescribing information (Cardinal Health 107, LLC). DailyMed, U.S. National Library of Medicine. Version effective September 18, 2026. Opened 2026-10-07.
- Levothyroxine sodium tablets: prescribing information (Alvogen). DailyMed, U.S. National Library of Medicine. Effective September 15, 2026. Opened 2026-10-07.
- Office of Dietary Supplements, National Institutes of Health. Calcium: Fact Sheet for Consumers. Updated September 14, 2023. Opened 2026-10-07.
- Office of Dietary Supplements, National Institutes of Health. Calcium: Fact Sheet for Health Professionals. Updated June 22, 2026. Opened 2026-10-07.
- MedlinePlus, U.S. National Library of Medicine. Levothyroxine: MedlinePlus Drug Information. Last revised February 15, 2019. Opened 2026-10-07.
- Zamfirescu I, Carlson HE. Absorption of levothyroxine when coadministered with various calcium formulations. Thyroid. 2011;21(5):483-486. Opened 2026-10-07.
- Office of Dietary Supplements, National Institutes of Health. Iron: Fact Sheet for Consumers. Updated August 17, 2023. Opened 2026-10-07.
- Office of Dietary Supplements, National Institutes of Health. Iron: Fact Sheet for Health Professionals. Updated September 4, 2025. Opened 2026-10-07.
- MedlinePlus, U.S. National Library of Medicine. Levothyroxine: MedlinePlus Drug Information. Last revised February 15, 2019. Opened 2026-10-07.
- Levothyroxine sodium tablets: prescribing information (Cardinal Health 107). DailyMed, U.S. National Library of Medicine. Published September 24, 2026. Opened 2026-10-07.
- MedlinePlus. Levothyroxine. U.S. National Library of Medicine. Last revised February 15, 2019. Opened 2026-10-07.
- National Institutes of Health, Office of Dietary Supplements. Calcium: Fact Sheet for Consumers. Updated September 14, 2023. Opened 2026-10-07.
- National Institutes of Health, Office of Dietary Supplements. Iron: Fact Sheet for Health Professionals. Updated September 4, 2025. Opened 2026-10-07.
- Synthroid (levothyroxine sodium) tablets: prescribing information (AbbVie). DailyMed, U.S. National Library of Medicine. Label effective February 20, 2024. Opened 2026-10-07.
- Levothyroxine. MedlinePlus Drug Information, U.S. National Library of Medicine. Last revised February 15, 2019. Opened 2026-10-07.
- Attinger MC, von Felten S, Rodrigues CL, Krützfeldt J, Risch L, Bonzon J. Single Center, Open-Label, Randomized Crossover Trial on Drug-Drug Interactions of Levothyroxine/Magnesium-Citrate and Levothyroxine/Magnesium-Aspartate in Healthy Subjects: The ThyroMag Trial. Clin Transl Sci. 2025;18(11):e70409. Opened 2026-10-07.
- Office of Dietary Supplements, National Institutes of Health. Chromium: Fact Sheet for Consumers. Updated March 22, 2021. Opened 2026-10-07.
- National Institutes of Health, Office of Dietary Supplements. Chromium: Fact Sheet for Health Professionals. Updated June 2, 2022. Opened 2026-10-07.
- John-Kalarickal J, Pearlman G, Carlson HE. New medications which decrease levothyroxine absorption. Thyroid. 2007;17(8):763-765. Opened 2026-10-07.
- U.S. Food and Drug Administration. Questions and Answers on Dietary Fiber. Opened 2026-10-07.
- National Institutes of Health, Office of Dietary Supplements. Biotin: Fact Sheet for Health Professionals. Updated January 10, 2022. Opened 2026-10-07.
- Levothyroxine sodium tablets: prescribing information (Cardinal Health 107, LLC). DailyMed, U.S. National Library of Medicine. Published September 24, 2026. Opened 2026-10-07.
- National Institutes of Health, Office of Dietary Supplements. Biotin: Fact Sheet for Health Professionals. Updated January 10, 2022. Opened 2026-10-07.
- National Center for Complementary and Integrative Health. Ashwagandha. Last updated March 2023. Opened 2026-10-07.
- Office of Dietary Supplements, National Institutes of Health. Ashwagandha: Is it helpful for stress, anxiety, or sleep? Fact Sheet for Consumers. Updated December 4, 2023. Opened 2026-10-07.
- MedlinePlus. Levothyroxine. U.S. National Library of Medicine. Last revised February 15, 2019. Opened 2026-10-07.
- Sharma AK, Basu I, Singh S. Efficacy and Safety of Ashwagandha Root Extract in Subclinical Hypothyroid Patients: A Double-Blind, Randomized Placebo-Controlled Trial. J Altern Complement Med. 2018;24(3):243-248. Opened 2026-10-07.
This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.